Hi,
On 2020-04-10 13:09:13 +0530, Amit Khandekar wrote:
> On my Intel Xeon machine with 8 cores, I tried to test PAUSE also
> using a sample C program (attached spin.c). Here, many child processes
> (much more than CPUs) wait in a tight loop for a shared variable to
> become 0, while the parent process continuously increments a sequence
> number for a fixed amount of time, after which, it sets the shared
> variable to 0. The child's tight loop calls PAUSE in each iteration.
> What I hoped was that because of PAUSE in children, the parent process
> would get more share of the CPU, due to which, in a given time, the
> sequence number will reach a higher value. Also, I expected the CPU
> cycles spent by child processes to drop down, thanks to PAUSE. None of
> these happened. There was no change.
> Possibly, this testcase is not right. Probably the process preemption
> occurs only within the set of hyperthreads attached to a single core.
> And in my testcase, the parent process is the only one who is ready to
> run. Still, I have anyway attached the program (spin.c) for archival;
> in case somebody with a YIELD-supporting ARM machine wants to use it
> to test YIELD.
PAUSE doesn't operate on the level of the CPU scheduler. So the OS won't
just schedule another process - you won't see different CPU usage if you
measure it purely as the time running. You should be able to see a
difference if you measure with a profiler that shows you data from the
CPUs performance monitoring unit.
Greetings,
Andres Freund
