Contexts 3 5 3 – Fast Window Switcher System

• Contexts 3 5 3 – Fast Window Switcher Systems
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In the above figure, you can see that initially, the process P1 is in the running state and the process P2 is in the ready state. Now, when some interruption occurs then you have to switch the process P1 from running to the ready state after saving the context and the process P2 from ready to running state. Minimum update needed to support TLS 1.2 with.NET Framework 3.5; Windows 10 Windows Server 2016: Cumulative Update for Windows 10 Version 1511 and Windows Server 2016 Technical Preview 4: May 10, 2016: Windows 8.1 Windows Server 2012 R2: Support for TLS System Default Versions included in the.NET Framework 3.5 on Windows 8.1 and Windows. How to Display and Hide the Windows 10 Desktop. Use the Win+D shortcut to display and hide the desktop. This command forces Windows to switch to the desktop immediately and minimize all open windows to the taskbar. Use the same shortcut to bring back those open windows. CSwitch class.; 2 minutes to read; In this article. This class is the event type class for context switch events. The following syntax is simplified from MOF code.

In computing, a context switch is the process of storing the state of a process or thread, so that it can be restored and resume execution at a later point. This allows multiple processes to share a single central processing unit (CPU), and is an essential feature of a multitasking operating system.

The precise meaning of the phrase 'context switch' varies. In a multitasking context, it refers to the process of storing the system state for one task, so that task can be paused and another task resumed. A context switch can also occur as the result of an interrupt, such as when a task needs to access disk storage, freeing up CPU time for other tasks. Some operating systems also require a context switch to move between user mode and kernel mode tasks. The process of context switching can have a negative impact on system performance.^[1]:28

Cost[edit]

Context switches are usually computationally intensive, and much of the design of operating systems is to optimize the use of context switches. Switching from one process to another requires a certain amount of time for doing the administration – saving and loading registers and memory maps, updating various tables and lists, etc. What is actually involved in a context switch depends on the architectures, operating systems, and the number of resources shared (threads that belong to the same process share many resources whether compared to unrelated non-cooperating processes. For example, in the Linux kernel, context switching involves switching registers, stack pointer (it's typical stack-pointer register), program counter, flushing the translation lookaside buffer (TLB) and loading the page table of the next process to run (unless the old process shares the memory with the new).^[2][3] Furthermore, analogous context switching happens between user threads, notably green threads, and is often very lightweight, saving and restoring minimal context. In extreme cases, such as switching between goroutines in Go, a context switch is equivalent to a coroutine yield, which is only marginally more expensive than a subroutine call.

Switching cases[edit]

There are three potential triggers for a context switch:

Multitasking[edit]

Most commonly, within some scheduling scheme, one process must be switched out of the CPU so another process can run. This context switch can be triggered by the process making itself unrunnable, such as by waiting for an I/O or synchronization operation to complete. On a pre-emptive multitasking system, the scheduler may also switch out processes that are still runnable. To prevent other processes from being starved of CPU time, preemptive schedulers often configure a timer interrupt to fire when a process exceeds its time slice. This interrupt ensures that the scheduler will gain control to perform a context switch.

Interrupt handling[edit]

Modern architectures are interrupt driven. This means that if the CPU requests data from a disk, for example, it does not need to busy-wait until the read is over; it can issue the request (to the I/O device) and continue with some other task. When the read is over, the CPU can be interrupted (by a hardware in this case, which sends interrupt request to PIC) and presented with the read. For interrupts, a program called an interrupt handler is installed, and it is the interrupt handler that handles the interrupt from the disk.

When an interrupt occurs, the hardware automatically switches a part of the context (at least enough to allow the handler to return to the interrupted code). The handler may save additional context, depending on details of the particular hardware and software designs. Often only a minimal part of the context is changed in order to minimize the amount of time spent handling the interrupt. The kernel does not spawn or schedule a special process to handle interrupts, but instead the handler executes in the (often partial) context established at the beginning of interrupt handling. Once interrupt servicing is complete, the context in effect before the interrupt occurred is restored so that the interrupted process can resume execution in its proper state.

User and kernel mode switching[edit]

When the system transitions between user mode and kernel mode, a context switch is not necessary; a mode transition is not by itself a context switch. However, depending on the operating system, a context switch may also take place at this time.

Steps[edit]

In a switch, the state of the process currently executing must be saved somehow, so that when it is rescheduled, this state can be restored.

The process state includes all the registers that the process may be using, especially the program counter, plus any other operating system specific data that may be necessary. This is usually stored in a data structure called a process control block (PCB) or switchframe.

The PCB might be stored on a per-process stack in kernel memory (as opposed to the user-mode call stack), or there may be some specific operating system-defined data structure for this information. A handle to the PCB is added to a queue of processes that are ready to run, often called the ready queue.

Since the operating system has effectively suspended the execution of one process, it can then switch context by choosing a process from the ready queue and restoring its PCB. In doing so, the program counter from the PCB is loaded, and thus execution can continue in the chosen process. Process and thread priority can influence which process is chosen from the ready queue (i.e., it may be a priority queue).

Example[edit]

Considering a general arithmetic addition operation A = B+1. The instruction is stored in the instruction register and the program counter is incremented. A and B are read from memory and are stored in registers R1, R2 respectively. In this case, B+1 is calculated and written in R1 as the final answer. This operation as there are sequential reads and writes and there's no waits for function calls used, hence no context switch/wait takes place in this case.

However, certain special instructions require system calls that require context switch to wait/sleep processes. A system call handler is used for context switch to kernel mode. A display(data x) function may require data x from the Disk and a device driver in kernel mode, hence the display() function goes to sleep and waits on the READ operation to get the value of x from the disk, causing the program to wait and a wait for function call to tbe released setting the current statement to go to sleep and wait for the syscall to wake it up. To maintain concurrency however the program needs to re-execute the new value and the sleeping process together again.

Performance[edit]

Context switching itself has a cost in performance, due to running the task scheduler, TLB flushes, and indirectly due to sharing the CPU cache between multiple tasks.^[4] Switching between threads of a single process can be faster than between two separate processes, because threads share the same virtual memory maps, so a TLB flush is not necessary.^[5]

Hardware vs. software[edit]

Context switching can be performed primarily by software or hardware. Some processors, like the Intel 80386 and its successors,^[6] have hardware support for context switches, by making use of a special data segment designated the task state segment (TSS). A task switch can be explicitly triggered with a CALL or JMP instruction targeted at a TSS descriptor in the global descriptor table. It can occur implicitly when an interrupt or exception is triggered if there's a task gate in the interrupt descriptor table (IDT). When a task switch occurs the CPU can automatically load the new state from the TSS.

As with other tasks performed in hardware, one would expect this to be rather fast; however, mainstream operating systems, including Windows and Linux,^[7] do not use this feature. This is mainly due to two reasons:

• Hardware context switching does not save all the registers (only general-purpose registers, not floating point registers — although the TS bit is automatically turned on in the CR0control register, resulting in a fault when executing floating-point instructions and giving the OS the opportunity to save and restore the floating-point state as needed).
• Associated performance issues, e.g., software context switching can be selective and store only those registers that need storing, whereas hardware context switching stores nearly all registers whether they are required or not.

See also[edit]

References[edit]

1. ^Tanenbaum, Andrew S.; Bos, Herbert (March 20, 2014). Modern Operating Systems (4th ed.). Pearson. ISBN978-0133591620.
2. ^IA-64 Linux Kernel: Design and Implementation, 4.7 Switching Address Spaces
3. ^Operating Systems, 5.6 The Context Switch, p. 118
4. ^Chuanpeng Li; Chen Ding; Kai Shen. 'Quantifying The Cost of Context Switch'(PDF).Cite journal requires |journal= (help)
5. ^Ulrich Drepper (9 October 2014). 'Memory part 3: Virtual Memory'. LWN.net.
6. ^'Context Switch definition'. Linfo.org. Archived from the original on 2010-02-18. Retrieved 2013-09-08.
7. ^Bovet, Daniel Pierre; Cesati, Marco (2006). Understanding the Linux Kernel, Third Edition. O'Reilly Media. p. 104. ISBN978-0-596-00565-8. Retrieved 2009-11-23.

External links[edit]

Contexts 3 5 3 – Fast Window Switcher Systems

• Context Switching at OSDev.org
• Context Switch Definition by The Linux Information Project (LINFO)
• Context Switches from the Microsoft Developer Network (MSDN)
• General Architecture and Design -Interrupt Handling at FreeBSD.org

Windows Installer can install a package on a computer into two installation contexts: per-machine and per-user. A per-machine installation of the package is required to enable all users of the computer to access and use the application. Because a per-machine installation makes changes to the system that affect all users, standard users having limited privileges may be prevented from installing a package into the per-machine context without first obtaining permission.

Contexts 3 5 3 – Fast Window Switcher System Download

You can specify installation context by authoring the package for per-user or per-machine installation and using the ALLUSERS and MSIINSTALLPERUSER properties. Based on these properties, Windows Installer automatically redirects the values of folder properties and registrations to locations for the per-user or per-machine context.

Note

The MSIINSTALLPERUSER property, available beginning with Windows Installer 5.0 and Windows Server 2008 R2 and Windows 7, can facilitate the development of a single package capable of being installed in either the per-machine or per-user context. For information about developing a dual-purpose package that gives the user the capability to choose the installation context at installation time, see Single Package Authoring. Windows Installer ignores the MSIINSTALLPERUSER property if the value of ALLUSERS is not 2. Windows Installer always resets the value of ALLUSERS to 1 when it installs in the per-machine context and resets the value of ALLUSERS to an empty string (') when it installs in the per-user context.

Shortcut Redirection

Contexts 3 5 3 – Fast Window Switcher System Diagram

The following table compares the locations of shortcuts for per-machine and per-user installation contexts.

Registry Redirection

Contexts 3 5 3 – Fast Window Switcher System Installation

The following table compares the locations of registry entries for the per-machine and per-user installation contexts.

Folder Redirection

Windows Installer sets the values of the folder properties to the full path of the respective folder for the installation context.

Note

Folders are identified by their KNOWNFOLDERID and CSIDL constants. Beginning with Windows Vista, applications should use the SHGetKnownFolderPath function and the KNOWNFOLDERID to determine the full path to the special folders. Existing applications that use the SHGetFolderPath function and constant special item IDs (CSIDL) will continue to work.

The following table compares the locations of folders that are used when Windows Installer installs the package in the per-machine or per-user installation contexts.

Note

An application can call the MsiEnumProducts or MsiEnumProductsEx functions to enumerate all the products installed on the system. The application can then retrieve information about the installation context of these products by calling the MsiGetProductInfoEx or MsiGetProductInfo functions. For information see Determining Installation Context.