OK.This explains that if a process gets "migrated" from one CPU to another,
the time is not "affected". But it still doesn't explain if the process
gets scheduled back to the same CPU.
Just in case I have not explained my question clearly, let me explain it
from the schedular's perspective.
Lets assume that there are 3 processes, P1 an MPI process, P2 and P3 are
otehr active processes. Assume that the timeslice for each process is 10ms.
During the first time slice that P1 gets, it calls the MPI_Wtime(), stores
it in "start" variable and does half of the solver. Unfortunately, at this
ponit the timeslice of P1 got over, and the OS schedules P2 and P3 before
getting back to P1. So it takes 20ms, before P1 gets its share of CPU back,
at which point it completes the remaining half of the solver, and
calculated MPI_Wtime() and stores it in end. In short, it required two
timeslice for the MPI process to complete.
If the "wall clock" (a global counter) and there is no way for me to "save"
the value when the context switch occured from P1 to P2 and re-initialize
counting as soon as context switches from P3 back to P1, then it would
include the time spent by the process P2 and P3 as well (20ms in this case)
PS: To keep it simple, I am not considering the preemption that might
happen due to an interrupt, or the overhead of context switching.
On Sat, May 5, 2012 at 9:04 AM, Eugene Loh <eugene.loh_at_[hidden]> wrote:
> MPI_Wtime() returns the elapsed time since some arbitrary time in the
> past. It is a measure of "wallclock" time, not of CPU time or anything.
> On 5/4/2012 3:08 PM, Jingcha Joba wrote:
> Lets say I have a code like this
> start = MPI_Wtime()
> <Run the solver>
> stop = MPI_Wtime();
> What happens when right after start=MPI_Wtime(), the timeslice of the
> process ( from the operating system's perspective not the MPI process) is
> over, and the operating system schedules a next process, after saving the
> context switch, and eventually this application would resume, once its
> process is scheduled back by the os.
> Does MPI_Wtime() takes care of storing/updating the time when this
> Of course, part of the answer lies in the implementation of Wtime.
> On Fri, May 4, 2012 at 3:53 AM, Jeff Squyres <jsquyres_at_[hidden]> wrote:
>> On May 3, 2012, at 2:02 PM, Jingcha Joba wrote:
>> > Not related to this question , but just curious, is Wtime context
>> switch safe ?
>> Not sure exactly what you're asking here...?
>> Jeff Squyres
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