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

Paul E. McKenney
2009-04-30T21:24:26Z

On Thu, Apr 30, 2009 at 04:08:48PM +0200, Eric Dumazet wrote:
> Ingo Molnar a écrit :
> > * Eric Dumazet <dada1@cosmosbay.com> wrote:
> >
> >> 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)
> >
> > What does "perf stat -l -a sleep 10" show? I suspect your counters
> > are scaled by about 67%, due to counter over-commit. -l will show
> > the scaling factor (and will scale up the results).
>
> Only difference I see with '-l' is cache misses not counted.
>
> (tbench 8 running, so not one instruction per cycle)
>
> # perf stat -l -a sleep 10
>
> Performance counter stats for 'sleep':
>
> 80007.128844 task clock ticks (msecs)
> 6754642 context switches (events)
> 2 CPU migrations (events)
> 474 pagefaults (events)
> 160925719143 CPU cycles (events)
> 108482003620 instructions (events)
> 7584035056 cache references (events)
> <not counted> cache misses
>
> Wall-clock time elapsed: 10000.595448 msecs
>
> # perf stat -a sleep 10
>
> Performance counter stats for 'sleep':
>
> 80702.908287 task clock ticks (msecs)
> 6792588 context switches (events)
> 24 CPU migrations (events)
> 4867 pagefaults (events)
> 161957342744 CPU cycles (events)
> 109147553984 instructions (events)
> 7633190481 cache references (events)
> 22996234 cache misses (events)
>
> Wall-clock time elapsed: 10087.502391 msecs
>
>
>
> >
> > If so then i think this behavior is confusing, and i'll make -l
> > default-enabled. (in fact i just committed this change to latest
> > -tip and pushed it out)
> >
> > To get only instructions and cycles, do:
> >
> > perf stat -e instructions -e cycles
> >
>
> # perf stat -e instructions -e cycles -a sleep 10
>
> Performance counter stats for 'sleep':
>
> 109469842392 instructions (events)
> 162012922122 CPU cycles (events)
>
> Wall-clock time elapsed: 10124.943544 msecs
>
> I am wondering if cpus are not running at 2 GHz ;)
>
>
> >> 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.
> >
> > Hm, that would be a bug. Could you send the dmesg output of:
> >
> > echo p > /proc/sysrq-trigger
> > echo p > /proc/sysrq-trigger
> >
> > with counters running it will show something like:
> >
> > [ 868.105712] SysRq : Show Regs
> > [ 868.106544]
> > [ 868.106544] CPU#1: ctrl: ffffffffffffffff
> > [ 868.106544] CPU#1: status: 0000000000000000
> > [ 868.106544] CPU#1: overflow: 0000000000000000
> > [ 868.106544] CPU#1: fixed: 0000000000000000
> > [ 868.106544] CPU#1: used: 0000000000000000
> > [ 868.106544] CPU#1: gen-PMC0 ctrl: 00000000001300c0
> > [ 868.106544] CPU#1: gen-PMC0 count: 000000ffee889194
> > [ 868.106544] CPU#1: gen-PMC0 left: 0000000011e1791a
> > [ 868.106544] CPU#1: gen-PMC1 ctrl: 000000000013003c
> > [ 868.106544] CPU#1: gen-PMC1 count: 000000ffd2542438
> > [ 868.106544] CPU#1: gen-PMC1 left: 000000002dd17a8e
>
> They stay fix (but only CPU#0 is displayed)
>
> Is perf able to display per cpu counters, and not aggregated values ?
>
> [ 7894.426787] CPU#0: ctrl: ffffffffffffffff
> [ 7894.426788] CPU#0: status: 0000000000000000
> [ 7894.426790] CPU#0: overflow: 0000000000000000
> [ 7894.426792] CPU#0: fixed: 0000000000000000
> [ 7894.426793] CPU#0: used: 0000000000000000
> [ 7894.426796] CPU#0: gen-PMC0 ctrl: 0000000000134f2e
> [ 7894.426798] CPU#0: gen-PMC0 count: 000000ffb91e31e1
> [ 7894.426799] CPU#0: gen-PMC0 left: 000000007fffffff
> [ 7894.426802] CPU#0: gen-PMC1 ctrl: 000000000013412e
> [ 7894.426804] CPU#0: gen-PMC1 count: 000000ff80312b23
> [ 7894.426805] CPU#0: gen-PMC1 left: 000000007fffffff
> [ 7894.426807] CPU#0: fixed-PMC0 count: 000000ffacf54a68
> [ 7894.426809] CPU#0: fixed-PMC1 count: 000000ffb71cfe02
> [ 7894.426811] CPU#0: fixed-PMC2 count: 0000000000000000
> [ 7905.522262] SysRq : Show Regs
> [ 7905.522277]
> [ 7905.522279] CPU#0: ctrl: ffffffffffffffff
> [ 7905.522281] CPU#0: status: 0000000000000000
> [ 7905.522283] CPU#0: overflow: 0000000000000000
> [ 7905.522284] CPU#0: fixed: 0000000000000000
> [ 7905.522286] CPU#0: used: 0000000000000000
> [ 7905.522288] CPU#0: gen-PMC0 ctrl: 0000000000134f2e
> [ 7905.522290] CPU#0: gen-PMC0 count: 000000ffb91e31e1
> [ 7905.522292] CPU#0: gen-PMC0 left: 000000007fffffff
> [ 7905.522294] CPU#0: gen-PMC1 ctrl: 000000000013412e
> [ 7905.522296] CPU#0: gen-PMC1 count: 000000ff80312b23
> [ 7905.522298] CPU#0: gen-PMC1 left: 000000007fffffff
> [ 7905.522299] CPU#0: fixed-PMC0 count: 000000ffacf54a68
> [ 7905.522301] CPU#0: fixed-PMC1 count: 000000ffb71cfe02
> [ 7905.522303] CPU#0: fixed-PMC2 count: 0000000000000000
>
>
> >
> > the counts should stay put (i.e. all counters should be disabled).
> > If they move around - despite there being no 'perf stat -a' session
> > running, that would be a bug.
>
> I rebooted my machine then got good results
>
> # perf stat -e instructions -e cycles -a sleep 10
>
> Performance counter stats for 'sleep':
>
> 240021659058 instructions (events)
> 240997984836 CPU cycles (events) << OK >>
>
> Wall-clock time elapsed: 10041.499326 msecs
>
> But if I use plain "perf stat -a sleep 10"
> it seems I get wrong values again (16 G cycles/sec) for all next perf sessions

I have to ask...

Is it possible that the machine runs at 3GHz initially, but slows down
to 2GHz for cooling reasons? One thing to try would be to run powertop,
which displays the frequencies. I get the following if mostly idle:

PowerTOP version 1.8 (C) 2007 Intel Corporation

Cn Avg residency P-states (frequencies)
C0 (cpu running) (14.1%) 2.17 Ghz 4.3%
C1 0.0ms ( 0.0%) 1.67 Ghz 0.0%
C2 0.5ms (16.2%) 1333 Mhz 0.0%
C3 0.5ms (69.8%) 1000 Mhz 95.7%

And the following with an infinite loop running:

PowerTOP version 1.8 (C) 2007 Intel Corporation

Cn Avg residency P-states (frequencies)
C0 (cpu running) (54.3%) 2.17 Ghz 100.0%
C1 0.0ms ( 0.0%) 1.67 Ghz 0.0%
C2 1.2ms ( 1.7%) 1333 Mhz 0.0%
C3 1.3ms (44.0%) 1000 Mhz 0.0%

But I am probably missing the point here...

Thanx, Paul

> # perf stat -a sleep 10
>
> Performance counter stats for 'sleep':
>
> 80332.718661 task clock ticks (msecs)
> 80602 context switches (events)
> 4 CPU migrations (events)
> 473 pagefaults (events)
> 160665397757 CPU cycles (events) << bad >>
> 160079277974 instructions (events)
> 793665 cache references (events)
> 226829 cache misses (events)
>
> Wall-clock time elapsed: 10041.302969 msecs
>
> # perf stat -e cycles -a sleep 10
>
> Performance counter stats for 'sleep':
>
> 160665176575 CPU cycles (events) << bad >>
>
> Wall-clock time elapsed: 10041.491503 msecs
>
>
> >
> > Also, the overhead might be profile-able, via:
> >
> > perf record -m 1024 sleep 10
> >
> > (this records the profile into output.perf.)
> >
> > followed by:
> >
> > ./perf-report | tail -20
> >
> > to display a histogram, with kernel-space and user-space symbols
> > mixed into a single profile.
> >
> > (Pick up latest -tip to get perf-report built by default.)
>
> Thanks, this is what I use currently
>
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