Re: [PATCH 5/6] mm/page_alloc: Free pages in a single pass during bulk free

[Aaron Lu <aaron.lu@intel.com>]

On Mon, Feb 21, 2022 at 09:38:22PM +0800, Aaron Lu wrote:
> On Fri, Feb 18, 2022 at 12:20:03PM +0800, Aaron Lu wrote:
> > On Thu, Feb 17, 2022 at 09:31:13AM +0000, Mel Gorman wrote:
> > > On Thu, Feb 17, 2022 at 09:53:08AM +0800, Aaron Lu wrote:
> > > > > 2-socket CascadeLake (40 cores, 80 CPUs HT enabled)
> > > > >                                                     5.17.0-rc3                 5.17.0-rc3
> > > > >                                                        vanilla           mm-highpcpopt-v2
> > > > > Hmean     page_fault1-processes-2        2694662.26 (   0.00%)      2695780.35 (   0.04%)
> > > > > Hmean     page_fault1-processes-5        6425819.34 (   0.00%)      6435544.57 *   0.15%*
> > > > > Hmean     page_fault1-processes-8        9642169.10 (   0.00%)      9658962.39 (   0.17%)
> > > > > Hmean     page_fault1-processes-12      12167502.10 (   0.00%)     12190163.79 (   0.19%)
> > > > > Hmean     page_fault1-processes-21      15636859.03 (   0.00%)     15612447.26 (  -0.16%)
> > > > > Hmean     page_fault1-processes-30      25157348.61 (   0.00%)     25169456.65 (   0.05%)
> > > > > Hmean     page_fault1-processes-48      27694013.85 (   0.00%)     27671111.46 (  -0.08%)
> > > > > Hmean     page_fault1-processes-79      25928742.64 (   0.00%)     25934202.02 (   0.02%) <--
> > > > > Hmean     page_fault1-processes-110     25730869.75 (   0.00%)     25671880.65 *  -0.23%*
> > > > > Hmean     page_fault1-processes-141     25626992.42 (   0.00%)     25629551.61 (   0.01%)
> > > > > Hmean     page_fault1-processes-172     25611651.35 (   0.00%)     25614927.99 (   0.01%)
> > > > > Hmean     page_fault1-processes-203     25577298.75 (   0.00%)     25583445.59 (   0.02%)
> > > > > Hmean     page_fault1-processes-234     25580686.07 (   0.00%)     25608240.71 (   0.11%)
> > > > > Hmean     page_fault1-processes-265     25570215.47 (   0.00%)     25568647.58 (  -0.01%)
> > > > > Hmean     page_fault1-processes-296     25549488.62 (   0.00%)     25543935.00 (  -0.02%)
> > > > > Hmean     page_fault1-processes-320     25555149.05 (   0.00%)     25575696.74 (   0.08%)
> > > > > 
> > > > > The differences are mostly within the noise and the difference close to
> > > > > $nr_cpus is negligible.
> > > > 
> > > > I have queued will-it-scale/page_fault1/processes/$nr_cpu on 2 4-sockets
> > > > servers: CascadeLake and CooperLaker and will let you know the result
> > > > once it's out.
> > > > 
> > > 
> > > Thanks, 4 sockets and a later generation would be nice to cover.
> > > 
> > > > I'm using 'https://github.com/hnaz/linux-mm master' and doing the
> > > > comparison with commit c000d687ce22("mm/page_alloc: simplify how many
> > > > pages are selected per pcp list during bulk free") and commit 8391e0a7e172
> > > > ("mm/page_alloc: free pages in a single pass during bulk free") there.
> > > > 
> > > 
> > > The baseline looks fine. It's different to what I used but the page_alloc
> > > shouldn't have much impact.
> > > 
> > > When looking at will-it-scale, please pay attention to lower CPU counts
> > > as well and take account changes in standard deviation. Looking at the
> > 
> > I'll also test nr_task=4/16/64 on the 4sockets CooperLake(nr_cpu=144) then.
> > 
> 
> For the record, these tests don't show any visible performance changes
> on CooperLake.

One thing I just noticed is that, zone lock contention increased to some
extent. I'm not sure if this is worrisome so I suppose I should at least
mention it here.

The nr_task=100% test on the 4 sockets Cooper Lake showed that zone lock
contention increased from 13.56% to 20.16% and for nr_task=16, it
increased from 4.75% to 6.18%.

The reason is probably due to more code are now inside the lock and when
there is contention, it will make things worse. I'm aware of that
nr_task=100% is a rare case and this patchset is meant to improve things
when there is very little contention, which should be the common case.
So I guess that's just the tradeoff we have to make...

Here are the results on performance metric and zone lock metrics:

nr_task=100%
=========================================================================================
tbox_group/testcase/rootfs/kconfig/compiler/nr_task/mode/test/thp_enabled/cpufreq_governor:
  lkp-cpl-4sp1/will-it-scale/debian-10.4-x86_64-20200603.cgz/x86_64-rhel-8.3/gcc-9/100%/process/page_fault1/never/performance

commit/ucode:
  8391e0a7e1728d74faecebf096b446ac5d0a5709/0x7002302 (mm/page_alloc: free pages in a single pass during bulk free)
  c000d687ce22252c8ea96e47b4a2add592fbad6c/0x7002302 (mm/page_alloc: simplify how many pages are selected per pcp list during bulk free)
  7decb609034044e56cffd1c9971738878467ee96/0x7002402 (mm/page_alloc: Do not prefetch buddies during bulk free)

8391e0a7e1728d74 c000d687ce22252c8ea96e47b4a 7decb609034044e56cffd1c9971
---------------- --------------------------- ---------------------------
         %stddev     %change         %stddev     %change         %stddev
             \          |                \          |                \
  11807831            -0.5%   11750578            -0.3%   11778047        will-it-scale.144.processes
     15.44 ± 10%      -4.9       10.58 ±  8%      +0.6       16.01 ±  5%  perf-profile.calltrace.cycles-pp.native_queued_spin_lock_slowpath._raw_spin_lock.rmqueue_bulk.get_page_from_freelist.__alloc_pages
      4.72 ±  8%      -1.7        2.98            -0.1        4.63 ±  3%  perf-profile.calltrace.cycles-pp.native_queued_spin_lock_slowpath._raw_spin_lock.free_pcppages_bulk.free_unref_page_list.release_pages


nr_task=16
=========================================================================================
tbox_group/testcase/rootfs/kconfig/compiler/nr_task/mode/test/thp_enabled/cpufreq_governor/ucode:
  lkp-cpl-4sp1/will-it-scale/debian-10.4-x86_64-20200603.cgz/x86_64-rhel-8.3/gcc-9/16/process/page_fault1/never/performance/0x7002402

commit:
  8391e0a7e1728d74faecebf096b446ac5d0a5709 (mm/page_alloc: free pages in a single pass during bulk free)
  c000d687ce22252c8ea96e47b4a2add592fbad6c (mm/page_alloc: simplify how many pages are selected per pcp list during bulk free)
  7decb609034044e56cffd1c9971738878467ee96 (mm/page_alloc: Do not prefetch buddies during bulk free)

8391e0a7e1728d74 c000d687ce22252c8ea96e47b4a 7decb609034044e56cffd1c9971
---------------- --------------------------- ---------------------------
         %stddev     %change         %stddev     %change         %stddev
             \          |                \          |                \
   3410615            +0.2%    3416565            +0.2%    3415846        will-it-scale.16.processes
      4.83 ±  3%      -1.1        3.76 ±  9%      -0.4        4.40 ±  4%  perf-profile.calltrace.cycles-pp.native_queued_spin_lock_slowpath._raw_spin_lock.rmqueue_bulk.get_page_from_freelist.__alloc_pages
      1.35 ±  9%      -0.4        0.99 ± 14%      -0.2        1.17 ±  3%  perf-profile.calltrace.cycles-pp.native_queued_spin_lock_slowpath._raw_spin_lock.free_pcppages_bulk.free_unref_page_list.release_pages

Regards,
Aaron