Re: [Qemu-devel] [PATCH 01/15] qemu coroutine: support bypass mode

[Paolo Bonzini]

Il 31/07/2014 10:59, Ming Lei ha scritto:
>> > No guesses please.  Actually that's also my guess, but since you are
>> > submitting the patch you must do better and show profiles where stack
>> > switching disappears after the patches.
> Follows the below hardware events reported by 'perf stat' when running
> fio randread benchmark for 2min in VM(single vq, 2 jobs):
> 
> sudo ~/bin/perf stat -e
> L1-dcache-loads,L1-dcache-load-misses,cpu-cycles,instructions,branch-instructions,branch-misses,branch-loads,branch-load-misses,dTLB-loads,dTLB-load-misses
> ./nqemu-start-mq 4 1
> 
> 1), without bypassing coroutine via forcing to set 's->raw_format ' as
> false, see patch 5/15
> 
> - throughout: 95K
>    232,564,905,115      instructions
>      161.991075781 seconds time elapsed
> 
> 
> 2), with bypassing coroutinue
> - throughput: 115K
>    255,526,629,881      instructions
>      162.333465490 seconds time elapsed

Ok, so you are saving 10% instructions per iop: before 232G / 95K =
2.45M instructions/iop, 255G / 115K = 2.22M instructions/iop.

That's not small, and it's a good thing for CPU utilization even if you
were not increasing iops.  On top of this, can you provide the stack
traces to see the difference in the profiles?

Paolo