Thread context switch Vs. process context switch

Could any one tell me what is exactly done in both situations? What is the main cost each of them?

The main distinction between a thread switch and a process switch is that during a thread switch, the virtual memory space remains the same, while it does not during a process switch. Both types involve handing control over to the operating system kernel to perform the context switch. The process of switching in and out of the OS kernel along with the cost of switching out the registers is the largest fixed cost of performing a context switch.

A more fuzzy cost is that a context switch messes with the processors cacheing mechanisms. Basically, when you context switch, all of the memory addresses that the processor "remembers" in its cache effectively become useless. The one big distinction here is that when you change virtual memory spaces, the processor's Translation Lookaside Buffer (TLB) or equivalent gets flushed making memory accesses much more expensive for a while. This does not happen during a thread switch.

Process context switching involves switching the memory address space. This includes memory addresses, mappings, page tables, and kernel resources—a relatively expensive operation. On some architectures, it even means flushing various processor caches that aren't sharable across address spaces. For example, x86 has to flush the TLB and some ARM processors have to flush the entirety of the L1 cache!

Thread switching is context switching from one thread to another in the same process (switching from thread to thread across processes is just process switching).Switching processor state (such as the program counter and register contents) is generally very efficient.

First of all, operating system brings outgoing thread in a kernel mode if it is not already there, because thread switch can be performed only between threads, that runs in kernel mode. Then the scheduler is invoked to make a decision about thread to which will be performed switching. After decision is made, kernel saves part of the thread context that is located in CPU (CPU registers) into the dedicated place in memory (frequently on the top of the kernel stack of outgoing thread). Then the kernel performs switch from kernel stack of outgoing thread on to kernel stack of the incoming thread. After that, kernel loads previously stored context of incoming thread from memory into CPU registers. And finally returns control back into user mode, but in user mode of the new thread. In the case when OS has determined that incoming thread runs in another process, kernel performs one additional step: sets new active virtual address space.

The main cost in both scenarios is related to a cache pollution. In most cases, the working set used by the outgoing thread will differ significantly from working set which is used by the incoming thread. As a result, the incoming thread will start its life with avalanche of cache misses, thus flushing old and useless data from the caches and loading the new data from memory. The same is true for TLB (Translation Look Aside Buffer, which is on the CPU). In the case of reset of virtual address space (threads run in different processes) the penalty is even worse, because reset of virtual address space leads to the flushing of the entire TLB, even if new thread actually needs to load only few new entries. As a result, the new thread will start its time quantum with lots TLB misses and frequent page walking. Direct cost of threads switch is also not negligible (from ~250 and up to ~1500-2000 cycles) and depends on the CPU complexity, states of both threads and sets of registers which they actually use.

P.S.: Good post about context switch overhead: http://blog.tsunanet.net/2010/11/how-long-does-it-take-to-make-context.html

• process switching: it is a transition between two memory resident of process in a multiprogramming environment;
• context switching: it is a changing context from an executing program to an interrupt service routine (ISR).

In Thread Context Switching, the virtual memory space remains the same while it is not in the case of Process Context Switch. Also, Process Context Switch is costlier than Thread Context Switch.

I think main difference is when calling switch_mm() which handles memory descriptors of old and new task. In the case of threads, the virtual memory address space is unchanged (threads share virtual memory), so very little has to be done, and therefore less costly.

Though thread context switching needs to change the execution context (registers, stack pointers, program counters), they don't need to change address space as processes context switches do. There's an additional cost when you switch address space, more memory access (paging, segmentation, etc) and you have to flush TLB when entering or exiting a new process...

In short, the thread context switch does not assign a brand new set of memory and pid, it uses the same as the parent since it is running within the same process. A process one spawns a new process and thus assigns new mem and pid.

There is a loooooot more to it. They have written books on it.

As for cost, a process context switch >>>> thread as you have to reset all of the stack counters etc.

Assuming that The CPU the OS runs has got Some High Latency Devices Attached,

It makes sense to run another thread Of the Process's Address Space, while the high latency device responds back.

But, if the High Latency Device is responding faster than the time to need do set up of table + translation of Virtual To Physical memories for a NEW Process, then it is questionable if a switch is essential at all.

Also, HOT cache(data needed for running the process/thread is reachable in less time) is better choice.