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

From: Ingo Molnar <[email-masked]>
Date: Thu, 30 Apr 2009 13:57:36 +0200

* Eric Dumazet <[email-masked]> wrote:

> Ingo Molnar a écrit :
> > * Eric Dumazet <[email-masked]> 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).

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

> 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

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.

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.)

	Ingo