Re: [PATCH] memcg: add pgfault latency histograms

[Ying Han <yinghan@google.com>]

On Thu, May 26, 2011 at 7:11 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@jp.fujitsu.com> wrote:
> On Thu, 26 May 2011 18:40:44 -0700
> Ying Han <yinghan@google.com> wrote:
>
>> On Thu, May 26, 2011 at 5:31 PM, KAMEZAWA Hiroyuki
>> <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>> > On Thu, 26 May 2011 17:23:20 -0700
>> > Ying Han <yinghan@google.com> wrote:
>> >
>> >> On Thu, May 26, 2011 at 5:05 PM, KAMEZAWA Hiroyuki <
>> >> kamezawa.hiroyu@jp.fujitsu.com> wrote:
>> >>
>> >> > On Thu, 26 May 2011 14:07:49 -0700
>> >> > Ying Han <yinghan@google.com> wrote:
>> >> >
>> >> > > This adds histogram to capture pagefault latencies on per-memcg basis. I
>> >> > used
>> >> > > this patch on the memcg background reclaim test, and figured there could
>> >> > be more
>> >> > > usecases to monitor/debug application performance.
>> >> > >
>> >> > > The histogram is composed 8 bucket in ns unit. The last one is infinite
>> >> > (inf)
>> >> > > which is everything beyond the last one. To be more flexible, the buckets
>> >> > can
>> >> > > be reset and also each bucket is configurable at runtime.
>> >> > >
>> >> > > memory.pgfault_histogram: exports the histogram on per-memcg basis and
>> >> > also can
>> >> > > be reset by echoing "reset". Meantime, all the buckets are writable by
>> >> > echoing
>> >> > > the range into the API. see the example below.
>> >> > >
>> >> > > /proc/sys/vm/pgfault_histogram: the global sysfs tunablecan be used to
>> >> > turn
>> >> > > on/off recording the histogram.
>> >> > >
>> >> > > Functional Test:
>> >> > > Create a memcg with 10g hard_limit, running dd & allocate 8g anon page.
>> >> > > Measure the anon page allocation latency.
>> >> > >
>> >> > > $ mkdir /dev/cgroup/memory/B
>> >> > > $ echo 10g >/dev/cgroup/memory/B/memory.limit_in_bytes
>> >> > > $ echo $$ >/dev/cgroup/memory/B/tasks
>> >> > > $ dd if=/dev/zero of=/export/hdc3/dd/tf0 bs=1024 count=20971520 &
>> >> > > $ allocate 8g anon pages
>> >> > >
>> >> > > $ echo 1 >/proc/sys/vm/pgfault_histogram
>> >> > >
>> >> > > $ cat /dev/cgroup/memory/B/memory.pgfault_histogram
>> >> > > pgfault latency histogram (ns):
>> >> > > < 600            2051273
>> >> > > < 1200           40859
>> >> > > < 2400           4004
>> >> > > < 4800           1605
>> >> > > < 9600           170
>> >> > > < 19200          82
>> >> > > < 38400          6
>> >> > > < inf            0
>> >> > >
>> >> > > $ echo reset >/dev/cgroup/memory/B/memory.pgfault_histogram
>> >> > > $ cat /dev/cgroup/memory/B/memory.pgfault_histogram
>> >> > > pgfault latency histogram (ns):
>> >> > > < 600            0
>> >> > > < 1200           0
>> >> > > < 2400           0
>> >> > > < 4800           0
>> >> > > < 9600           0
>> >> > > < 19200          0
>> >> > > < 38400          0
>> >> > > < inf            0
>> >> > >
>> >> > > $ echo 500 520 540 580 600 1000 5000
>> >> > >/dev/cgroup/memory/B/memory.pgfault_histogram
>> >> > > $ cat /dev/cgroup/memory/B/memory.pgfault_histogram
>> >> > > pgfault latency histogram (ns):
>> >> > > < 500            50
>> >> > > < 520            151
>> >> > > < 540            3715
>> >> > > < 580            1859812
>> >> > > < 600            202241
>> >> > > < 1000           25394
>> >> > > < 5000           5875
>> >> > > < inf            186
>> >> > >
>> >> > > Performance Test:
>> >> > > I ran through the PageFaultTest (pft) benchmark to measure the overhead
>> >> > of
>> >> > > recording the histogram. There is no overhead observed on both
>> >> > "flt/cpu/s"
>> >> > > and "fault/wsec".
>> >> > >
>> >> > > $ mkdir /dev/cgroup/memory/A
>> >> > > $ echo 16g >/dev/cgroup/memory/A/memory.limit_in_bytes
>> >> > > $ echo $$ >/dev/cgroup/memory/A/tasks
>> >> > > $ ./pft -m 15g -t 8 -T a
>> >> > >
>> >> > > Result:
>> >> > > "fault/wsec"
>> >> > >
>> >> > > $ ./ministat no_histogram histogram
>> >> > > x no_histogram
>> >> > > + histogram
>> >> > >
>> >> > +--------------------------------------------------------------------------+
>> >> > >    N           Min           Max        Median           Avg
>> >> >  Stddev
>> >> > > x   5     813404.51     824574.98      821661.3     820470.83
>> >> > 4202.0758
>> >> > > +   5     821228.91     825894.66     822874.65     823374.15
>> >> > 1787.9355
>> >> > >
>> >> > > "flt/cpu/s"
>> >> > >
>> >> > > $ ./ministat no_histogram histogram
>> >> > > x no_histogram
>> >> > > + histogram
>> >> > >
>> >> > +--------------------------------------------------------------------------+
>> >> > >    N           Min           Max        Median           Avg
>> >> >  Stddev
>> >> > > x   5     104951.93     106173.13     105142.73      105349.2
>> >> > 513.78158
>> >> > > +   5     104697.67      105416.1     104943.52     104973.77
>> >> > 269.24781
>> >> > > No difference proven at 95.0% confidence
>> >> > >
>> >> > > Signed-off-by: Ying Han <yinghan@google.com>
>> >> >
>> >> > Hmm, interesting....but isn't it very very very complicated interface ?
>> >> > Could you make this for 'perf' ? Then, everyone (including someone who
>> >> > don't use memcg)
>> >> > will be happy.
>> >> >
>> >>
>> >> Thank you for looking at it.
>> >>
>> >> There is only one per-memcg API added which is basically exporting the
>> >> histogram. The "reset" and reconfiguring the bucket is not "must" but make
>> >> it more flexible. Also, the sysfs API can be reduced if necessary since
>> >> there is no over-head observed by always turning it on anyway.
>> >>
>> >> I am not familiar w/ perf, any suggestions how it is supposed to be look
>> >> like?
>> >>
>> >> Thanks
>> >>
>> >
>> > IIUC, you can record "all" latency information by perf record. Then, latency
>> > information can be dumped out to some file.
>> >
>> > You can add a python? script for perf as
>> >
>> >  # perf report memory-reclaim-latency-histgram -f perf.data
>> >                -o 500,1000,1500,2000.....
>> >   ...show histgram in text.. or report the histgram in graphic.
>> >
>> > Good point is
>> >  - you can reuse perf.data and show histgram from another point of view.
>> >
>> >  - you can show another cut of view, for example, I think you can write a
>> >    parser to show "changes in hisgram by time", easily.
>> >    You may able to generate a movie ;)
>> >
>> >  - Now, perf cgroup is supported. Then,
>> >    - you can see per task histgram
>> >    - you can see per cgroup histgram
>> >    - you can see per system-wide histgram
>> >      (If you record latency of usual kswapd/alloc_pages)
>> >
>> >  - If you record latency within shrink_zone(), you can show per-zone
>> >    reclaim latency histgram. record parsers can gather them and
>> >    show histgram. This will be benefical to cpuset users.
>> >
>> >
>> > I'm sorry if I miss something.
>>
>> After study a bit on perf, it is not feasible in this casecase. The
>> cpu & memory overhead of perf is overwhelming.... Each page fault will
>> generate a record in the buffer and how many data we can record in the
>> buffer, and how many data will be processed later.. Most of the data
>> that is recorded by the general perf framework is not needed here.
>>
>
> I disagree. "each page fault" is not correct. "every lru scan" is correct.
> Then, record to buffer will be at most memory.failcnt times.

Hmm. Sorry I might miss something here... :(

The page fault histogram recorded is per page-fault, only the ones
trigger reclaim. The background reclaim testing is just one usecase of
it, and we need this information for more
general usage to monitor application performance. So i recorded the
latency for each single page fault.

--Ying

>
> please consider more.
>
>
> Thanks,
> -Kame
>
>

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