Leveraging System Context in Windows

1. Contexts 3 5 3 – Fast Window Switcher Systems Installation
2. Contexts 3 5 3 – Fast Window Switcher Systems Inc
3. Contexts 3 5 3 – Fast Window Switcher Systems System

The aim of system context analysis is to determine all persons, groups, organizations, processes, events and documents relevant for a system. Demarcating the system boundary defines what functionalities a system is supposed to offer and what interfaces to external systems exist.

I’d like to take things away from App-V for this blog if possible. Give the site a little more variety. I have been working with App-v mainly, however, I have also obviously been involved with MSI packaging and OS migration. With Windows 7, some features have been deprecated and as an IT professional, learning the little nuances in the new platform is part of the challenge and fun!

• 1-1 Chapter 1 Introduction to CNC Systems This chapter introduces you to terminology used in the rest of this manual and explains the purpose of.
• For context switching to happen, two processes are at least required in general, and in the case of the round-robin algorithm, you can perform context switching with the help of one process only. The time involved in the context switching of one process by other is called the Context Switching Time. Advantage of Context Switching.

One of the first walls I ran into, with Windows 7, was met when I was working on a project that required me to perform my test install in system context. The reason why I wanted to do this was to mimic the install in the context it would be deployed in. Previously, when testing on XP, I would have used the ‘At interactive’ method to do this, but this is no more. A colleague of mine actually told me about this nice little tool for launching the command window in System context on Win7. I shall dispense this advice now(Crappy Sunscreen song reference, sorry)

How To Test an application install in the system context mode to mimic Deployment tools

You can open up an entry point in System context by following the below four steps.

Step 1: Run Command Window as Administrator

Step 2: Use psexec.exe

You will need to use the SysInternals Tool ‘psexec.exe’. These tools are a free set of troubleshooting and development tools that can be found on the Microsoft Technet site at the following location: – https://technet.microsoft.com/en-us/sysinternals/default.aspx The Sysinternals tools get updated so you should ensure you have the latest set.

Run the command as seen below (NOTE: Location of your psexec.exe may differ in this case I have copied the psexec.exe to the desktop as illustrated below):

A new CMD window should appear:

Step 3: Verify Context

Close the original cmd window as seen in Figure 2 and check Task Manager (Task Manager can be launched by right-clicking the taskbar and choosing Task Manager)(Explain how you launch task manager) to ensure your command window is now running in System context:

Step 4: Perform Actions

Run your install or do whatever it is that you aim to do in the System Context. This should help you to mimic the customer’s deployment method by installing in the correct context.

Get the App-V Decison Matrix and Interactive Tool.

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 only requires reads and writes that happen in a sequence/order and there are no 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) function may require data from disk and a device driver in kernel mode, hence the display() function goes to sleep and waits on the 'read data' operation for the system call to wake it up when the “data” is received from the disk. i.e It waits for the function call to be released. After the “data” is received, the display(data) function is re-executed with the new value in order to prevent inconsistencies.

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]

Contexts 3 5 3 – Fast Window Switcher Systems Installation

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.

Contexts 3 5 3 – Fast Window Switcher Systems Inc

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]

• 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

Contexts 3 5 3 – Fast Window Switcher Systems System