• <ins id="pjuwb"></ins>
    <blockquote id="pjuwb"><pre id="pjuwb"></pre></blockquote>
    <noscript id="pjuwb"></noscript>
          <sup id="pjuwb"><pre id="pjuwb"></pre></sup>
            <dd id="pjuwb"></dd>
            <abbr id="pjuwb"></abbr>
            posts - 297,  comments - 15,  trackbacks - 0

            A context switch (also sometimes referred to as a process switch or a task switch ) is the switching of the CPU (central processing unit) from one process or thread to another.

            A process (also sometimes referred to as a task ) is an executing (i.e., running) instance of a program. In Linux, threads are lightweight processes that can run in parallel and share an address space (i.e., a range of memory locations) and other resources with their parent processes (i.e., the processes that created them).

            A context is the contents of a CPU's registers and program counter at any point in time. A register is a small amount of very fast memory inside of a CPU (as opposed to the slower RAM main memory outside of the CPU) that is used to speed the execution of computer programs by providing quick access to commonly used values, generally those in the midst of a calculation. A program counter is a specialized register that indicates the position of the CPU in its instruction sequence and which holds either the address of the instruction being executed or the address of the next instruction to be executed, depending on the specific system.

            Context switching can be described in slightly more detail as the kernel operating system) performing the following activities with regard to processes (including threads) on the CPU: (1) suspending the progression of one process and storing the CPU's state (i.e., the context) for that process somewhere in memory, (2) retrieving the context of the next process from memory and restoring it in the CPU's registers and (3) returning to the location indicated by the program counter (i.e., returning to the line of code at which the process was interrupted) in order to resume the process. (i.e., the core of the

            A context switch is sometimes described as the kernel suspending execution of one process on the CPU and resuming execution of some other process that had previously been suspended. Although this wording can help clarify the concept, it can be confusing in itself because a process is , by definition, an executing instance of a program. Thus the wording suspending progression of a process might be preferable.

            Context Switches and Mode Switches

            Context switches can occur only in kernel mode . Kernel mode is a privileged mode of the CPU in which only the kernel runs and which provides access to all memory locations and all other system resources. Other programs, including applications, initially operate in user mode , but they can run portions of the kernel code via system calls . A system call is a request in a Unix-like operating system by an active process (i.e., a process currently progressing in the CPU) for a service performed by the kernel, such as input/output (I/O) or process creation (i.e., creation of a new process). I/O can be defined as any movement of information to or from the combination of the CPU and main memory (i.e. RAM), that is, communication between this combination and the computer's users (e.g., via the keyboard or mouse), its storage devices (e.g., disk or tape drives), or other computers.

            The existence of these two modes in Unix-like operating systems means that a similar, but simpler, operation is necessary when a system call causes the CPU to shift to kernel mode. This is referred to as a mode switch rather than a context switch, because it does not change the current process.

            Context switching is an essential feature of multitasking operating systems. A multitasking operating system is one in which multiple processes execute on a single CPU seemingly simultaneously and without interfering with each other. This illusion of concurrency is achieved by means of context switches that are occurring in rapid succession (tens or hundreds of times per second). These context switches occur as a result of processes voluntarily relinquishing their time in the CPU or as a result of the scheduler making the switch when a process has used up its CPU time slice .

            A context switch can also occur as a result of a hardware interrupt , which is a signal from a hardware device (such as a keyboard, mouse, modem or system clock) to the kernel that an event (e.g., a key press, mouse movement or arrival of data from a network connection) has occurred.

            Intel 80386 and higher CPUs contain hardware support for context switches. However, most modern operating systems perform software context switching , which can be used on any CPU, rather than hardware context switching in an attempt to obtain improved performance. Software context switching was first implemented in Linux for Intel-compatible processors with the 2.4 kernel.

            One major advantage claimed for software context switching is that, whereas the hardware mechanism saves almost all of the CPU state, software can be more selective and save only that portion that actually needs to be saved and reloaded. However, there is some question as to how important this really is in increasing the efficiency of context switching. Its advocates also claim that software context switching allows for the possibility of improving the switching code, thereby further enhancing efficiency, and that it permits better control over the validity of the data that is being loaded.

            The Cost of Context Switching

            Context switching is generally computationally intensive. That is, it requires considerable processor time, which can be on the order of nanoseconds for each of the tens or hundreds of switches per second. Thus, context switching represents a substantial cost to the system in terms of CPU time and can, in fact, be the most costly operation on an operating system.

            Consequently, a major focus in the design of operating systems has been to avoid unnecessary context switching to the extent possible. However, this has not been easy to accomplish in practice. In fact, although the cost of context switching has been declining when measured in terms of the absolute amount of CPU time consumed, this appears to be due mainly to increases in CPU clock speeds rather than to improvements in the efficiency of context switching itself.

            One of the many advantages claimed for Linux as compared with other operating systems, including some other Unix-like systems, is its extremely low cost of context switching and mode switching.


            from:
            http://blog.csdn.net/wave_1102/archive/2007/09/04/1771745.aspx

            posted on 2010-02-25 23:09 chatler 閱讀(383) 評論(0)  編輯 收藏 引用 所屬分類: OS
            <2025年7月>
            293012345
            6789101112
            13141516171819
            20212223242526
            272829303112
            3456789

            常用鏈接

            留言簿(10)

            隨筆分類(307)

            隨筆檔案(297)

            algorithm

            Books_Free_Online

            C++

            database

            Linux

            Linux shell

            linux socket

            misce

            • cloudward
            • 感覺這個博客還是不錯,雖然做的東西和我不大相關,覺得看看還是有好處的

            network

            OSS

            • Google Android
            • Android is a software stack for mobile devices that includes an operating system, middleware and key applications. This early look at the Android SDK provides the tools and APIs necessary to begin developing applications on the Android platform using the Java programming language.
            • os161 file list

            overall

            搜索

            •  

            最新評論

            閱讀排行榜

            評論排行榜

            久久99热国产这有精品| 久久综合久久综合久久| 日本久久久久久中文字幕| 久久国产精品成人片免费| 久久精品亚洲精品国产色婷| 国产精品久久久久久吹潮| 亚洲欧美精品伊人久久| 国产精品久久久久蜜芽| 无码AV波多野结衣久久| 欧洲人妻丰满av无码久久不卡| 久久久久人妻精品一区二区三区| 久久国产乱子伦免费精品| 天天久久狠狠色综合| 一本色道久久综合| 久久久无码人妻精品无码| 久久精品国产精品青草app| 久久精品国产精品亚洲下载 | 久久99精品国产99久久6| 久久久久亚洲av综合波多野结衣| 久久97精品久久久久久久不卡| 国产激情久久久久影院老熟女| 久久人爽人人爽人人片AV| 精品久久久久久国产牛牛app| 久久99这里只有精品国产| 色偷偷888欧美精品久久久| 久久久久久久久66精品片| 一本久久a久久精品综合夜夜| 99久久无色码中文字幕人妻| 漂亮人妻被中出中文字幕久久| 国产精品对白刺激久久久| 久久无码AV一区二区三区| 91久久成人免费| 天天久久狠狠色综合| 亚洲精品无码专区久久久| 国产成人综合久久精品红| 久久99精品国产99久久6男男| 亚洲中文字幕无码久久2020| 久久久久国产精品麻豆AR影院| 国产精品熟女福利久久AV| 99久久精品午夜一区二区| 久久人爽人人爽人人片AV|