Re: [PATCH] poll: Avoid extra wakeups in select/poll

Eric Dumazet
2009-04-30T10:50:16Z

Ingo Molnar a écrit :
> * Eric Dumazet <dada1@cosmosbay.com> wrote:
>
>> On uddpping, I had prior to the patch about 49000 wakeups per
>> second, and after patch about 26000 wakeups per second (matches
>> number of incoming udp messages per second)
>
> very nice. It might not show up as a real performance difference if
> the CPUs are not fully saturated during the test - but it could show
> up as a decrease in CPU utilization.
>
> Also, if you run the test via 'perf stat -a ./test.sh' you should
> see a reduction in instructions executed:
>
> aldebaran:~/linux/linux> perf stat -a sleep 1
>
> Performance counter stats for 'sleep':
>
> 16128.045994 task clock ticks (msecs)
> 12876 context switches (events)
> 219 CPU migrations (events)
> 186144 pagefaults (events)
> 20911802763 CPU cycles (events)
> 19309416815 instructions (events)
> 199608554 cache references (events)
> 19990754 cache misses (events)
>
> Wall-clock time elapsed: 1008.882282 msecs
>
> With -a it's measured system-wide, from start of test to end of test
> - the results will be a lot more stable (and relevant) statistically
> than wall-clock time or CPU usage measurements. (both of which are
> rather imprecise in general)

I tried this perf stuff and got strange results on a cpu burning bench,
saturating my 8 cpus with a "while (1) ;" loop

# perf stat -a sleep 10

Performance counter stats for 'sleep':

80334.709038 task clock ticks (msecs)
80638 context switches (events)
4 CPU migrations (events)
468 pagefaults (events)
160694681969 CPU cycles (events)
160127154810 instructions (events)
686393 cache references (events)
230117 cache misses (events)

Wall-clock time elapsed: 10041.531644 msecs

So its about 16069468196 cycles per second for 8 cpus
Divide by 8 to get 2008683524 cycles per second per cpu,
which is not 3000000000 (E5450 @ 3.00GHz)

It seems strange a "jmp myself" uses one unhalted cycle per instruction
and 0.5 halted cycle ...

Also, after using "perf stat", tbench results are 1778 MB/S
instead of 2610 MB/s. Even if no perf stat running.