Đang tải... (xem toàn văn)
Part 2 book “Operating systems - Internals and designprinciples” has contents: I/O management and disk scheduling, file management, embedded operating systems, virtual machines, operating system securit, cloud and IoT operating systems.
www.downloadslide.net Part Input/Output and Files Chapter I/O Management and Disk Scheduling 11.1 11.2 I/O Devices Organization of the I/O Function The Evolution of the I/O Function Direct Memory Access 11.3 Operating System Design Issues Design Objectives Logical Structure of the I/O Function 11.4 I/O Buffering Single Buffer Double Buffer Circular Buffer The Utility of Buffering 11.5 Disk Scheduling Disk Performance Parameters Disk Scheduling Policies 11.6 RAID RAID Level RAID Level RAID Level RAID Level RAID Level RAID Level RAID Level 11.7 Disk Cache Design Considerations Performance Considerations 11.8 UNIX SVR4 I/O Buffer Cache Character Queue Unbuffered I/O UNIX Devices 11.9 Linux I/O Disk Scheduling Linux Page Cache 11.10 Windows I/O Basic I/O Facilities Asynchronous and Synchronous I/O Software RAID Volume Shadow Copies Volume Encryption 11.11 Summary 11.12 Key Terms, Review Questions, and Problems M11_STAL4290_09_GE_C11.indd 505 505 5/9/17 4:32 PM www.downloadslide.net 506 Chapter 11 / I/O Management and Disk Scheduling Learning Objectives After studying this chapter, you should be able to: • Summarize key categories of I/O devices on computers • Discuss the organization of the I/O function • Explain some of the key issues in the design of OS support for I/O • Analyze the performance implications of various I/O buffering alternatives • Understand the performance issues involved in magnetic disk access • Explain the concept of RAID and describe the various levels • Understand the performance implications of disk cache • Describe the I/O mechanisms in UNIX, Linux, and Windows Perhaps the messiest aspect of operating system design is input/output Because there is such a wide variety of devices and applications of those devices, it is difficult to develop a general, consistent solution We begin with a brief discussion of I/O devices and the organization of the I/O function These topics, which generally come within the scope of computer architecture, set the stage for an examination of I/O from the point of view of the OS The next section examines operating system design issues, including design objectives, and the way in which the I/O function can be structured Then I/O buffering is examined; one of the basic I/O services provided by the operating system is a buffering function, which improves overall performance The next sections of the chapter are devoted to magnetic disk I/O In contemporary systems, this form of I/O is the most important and is key to the performance as perceived by the user We begin by developing a model of disk I/O performance then examine several techniques that can be used to enhance performance Appendix J summarizes characteristics of secondary storage devices, including magnetic disk and optical memory 11.1 I/O DEVICES As was mentioned in Chapter 1, external devices that engage in I/O with computer systems can be roughly grouped into three categories: Human readable: Suitable for communicating with the computer user Examples include printers and terminals, the latter consisting of video display, keyboard, and perhaps other devices such as a mouse Machine readable: Suitable for communicating with electronic equipment Examples are disk drives, USB keys, sensors, controllers, and actuators Communication: Suitable for communicating with remote devices Examples are digital line drivers and modems M11_STAL4290_09_GE_C11.indd 506 5/9/17 4:32 PM www.downloadslide.net 11.1 / I/O DEVICES 507 Gigabit ethernet Graphics display Hard disk Ethernet Optical disk Scanner Laser printer Floppy disk Modem Mouse Keyboard 101 102 103 104 105 Data Rate (bps) 106 107 108 109 Figure 11.1 Typical I/O Device Data Rates There are great differences across classes and even substantial differences within each class Among the key differences are the following: • Data rate: There may be differences of several orders of magnitude between the data transfer rates Figure 11.1 gives some examples • Application: The use to which a device is put has an influence on the software and policies in the OS and supporting utilities For example, a disk used for files requires the support of file management software A disk used as a backing store for pages in a virtual memory scheme depends on the use of virtual memory hardware and software Furthermore, these applications have an impact on disk scheduling algorithms (discussed later in this chapter) As another example, a terminal may be used by an ordinary user or a system administrator These uses imply different privilege levels and perhaps different priorities in the OS • Complexity of control: A printer requires a relatively simple control interface A disk is much more complex The effect of these differences on the OS is filtered to some extent by the complexity of the I/O module that controls the device, as discussed in the next section • Unit of transfer: Data may be transferred as a stream of bytes or characters (e.g., terminal I/O) or in larger blocks (e.g., disk I/O) • Data representation: Different data encoding schemes are used by different devices, including differences in character code and parity conventions M11_STAL4290_09_GE_C11.indd 507 5/9/17 4:32 PM www.downloadslide.net 508 Chapter 11 / I/O Management and Disk Scheduling • Error conditions: The nature of errors, the way in which they are reported, their consequences, and the available range of responses differ widely from one device to another This diversity makes a uniform and consistent approach to I/O, both from the point of view of the operating system and from the point of view of user processes, difficult to achieve 11.2 ORGANIZATION OF THE I/O FUNCTION Appendix C summarizes three techniques for performing I/O: Programmed I/O: The processor issues an I/O command, on behalf of a process, to an I/O module; that process then busy waits for the operation to be completed before proceeding Interrupt-driven I/O: The processor issues an I/O command on behalf of a process There are then two possibilities If the I/O instruction from the process is nonblocking, then the processor continues to execute instructions from the process that issued the I/O command If the I/O instruction is blocking, then the next instruction that the processor executes is from the OS, which will put the current process in a blocked state and schedule another process Direct memory access (DMA): A DMA module controls the exchange of data between main memory and an I/O module The processor sends a request for the transfer of a block of data to the DMA module, and is interrupted only after the entire block has been transferred Table 11.1 indicates the relationship among these three techniques In most computer systems, DMA is the dominant form of transfer that must be supported by the operating system The Evolution of the I/O Function As computer systems have evolved, there has been a pattern of increasing complexity and sophistication of individual components Nowhere is this more evident than in the I/O function The evolutionary steps can be summarized as follows: The processor directly controls a peripheral device This is seen in simple microprocessor-controlled devices A controller or I/O module is added The processor uses programmed I/O without interrupts With this step, the processor becomes somewhat divorced from the specific details of external device interfaces Table 11.1 I/O Techniques I/O-to-Memory Transfer through Processor Direct I/O-to-Memory Transfer M11_STAL4290_09_GE_C11.indd 508 No Interrupts Use of Interrupts Programmed I/O Interrupt-driven I/O Direct memory access (DMA) 5/9/17 4:32 PM www.downloadslide.net 11.2 / ORGANIZATION OF THE I/O FUNCTION 509 The same configuration as step is used, but now interrupts are employed The processor need not spend time waiting for an I/O operation to be performed, thus increasing efficiency The I/O module is given direct control of memory via DMA It can now move a block of data to or from memory without involving the processor, except at the beginning and end of the transfer The I/O module is enhanced to become a separate processor, with a specialized instruction set tailored for I/O The central processing unit (CPU) directs the I/O processor to execute an I/O program in main memory The I/O processor fetches and executes these instructions without processor intervention This allows the processor to specify a sequence of I/O activities and to be interrupted only when the entire sequence has been performed The I/O module has a local memory of its own and is, in fact, a computer in its own right With this architecture, a large set of I/O devices can be controlled, with minimal processor involvement A common use for such an architecture has been to control communications with interactive terminals The I/O processor takes care of most of the tasks involved in controlling the terminals As one proceeds along this evolutionary path, more and more of the I/O function is performed without processor involvement The central processor is increasingly relieved of I/O-related tasks, improving performance With the last two steps (5 and 6), a major change occurs with the introduction of the concept of an I/O module capable of executing a program A note about terminology: For all of the modules described in steps through 6, the term direct memory access is appropriate, because all of these types involve direct control of main memory by the I/O module Also, the I/O module in step is often referred to as an I/O channel, and that in step as an I/O processor; however, each term is, on occasion, applied to both situations In the latter part of this section, we will use the term I/O channel to refer to both types of I/O modules Direct Memory Access Figure 11.2 indicates, in general terms, the DMA logic The DMA unit is capable of mimicking the processor and, indeed, of taking over control of the system bus just like a processor It needs to this to transfer data to and from memory over the system bus The DMA technique works as follows When the processor wishes to read or write a block of data, it issues a command to the DMA module by sending to the DMA module the following information: • Whether a read or write is requested, using the read or write control line between the processor and the DMA module • The address of the I/O device involved, communicated on the data lines • The starting location in memory to read from or write to, communicated on the data lines and stored by the DMA module in its address register • The number of words to be read or written, again communicated via the data lines and stored in the data count register M11_STAL4290_09_GE_C11.indd 509 5/9/17 4:32 PM www.downloadslide.net 510 Chapter 11 / I/O Management and Disk Scheduling Data count Data lines Data register Address lines Address register Request to DMA Acknowledge from DMA Interrupt Read Write Control logic Figure 11.2 Typical DMA Block Diagram The processor then continues with other work It has delegated this I/O operation to the DMA module The DMA module transfers the entire block of data, one word at a time, directly to or from memory, without going through the processor When the transfer is complete, the DMA module sends an interrupt signal to the processor Thus, the processor is involved only at the beginning and end of the transfer (see Figure C.4c) The DMA mechanism can be configured in a variety of ways Some possibilities are shown in Figure 11.3 In the first example, all modules share the same system bus The DMA module, acting as a surrogate processor, uses programmed I/O to exchange data between memory and an I/O module through the DMA module This configuration, while it may be inexpensive, is clearly inefficient: As with processor-controlled programmed I/O, each transfer of a word consumes two bus cycles (transfer request followed by transfer) The number of required bus cycles can be cut substantially by integrating the DMA and I/O functions As Figure 11.3b indicates, this means there is a path between the DMA module and one or more I/O modules that does not include the system bus The DMA logic may actually be a part of an I/O module, or it may be a separate module that controls one or more I/O modules This concept can be taken one step further by connecting I/O modules to the DMA module using an I/O bus (see Figure 11.3c) This reduces the number of I/O interfaces in the DMA module to one and provides for an easily expandable configuration In all of these cases (see Figures 11.3b and 11.3c), the system bus that the DMA module shares with the processor and main memory is used by the DMA module only to exchange data with memory and to exchange control signals with the processor The exchange of data between the DMA and I/O modules takes place off the system bus M11_STAL4290_09_GE_C11.indd 510 5/9/17 4:32 PM www.downloadslide.net 11.3 / OPERATING SYSTEM DESIGN ISSUES 511 Processor DMA I/O Memory I/O (a) Single-bus, detached DMA Processor DMA Memory DMA I/O I/O I/O (b) Single-bus, integrated DMA-I/O System bus Processor Memory DMA I/O bus I/O I/O I/O (c) I/O bus Figure 11.3 Alternative DMA Configurations 11.3 OPERATING SYSTEM DESIGN ISSUES Design Objectives Two objectives are paramount in designing the I/O facility: efficiency and generality Efficiency is important because I/O operations often form a bottleneck in a computing system Looking again at Figure 11.1, we see that most I/O devices are extremely slow compared with main memory and the processor One way to tackle this problem is multiprogramming, which, as we have seen, allows some processes to be waiting on I/O operations while another process is executing However, even with the vast size of main memory in today’s machines, it will still often be the case that I/O is not keeping up with the activities of the processor Swapping is used to bring in additional ready processes to keep the processor busy, but this in itself is an I/O operation Thus, a major effort in I/O design has been schemes for improving the efficiency of the I/O The area that has received the most attention, because of its importance, is disk I/O, and much of this chapter will be devoted to a study of disk I/O efficiency M11_STAL4290_09_GE_C11.indd 511 5/9/17 4:32 PM www.downloadslide.net 512 Chapter 11 / I/O Management and Disk Scheduling The other major objective is generality In the interests of simplicity and freedom from error, it is desirable to handle all devices in a uniform manner This applies both to the way in which processes view I/O devices, and to the way in which the OS manages I/O devices and operations Because of the diversity of device characteristics, it is difficult in practice to achieve true generality What can be done is to use a hierarchical, modular approach to the design of the I/O function This approach hides most of the details of device I/O in lower-level routines so user processes and upper levels of the OS see devices in terms of general functions such as read, write, open, close, lock, and unlock We turn now to a discussion of this approach Logical Structure of the I/O Function In Chapter 2, in the discussion of system structure, we emphasized the hierarchical nature of modern operating systems The hierarchical philosophy is that the functions of the OS should be separated according to their complexity, their characteristic time scale, and their level of abstraction Applying this philosophy specifically to the I/O facility leads to the type of organization suggested by Figure 11.4 The details of the organization will depend on the type of device and the application The three most important logical structures are presented in the figure Of course, a particular operating system may not conform exactly to these structures However, the general principles are valid, and most operating systems approach I/O in approximately this way Let us consider the simplest case first, that of a local peripheral device that communicates in a simple fashion, such as a stream of bytes or records (see Figure 11.4a) The following layers are involved: • Logical I/O: The logical I/O module deals with the device as a logical resource and is not concerned with the details of actually controlling the device The logical I/O module is concerned with managing general I/O functions on behalf of user processes, allowing them to deal with the device in terms of a device identifier and simple commands such as open, close, read, and write • Device I/O: The requested operations and data (buffered characters, records, etc.) are converted into appropriate sequences of I/O instructions, channel commands, and controller orders Buffering techniques may be used to improve utilization • Scheduling and control: The actual queueing and scheduling of I/O operations occurs at this layer, as well as the control of the operations Thus, interrupts are handled at this layer and I/O status is collected and reported This is the layer of software that actually interacts with the I/O module and hence the device hardware For a communications device, the I/O structure (see Figure 11.4b) looks much the same as that just described The principal difference is that the logical I/O module is replaced by a communications architecture, which may itself consist of a number of layers An example is TCP/IP, which will be discussed in Chapter 17 Figure 11.4c shows a representative structure for managing I/O on a secondary storage device that supports a file system The three layers not previously discussed are as follows: Directory management: At this layer, symbolic file names are converted to identifiers that either reference the file directly or indirectly through a file M11_STAL4290_09_GE_C11.indd 512 5/9/17 4:32 PM www.downloadslide.net 11.3 / OPERATING SYSTEM DESIGN ISSUES 513 User processes User processes User processes Directory management Logical I/O Communication architecture File system Physical organization Device I/O Device I/O Device I/O Scheduling & control Scheduling & control Scheduling & control Hardware Hardware Hardware (a) Local peripheral device (b) Communications port (c) File system Figure 11.4 A Model of I/O Organization descriptor or index table This layer is also concerned with user operations that affect the directory of files, such as add, delete, and reorganize File system: This layer deals with the logical structure of files and with the operations that can be specified by users, such as open, close, read, and write Access rights are also managed at this layer Physical organization: Just as virtual memory addresses must be converted into physical main memory addresses, taking into account the segmentation and paging structure, logical references to files and records must be converted to physical secondary storage addresses, taking into account the physical track and sector structure of the secondary storage device Allocation of secondary storage space and main storage buffers is generally treated at this layer as well Because of the importance of the file system, we will spend some time, in this chapter and the next, looking at its various components The discussion in this chapter focuses on the lower three layers, while the upper two layers will be examined in Chapter 12 M11_STAL4290_09_GE_C11.indd 513 5/9/17 4:32 PM www.downloadslide.net 514 Chapter 11 / I/O Management and Disk Scheduling 11.4 I/O BUFFERING Suppose a user process wishes to read blocks of data from a disk one at a time, with each block having a length of 512 bytes The data are to be read into a data area within the address space of the user process at virtual location 1000 to 1511 The simplest way would be to execute an I/O command (something like Read_Block[1000, disk]) to the disk unit then wait for the data to become available The waiting could either be busy waiting (continuously test the device status) or, more practically, process suspension on an interrupt There are two problems with this approach First, the program is up waiting for the relatively slow I/O to complete The second problem is that this approach to I/O interferes with swapping decisions by the OS Virtual locations 1000 to 1511 must remain in main memory during the course of the block transfer Otherwise, some of the data may be lost If paging is being used, at least the page containing the target locations must be locked into main memory Thus, although portions of the process may be paged out to disk, it is impossible to swap the process out completely, even if this is desired by the operating system Notice also there is a risk of single-process deadlock If a process issues an I/O command, is suspended awaiting the result, and then is swapped out prior to the beginning of the operation, the process is blocked waiting on the I/O event, and the I/O operation is blocked waiting for the process to be swapped in To avoid this deadlock, the user memory involved in the I/O operation must be locked in main memory immediately before the I/O request is issued, even though the I/O operation is queued and may not be executed for some time The same considerations apply to an output operation If a block is being transferred from a user process area directly to an I/O module, then the process is blocked during the transfer and the process may not be swapped out To avoid these overheads and inefficiencies, it is sometimes convenient to perform input transfers in advance of requests being made, and to perform output transfers some time after the request is made This technique is known as buffering In this section, we look at some of the buffering schemes that are supported by operating systems to improve the performance of the system In discussing the various approaches to buffering, it is sometimes important to make a distinction between two types of I/O devices: block-oriented and streamoriented A block-oriented device stores information in blocks that are usually of fixed size, and transfers are made one block at a time Generally, it is possible to reference data by its block number Disks and USB keys are examples of blockoriented devices A stream-oriented device transfers data in and out as a stream of bytes, with no block structure Terminals, printers, communications ports, mouse and other pointing devices, and most other devices that are not secondary storage are stream-oriented Single Buffer The simplest type of support that the OS can provide is single buffering (see Figure 11.5b) When a user process issues an I/O request, the OS assigns a buffer in the system portion of main memory to the operation M11_STAL4290_09_GE_C11.indd 514 5/9/17 4:32 PM www.downloadslide.net Q-14 Appendix Q / eCos cyg_mutex_t res_lock; res_t res_pool[RES_MAX]; int res_count = RES_MAX; void res_init(void) { cyg_mutex_init(&res_lock); } res_t res_allocate(void) { res_t res; cyg_mutex_lock(&res_lock); if( res_count == ) res =RES_NONE; else { res_count ; res =res_pool[res_count]; } cyg_mutex_unlock(&res_lock); return res; } void res_free(res_t res) { cyg_mutex_lock(&res_lock); res_pool[res_count] =res; res_count++; cyg_mutex_unlock(&res_lock); } // lock the mutex // check for free resource // return RES_NONE if none // allocate a resources // unlock the mutex // lock the mutex // free the resource // unlock the mutex Figure Q.7 Controlling Access to a Pool of Resources Using Mutexes Figure Q.8 accomplishes this with the use of a condition variable associated with the mutex When res_allocate detects that there are no resources, it calls cyg_cond_ wait This latter function unlocks the mutex and puts the calling thread to sleep on the condition variable When res_free is eventually called, it puts a resource back into the pool and calls cyg_cond_signal to wake up any thread waiting on the condition variable When the waiting thread eventually gets to run again, it will relock the mutex before returning from cyg_cond_wait There are two significant features of this example, and of the use of condition variables in general First, the mutex unlock and wait in cyg_cond_wait are atomic: No other thread can run between the unlock and the wait If this were not the case, then a call to res_free by some other thread would release the resource, but the call to cyg_cond_signal would be lost, and the first thread would end up waiting when there were resources available Z22_STAL4290_09_GE_APPQ.indd 14 4/18/17 7:36 AM www.downloadslide.net Q.4 / eCOS THREAD SYNCHRONIZATION Q-15 cyg_mutex_t res_lock; cyg_cond_t res_wait; res_t res_pool[RES_MAX]; int res_count =RES_MAX; void res_init(void) { cyg_mutex_init(&res_lock); cyg_cond_init(&res_wait, &res_lock); } res_t res_allocate(void) { res_t res; cyg_mutex_lock(&res_lock); // lock the mutex while( res_count == ) // wait for a resources cyg_cond_wait(&res_wait); res_count ; // allocate a resource res =res_pool[res_count]; cyg_mutex_unlock(&res_lock); // unlock the mutex return res; } void res_free(res_t res) { cyg_mutex_lock(&res_lock); // lock the mutex res_pool[res_count] =res; // free the resource res_count++; cyg_cond_signal(&res_wait); // wake up any waiting allocators cyg_mutex_unlock(&res_lock); // unlock the mutex } Figure Q.8 Controlling Access to a Pool of Resources Using Mutexes and Condition Variables The second feature is that the call to cyg_cond_wait is in a while loop and not a simple if statement This is because of the need to relock the mutex in cyg_ cond_wait when the signaled thread reawakens If there are other threads already queued to claim the lock, then this thread must wait Depending on the scheduler and the queue order, many other threads may have entered the critical section before this one gets to run So the condition that it was waiting for may have been rendered false Using a loop around all condition variable wait operations is the only way to guarantee that the condition being waited for is still true after waiting Event Flags An event flag is a 32-bit word used as a synchronization mechanism Application code may associate a different event with each bit in a flag A thread can wait for either a single event or a combination of events by checking one or multiple bits in the Z22_STAL4290_09_GE_APPQ.indd 15 4/18/17 7:36 AM www.downloadslide.net Q-16 Appendix Q / eCos corresponding flag The thread is blocked until all of the required bits are set (AND) or until at least one of the bits is set (OR) A signaling thread can set or reset bits based on specific conditions or events so that the appropriate thread is unblocked For example, bit could represent completion of a specific I/O operation, making data available, and bit could indicate that the user has pressed a start button A producer thread or DSR could set these two bits, and a consumer thread waiting on these two events will be woken up A thread can wait on one or more events using the cyg_flag_wait command, which takes three arguments: a particular event flag, a combination of bit positions in the flag, and a mode parameter The mode parameter specifies whether the thread will block until all the bits are set (AND) or until at least one of the bits is set (OR) The mode parameter may also specify that when the wait succeeds, the entire event flag is cleared (set to all zeros) Mailboxes Mailboxes, also called message boxes, are an eCos synchronization mechanism that provides a means for two threads to exchange information Section 5.5 provides a general discussion of message-passing synchronization Here, we look at the specifics of the eCos version The eCos mailbox mechanism can be configured for blocking or nonblocking on both the send and receive side The maximum size of the message queue associated with a given mailbox can also be configured The send message primitive, called put, includes two arguments: a handle to the mailbox and a pointer for the message itself There are three variants to this primitive: cyg_mbox_put If there is a spare slot in the mailbox, then the new message is placed there; if there is a waiting thread, it will be woken up so it can receive the message If the mailbox is currently full, cyg_mbox_put blocks until there has been a corresponding get operation and a slot is available cyg_mbox_timed_put Same as cyg_mbox_put if there is a spare slot Other wise, the function will wait a specified time limit and place the message if a slot becomes available If the time limit expires, the operation returns false Thus, cyg_mbox_ timed_put is blocking only for less than or equal to a specified time interval cyg_mbox_tryput This is a nonblocking version, which returns true if the message is sent successfully and false if the mailbox is full Similarly, there are three variants to the get primitive cyg_mbox_get If there is a pending message in the specified mailbox, cyg_ mbox_get returns with the message that was put into the mailbox Otherwise this function blocks until there is a put operation cyg_mbox_timed_get Immediately returns a message if one is available Otherwise, the function will wait until either a message is available or until a number of clock ticks have occurred If the time limit expires, the operation returns a null pointer Thus, cyg_mbox_timed_get is blocking only for less than or equal to a specified time interval cyg_mbox_tryget This is a nonblocking version, which returns a message if one is available and a null pointer if the mailbox is empty Z22_STAL4290_09_GE_APPQ.indd 16 4/18/17 7:36 AM www.downloadslide.net Q.4 / eCOS THREAD SYNCHRONIZATION Q-17 Spinlocks A spinlock is a flag that a thread can check before executing a particular piece of code Recall from our discussion of Linux spinlocks in Chapter the basic operation of the spinlock: Only one thread at a time can acquire a spinlock Any other thread attempting to acquire the same lock will keep trying (spinning) until it can acquire the lock In essence, a spinlock is built on an integer location in memory that is checked by each thread before it enters its critical section If the value is 0, the thread sets the value to and enters its critical section If the value is nonzero, the thread continually checks the value until it is zero A spinlock should not be used on a single-processor system, which is why it is compiled away on Linux As an example of the danger, consider a uniprocessor system with preemptive scheduling, in which a higher-priority thread attempts to acquire a spinlock already held by a lower-priority thread The lower-priority thread cannot execute so as to finish its work and release the spinlock, because the higher-priority thread preempts it The higher-priority thread can execute but is stuck checking the spinlock As a result, the higher-priority thread will just loop forever and the lowerpriority thread will never get another chance to run and release the spinlock On an SMP system, the current owner of a spinlock can continue running on a different processor Z22_STAL4290_09_GE_APPQ.indd 17 4/18/17 7:36 AM www.downloadslide.net Glossary access method The method that is used to find a file, a record, or a set of records address space The range of addresses available to a computer program address translator A functional unit that transforms virtual addresses to real addresses application programming interface (API) A standardized library of programming tools used by software developers to write applications that are compatible with a specific operating system or graphic user interface asynchronous operation An operation that occurs without a regular or predictable time relationship to a specified event, for example, the calling of an error diagnostic routine that may receive control at any time during the execution of a computer program base address An address that is used as the origin in the calculation of addresses in the execution of a computer program batch processing Pertaining to the technique of executing a set of computer programs such that each is completed before the next program of the set is started Beowulf Defines a class of clustered computing that focuses on minimizing the price-to-performance ratio of the overall system without compromising its ability to perform the computation work for which it is being built Most Beowulf systems are implemented on Linux computers binary semaphore A semaphore that takes on only the values and A binary semaphore allows only one process or thread to have access to a shared critical resource at a time block (1) A collection of contiguous records that are recorded as a unit; the units are separated by interblock gaps (2) A group of bits that are transmitted as a unit B-tree A technique for organizing indexes In order to keep access time to a minimum, it stores the data keys in a balanced hierarchy that continually realigns itself as items are inserted and deleted Thus, all nodes always have a similar number of keys busy waiting The repeated execution of a loop of code while waiting for an event to occur cache memory A memory that is smaller and faster than main memory and that is interposed between the processor and main memory The cache acts as a buffer for recently used memory locations central processing unit (CPU) That portion of a computer that fetches and executes instructions It consists of an Arithmetic and Logic Unit (ALU), a control unit, and registers Often simply referred to as a processor chained list A list in which data items may be dispersed but in which each item contains an identifier for locating the next item client A process that requests services by sending messages to server processes GL-1 Z23_STAL4290_09_GE_GLOS.indd 4/18/17 7:36 AM www.downloadslide.net GL-2 Glossary cluster A group of interconnected, whole computers working together as a unified computing resource that can create the illusion of being one machine The term whole computer means a system that can run on its own, apart from the cluster communications architecture The hardware and software structure that implements the communications function compaction A technique used when memory is divided into variable-size partitions From time to time, the operating system shifts the partitions so they are contiguous and so all of the free memory is together in one block See external fragmentation concurrent Pertaining to processes or threads that take place within a common interval of time during which they may have to alternately share common resources consumable resource A resource that can be created (produced) and destroyed (consumed) When a resource is acquired by a process, the resource ceases to exist Examples of consumable resources are interrupts, signals, messages, and information in I/O buffers critical section In an asynchronous procedure of a computer program, a part that cannot be executed simultaneously with an associated critical section of another asynchronous procedure See mutual exclusion database A collection of interrelated data, often with controlled redundancy, organized according to a schema to serve one or more applications; the data are stored so they can be used by different programs without concern for the data structure or organization A common approach is used to add new data, and to modify and retrieve existing data deadlock (1) An impasse that occurs when multiple processes are waiting for the availability of a resource that will not become available because it is being held by another process that is in a similar wait state (2) An impasse that occurs when multiple processes are waiting for an action by or a response from another process that is in a similar wait state deadlock avoidance A dynamic technique that examines each new resource request for deadlock If the new request could lead to a deadlock, then the request is denied deadlock detection A technique in which requested resources are always granted when available Periodically, the operating system tests for deadlock deadlock prevention A technique that guarantees that a deadlock will not occur Prevention is achieved by assuring that one of the necessary conditions for deadlock is not met demand paging The transfer of a page from secondary memory to main memory storage at the moment of need Compare prepaging device driver An operating system module (usually in the kernel) that deals directly with a device or I/O module direct access The capability to obtain data from a storage device or to enter data into a storage device in a sequence independent of their relative position, by means of addresses that indicate the physical location of the data Z23_STAL4290_09_GE_GLOS.indd 4/18/17 7:36 AM www.downloadslide.net Glossary GL-3 direct memory access (DMA) A form of I/O in which a special module, called a DMA module, controls the exchange of data between main memory and an I/O device The processor sends a request for the transfer of a block of data to the DMA module, and is interrupted only after the entire block has been transferred disabled interrupt A condition, usually created by the operating system, during which the processor will ignore interrupt request signals of a specified class disk allocation table A table that indicates which blocks on secondary storage are free and available for allocation to files disk cache A buffer, usually kept in main memory, that functions as a cache of disk blocks between disk memory and the rest of main memory dispatch To allocate time on a processor to jobs or tasks that are ready for execution distributed operating system A common operating system shared by a network of computers The distributed operating system provides support for interprocess communication, process migration, mutual exclusion, and the prevention or detection of deadlock dynamic relocation A process that assigns new absolute addresses to a computer program during execution so the program may be executed from a different area of main storage enabled interrupt A condition, usually created by the operating system, during which the processor will respond to interrupt request signals of a specified class encryption The conversion of plain text or data into unintelligible form by means of a reversible mathematical computation execution context Same as process state external fragmentation Occurs when memory is divided into variable-size partitions corresponding to the blocks of data assigned to the memory (e.g., segments in main memory) As segments are moved into and out of the memory, gaps will occur between the occupied portions of memory field (1) Defined logical data that are part of a record (2) The elementary unit of a record that may contain a data item, a data aggregate, a pointer, or a link file A set of related records treated as a unit file allocation table (FAT) A table that indicates the physical location on secondary storage of the space allocated to a file There is one file allocation table for each file file management system A set of system software that provides services to users and applications in the use of files, including file access, directory maintenance, and access control file organization The physical order of records in a file, as determined by the access method used to store and retrieve them first-come-first-served (FCFS) Same as FIFO first-in-first-out (FIFO) A queueing technique in which the next item to be retrieved is the item that has been in the queue for the longest time Z23_STAL4290_09_GE_GLOS.indd 4/18/17 7:36 AM www.downloadslide.net GL-4 Glossary frame In paged virtual storage, a fixed-length block of main memory that is used to hold one page of virtual memory gang scheduling The scheduling of a set of related threads to run on a set of processors at the same time, on a one-to-one basis hash file A file in which records are accessed according to the values of a key field Hashing is used to locate a record on the basis of its key value hashing The selection of a storage location for an item of data by calculating the address as a function of the contents of the data This technique complicates the storage allocation function but results in rapid random retrieval hit ratio In a two-level memory, the fraction of all memory accesses that are found in the faster memory (e.g., the cache) indexed access Pertaining to the organization and accessing of the records of a storage structure through a separate index to the locations of the stored records indexed file A file in which records are accessed according to the value of key fields An index is required that indicates the location of each record on the basis of each key value indexed sequential access Pertaining to the organization and accessing of the records of a storage structure through an index of the keys that are stored in arbitrarily partitioned sequential files indexed sequential file A file in which records are ordered according to the values of a key field The main file is supplemented with an index file that contains a partial list of key values; the index provides a lookup capability to quickly reach the vicinity of a desired record instruction cycle The time period during which one instruction is fetched from memory and executed when a computer is given an instruction in machine language internal fragmentation Occurs when memory is divided into fixed-size partitions (e.g., page frames in main memory, physical blocks on disk) If a block of data is assigned to one or more partitions, then there may be wasted space in the last partition This will occur if the last portion of data is smaller than the last partition interrupt A suspension of a process, such as the execution of a computer program, caused by an event external to that process and performed in such a way that the process can be resumed interrupt handler A routine, generally part of the operating system When an interrupt occurs, control is transferred to the corresponding interrupt handler, which takes some action in response to the condition that caused the interrupt job A set of computational steps packaged to run as a unit job control language (JCL) A problem-oriented language that is designed to express statements in a job that are used to identify the job or to describe its requirements to an operating system kernel A portion of the operating system that includes the most heavily used portions of software Generally, the kernel is maintained permanently in main memory The kernel runs in a privileged mode and responds to calls from processes and interrupts from devices Z23_STAL4290_09_GE_GLOS.indd 4/18/17 7:36 AM www.downloadslide.net Glossary GL-5 kernel mode A privileged mode of execution reserved for the kernel of the operating system Typically, kernel mode allows access to regions of main memory that are unavailable to processes executing in a less-privileged mode, and also enables execution of certain machine instructions that are restricted to the kernel mode Also referred to as system mode or privileged mode last-in-first-out (LIFO) A queueing technique in which the next item to be retrieved is the item most recently placed in the queue lightweight process A thread livelock A condition in which two or more processes continuously change their state in response to changes in the other process(es) without doing any useful work This is similar to deadlock in that no progress is made, but it differs in that neither process is blocked or waiting for anything locality of reference The tendency of a processor to access the same set of memory locations repetitively over a short period of time logical address A reference to a memory location independent of the current assignment of data to memory A translation must be made to a physical address before the memory access can be achieved logical record A record independent of its physical environment; portions of one logical record may be located in different physical records or several logical records or parts of logical records may be located in one physical record macrokernel A large operating system core that provides a wide range of services mailbox A data structure shared among a number of processes that is used as a queue for messages Messages are sent to the mailbox and retrieved from the mailbox rather than passing directly from sender to receiver main memory Memory that is internal to the computer system, is program addressable, and can be loaded into registers for subsequent execution or processing malicious software Any software designed to cause damage to or use up the resources of a target computer Malicious software (malware) is frequently concealed within or masquerades as legitimate software In some cases, it spreads itself to other computers via e-mail or infected disks Types of malicious software include viruses, Trojan horses, worms, and hidden software for launching denial-of-service attacks memory cycle time The time it takes to read one word from or write one word to memory This is the inverse of the rate at which words can be read from or written to memory memory partitioning The subdividing of storage into independent sections message A block of information that may be exchanged between processes as a means of communication microkernel A small, privileged operating system core that provides process scheduling, memory management, and communication services and relies on other processes to perform some of the functions traditionally associated with the operating system kernel mode switch A hardware operation that occurs that causes the processor to execute in a different mode (kernel or process) When the mode switches from Z23_STAL4290_09_GE_GLOS.indd 4/18/17 7:36 AM www.downloadslide.net GL-6 Glossary process to kernel, the program counter, processor status word, and other registers are saved When the mode switches from kernel to process, this information is restored monitor A programming language construct that encapsulates variables, access procedures, and initialization code within an abstract data type The monitor’s variable may only be accessed via its access procedures and only one process may be actively accessing the monitor at any one time The access procedures are critical sections A monitor may have a queue of processes that are waiting to access it monolithic kernel A large kernel containing virtually the complete operating system, including scheduling, file system, device drivers, and memory management All the functional components of the kernel have access to all of its internal data structures and routines Typically, a monolithic kernel is implemented as a single process, with all elements sharing the same address space multilevel security A capability that enforces access control across multiple levels of classification of data multiprocessing A mode of operation that provides for parallel processing by two or more processors of a multiprocessor multiprocessor A computer with two or more processors that have common access to a main storage multiprogramming A mode of operation that provides for the interleaved execution of two or more computer programs by a single processor The same as multitasking, using different terminology multiprogramming level The number of processes that are partially or fully resident in main memory multithreading Multitasking within a single program It allows multiple streams of instructions (threads) to execute concurrently within the same program, each stream processing a different transaction or message Each stream is a “subprocess,” and the operating system typically cooperates with the application to handle the threads multitasking A mode of operation that provides for the concurrent performance or interleaved execution of two or more computer tasks The same as multiprogramming, using different terminology mutex A programming flag used to grab and release an object When data are acquired that cannot be shared or processing is started that cannot be performed simultaneously elsewhere in the system, the mutex is set to “lock,” which blocks other attempts to use it The mutex is set to “unlock” when the data are no longer needed or the routine is finished Similar to a binary semaphore A key difference between the two is that the process that locks the mutex (sets the value to zero) must be the one to unlock it (sets the value to 1) In contrast, it is possible for one process to lock a binary semaphore and for another to unlock it mutual exclusion A condition in which there is a set of processes, only one of which is able to access a given resource or perform a given function at any time See critical section Z23_STAL4290_09_GE_GLOS.indd 4/18/17 7:36 AM www.downloadslide.net Glossary GL-7 nonprivileged state An execution context that does not allow sensitive hardware instructions to be executed, such as the halt instruction and I/O instructions nonuniform memory access (NUMA) multiprocessor A shared-memory multiprocessor in which the access time from a given processor to a word in memory varies with the location of the memory word object request broker An entity in an object-oriented system that acts as an intermediary for requests sent from a client to a server operating system Software that controls the execution of programs and provides services such as resource allocation, scheduling, input/output control, and data management page In virtual storage, a fixed-length block that has a virtual address and is transferred as a unit between main memory and secondary memory page fault Occurs when the page containing a referenced word is not in main memory This causes an interrupt and requires the proper page be brought into main memory page frame A fixed-size contiguous block of main memory used to hold a page paging The transfer of pages between main memory and secondary memory physical address The absolute location of a unit of data in memory (e.g., word or byte in main memory, block on secondary memory) pipe A circular buffer allowing two processes to communicate on the producer– consumer model Thus, it is a first-in-first-out queue, written by one process and read by another In some systems, the pipe is generalized to allow any item in the queue to be selected for consumption preemption Reclaiming a resource from a process before the process has finished using it prepaging The retrieval of pages other than the one demanded by a page fault The hope is that the additional pages will be needed in the near future, conserving disk I/O Compare demand paging priority inversion A circumstance in which the operating system forces a higherpriority task to wait for a lower-priority task privileged instruction An instruction that can be executed only in a specific mode, usually by a supervisory program privileged mode Same as kernel mode process A program in execution A process is controlled and scheduled by the operating system Same as task process control block The manifestation of a process in an operating system It is a data structure containing information about the characteristics and state of the process process descriptor Same as process control block process image All of the ingredients of a process, including program, data, stack, and process control block process migration The transfer of a sufficient amount of the state of a process from one machine to another for the process to execute on the target machine Z23_STAL4290_09_GE_GLOS.indd 4/18/17 7:36 AM www.downloadslide.net GL-8 Glossary process spawning The creation of a new process by another process process state All of the information the operating system needs to manage a process and the processor needs to properly execute the process The process state includes the contents of the various processor registers, such as the program counter and data registers; it also includes information of use to the operating system, such as the priority of the process and whether the process is waiting for the completion of a particular I/O event Same as execution context process switch An operation that switches the processor from one process to another by saving all the process control block, registers, and other information for the first and replacing them with the process information for the second processor In a computer, a functional unit that interprets and executes instructions A processor consists of at least an instruction control unit and an arithmetic unit program counter Instruction address register program status word (PSW) A register or set of registers that contains condition codes, execution mode, and other status information that reflects the state of a process programmed I/O A form of I/O in which the CPU issues an I/O command to an I/O module and must then wait for the operation to be complete before proceeding race condition Situation in which multiple processes access and manipulate shared data with the outcome dependent on the relative timing of the processes real address A physical address in main memory real-time system An operating system that must schedule and manage real-time tasks real-time task A task that is executed in connection with some process or function or set of events external to the computer system, and must meet one or more deadlines to interact effectively and correctly with the external environment record A group of data elements treated as a unit reentrant procedure A routine that may be entered before the completion of a prior execution of the same routine and execute correctly registers High-speed memory internal to the CPU Some registers are user visible; that is, available to the programmer via the machine instruction set Other registers are used only by the CPU, for control purposes relative address An address calculated as a displacement from a base address remote procedure call (RPC) A technique by which two programs on different machines interact using procedure call/return syntax and semantics Both the called and calling program behave as if the partner program were running on the same machine rendezvous In message passing, a condition in which both the sender and receiver of a message are blocked until the message is delivered resident set That portion of a process that is actually in main memory at a given time Compare working set Z23_STAL4290_09_GE_GLOS.indd 4/18/17 7:36 AM www.downloadslide.net Glossary GL-9 response time In a data system, the elapsed time between the end of transmission of an enquiry message and the beginning of the receipt of a response message, measured at the enquiry terminal reusable resource A resource that can be safely used by only one process at a time and is not depleted by that use Processes obtain reusable resource units that they later release for reuse by other processes Examples of reusable resources include processors, I/O channels, main and secondary memory, devices, and data structures such as files, databases, and semaphores round robin A scheduling algorithm in which processes are activated in a fixed cyclic order; that is, all processes are in a circular queue A process that cannot proceed because it is waiting for some event (e.g., termination of a child process or an input/output operation) returns control to the scheduler scheduling To select jobs or tasks that are to be dispatched In some operating systems, other units of work, such as input/output operations, may also be scheduled secondary memory Memory located outside the computer system itself; that is, it cannot be processed directly by the processor It must first be copied into main memory Examples include disk and tape segment In virtual memory, a block that has a virtual address The blocks of a program may be of unequal length, and may even be of dynamically varying lengths segmentation The division of a program or application into segments as part of a virtual memory scheme semaphore An integer value used for signaling among processes Only three operations may be performed on a semaphore, all of which are atomic: initialize, decrement, and increment Depending on the exact definition of the semaphore, the decrement operation may result in the blocking of a process, and the increment operation may result in the unblocking of a process Also known as a counting semaphore or a general semaphore sequential access The capability to enter data into a storage device or a data medium in the same sequence as the data are ordered, or to obtain data in the same order as they were entered sequential file A file in which records are ordered according to the values of one or more key fields and processed in the same sequence from the beginning of the file server (1) A process that responds to request from clients via messages (2) In a network, a data station that provides facilities to other stations; for example, a file server, a print server, and a mail server session A collection of one or more processes that represents a single interactive user application or operating system function All keyboard and mouse input is directed to the foreground session, and all output from the foreground session is directed to the display screen shell The portion of the operating system that interprets interactive user commands and job control language commands It functions as an interface between the user and the operating system Z23_STAL4290_09_GE_GLOS.indd 4/18/17 7:36 AM www.downloadslide.net GL-10 Glossary spin lock Mutual exclusion mechanism in which a process executes in an infinite loop waiting for the value of a lock variable to indicate availability spooling The use of secondary memory as buffer storage to reduce processing delays when transferring data between peripheral equipment and the processors of a computer stack An ordered list in which items are appended to and deleted from the same end of the list, known as the top That is, the next item appended to the list is put on the top, and the next item to be removed from the list is the item that has been in the list the shortest time This method is characterized as last in first out starvation A condition in which a process is indefinitely delayed because other processes are always given preference strong semaphore A semaphore in which all processes waiting on the same semaphore are queued and will eventually proceed in the same order as they executed the wait (P) operations (FIFO order) swapping A process that interchanges the contents of an area of main storage with the contents of an area in secondary memory symmetric multiprocessing (SMP) A form of multiprocessing that allows the operating system to execute on any available processor or on several available processors simultaneously synchronous operation An operation that occurs regularly or predictably with respect to the occurrence of a specified event in another process, for example, the calling of an input/output routine that receives control at a precoded location in a computer program synchronization Situation in which two or more processes coordinate their activities based on a condition system bus A bus used to interconnect major computer components (CPU, memory, I/O) system mode Same as kernel mode task Same as process thrashing A phenomenon in virtual memory schemes, in which the processor spends most of its time swapping pieces rather than executing instructions thread A dispatchable unit of work It includes a processor context (which includes the program counter and stack pointer) and its own data area for a stack (to enable subroutine branching) A thread executes sequentially and is interruptible so the processor can turn to another thread A process may consist of multiple threads thread switch The act of switching processor control from one thread to another within the same process time sharing The concurrent use of a device by a number of users time slice The maximum amount of time that a process can execute before being interrupted time slicing A mode of operation in which two or more processes are assigned quanta of time on the same processor Z23_STAL4290_09_GE_GLOS.indd 10 4/18/17 7:36 AM www.downloadslide.net Glossary GL-11 trace A sequence of instructions that are executed when a process is running translation lookaside buffer (TLB) A high-speed cache used to hold recently referenced page table entries as part of a paged virtual memory scheme The TLB reduces the frequency of access to main memory to retrieve page table entries trap An unprogrammed conditional jump to a specified address that is automatically activated by hardware; the location from which the jump was made is recorded trap door Secret undocumented entry point into a program, used to grant access without normal methods of access authentication Trojan horse A computer program that appears to have a useful function, but also has a hidden and potentially malicious function that evades security mechanisms, sometimes by exploiting legitimate authorizations of a system entity that invokes the Trojan horse program trusted system A computer and operating system that can be verified to implement a given security policy user mode The least-privileged mode of execution Certain regions of main memory and certain machine instructions cannot be used in this mode virtual address The address of a storage location in virtual memory virtual machine One instance of an operating system along with one or more applications running in an isolated partition within the computer It enables different operating systems to run in the same computer at the same time as well as prevents applications from interfering with each other virtual memory The storage space that may be regarded as addressable main storage by the user of a computer system in which virtual addresses are mapped into real addresses The size of virtual storage is limited by the addressing scheme of the computer system and by the amount of secondary memory available and not by the actual number of main storage locations virus Software that, when executed, tries to replicate itself into other executable code; when it succeeds the code is said to be infected When the infected code is executed, the virus also executes weak semaphore A semaphore in which all processes waiting on the same semaphore proceed in an unspecified order (i.e., the order is unknown or indeterminate) word An ordered set of bytes or bits that is the normal unit in which information may be stored, transmitted, or operated on within a given computer Typically, if a processor has a fixed-length instruction set, then the instruction length equals the word length working set The working set with parameter Δ for a process at virtual time t, W(t, Δ) is the set of pages of that process that have been referenced in the last Δ time units Compare resident set worm A destructive program that replicates itself throughout a single computer or across a network, both wired and wireless It can damage by sheer reproduction, consuming internal disk and memory resources within a single computer or by exhausting network bandwidth It can also deposit a Trojan that turns a computer into a zombie for spam and other malicious purposes Very often, the terms “worm” and “virus” are used synonymously; however, worm implies an automatic method for reproducing itself in other computers Z23_STAL4290_09_GE_GLOS.indd 11 4/18/17 7:36 AM ... buffer For stream-oriented I/O, the single buffering scheme can be used in a line-at-atime fashion or a byte-at-a-time fashion Line-at-a-time operation is appropriate for scroll-mode terminals... 58 32 160 10 160 10 39 19 55 3 184 24 184 24 18 21 39 16 90 94 18 166 90 72 38 1 58 32 38 20 160 70 18 20 55 3 39 1 150 10 150 1 32 39 16 55 16 38 1 12 160 10... block block block block P( 4-7 ) Q( 4-7 ) block block block P( 8-1 1) Q( 8-1 1) block 10 block 11 block 12 P(1 2- 1 5) Q(1 2- 1 5) block 13 block 14 block 15 Q( 0-3 ) (g) RAID (block-interleaved dual distributed