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I was asked to anwer these questions about the OS context switch, the question is pretty tricky and I cannot find any answer in my textbook:

  1. How many PCBs exist in a system at a particular time?
  2. What are two situations that could cause a Context Switch to occur? (I think they are interrupt and termination of a process,but I am not sure )
  3. Hardware support can make a difference in the amount of time it takes to do the switch. What are two different approaches?
  4. What part of the OS is involved in managing the Context Switch?
Mihai Maruseac
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user2507728
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3 Answers3

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  1. There can be any number of PCBs in the system at a given moment in time. Each PCB is linked to a process.
  2. Timer interrupts in preemptive kernels or process renouncing control of processor in cooperative kernels. And, of course, process termination and blocking at I/O operations.
  3. I don't know the answer here, but see Marko's answer
  4. One of the schedulers from the kernel.

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Community
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Mihai Maruseac
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3: A whole number of possible hardware optimisations

  • Small register sets (therefore less to save and restore on context switch)
  • 'Dirty' flags for floating point/vector processor register set - allows the kernel to avoid saving the context if nothing has happened to it since it was switched in. FP/VP contexts are usually very large and a great many threads never use them. Some RTOSs provide an API to tell the kernel that a thread never uses FP/VP at all eliminating even more context restores and some saves - particularly when a thread handling an ISR pre-empts another, and then quickly completes, with the kernel immediately rescheduling the original thread.
  • Shadow register banks: Seen on small embedded CPUs with on-board singe-cycle SRAM. CPU registers are memory backed. As a result, switching bank is merely a case of switching base-address of the registers. This is usually achieved in a few instructions and is very cheap. Usually the number of context is severely limited in these systems.
  • Shadow interrupt registers: Shadow register banks for use in ISRs. An example is all ARM CPUs that have a shadow bank of about 6 or 7 registers for its fast interrupt handler and a slightly fewer shadowed for the regular one. Whilst not strictly a performance increase for context switching, this can help ith the cost of context switching on the back of an ISR.
  • Physically rather than virtually mapped caches. A virtually mapped cache has to be flushed on context switch if the MMU is changed - which it will be in any multi-process environment with memory protection. However, a physically mapped cache means that virtual-physical address translation is a critical-path activity on load and store operations, and a lot of gates are expended on caching to improve performance. Virtually mapped caches were therefore a design choice on some CPUs designed for embedded systems.
marko
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The scheduler is the part of the operating systems that manage context switching, it perform context switching in one of the following conditions:
1.Multitasking

2.Interrupt handling

3.User and kernel mode switching

and each process have its own PCB

Ankur
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