On Thu, Feb 17, 2022 at 09:53:08AM +0800, Aaron Lu wrote:
> On Thu, Feb 17, 2022 at 12:22:26AM +0000, Mel Gorman wrote:
> > free_pcppages_bulk() has taken two passes through the pcp lists since
> > commit 0a5f4e5b4562 ("mm/free_pcppages_bulk: do not hold lock when picking
> > pages to free") due to deferring the cost of selecting PCP lists until
> > the zone lock is held. Now that list selection is simplier, the main
> > cost during selection is bulkfree_pcp_prepare() which in the normal case
> > is a simple check and prefetching. As the list manipulations have cost
> > in itself, go back to freeing pages in a single pass.
> >
> > The series up to this point was evaulated using a trunc microbenchmark
> > that is truncating sparse files stored in page cache (mmtests config
> > config-io-trunc). Sparse files were used to limit filesystem interaction.
> > The results versus a revert of storing high-order pages in the PCP lists is
> >
> > 1-socket Skylake
> > 5.17.0-rc3 5.17.0-rc3 5.17.0-rc3
> > vanilla mm-reverthighpcp-v1 mm-highpcpopt-v2
> > Min elapsed 540.00 ( 0.00%) 530.00 ( 1.85%) 530.00 ( 1.85%)
> > Amean elapsed 543.00 ( 0.00%) 530.00 * 2.39%* 530.00 * 2.39%*
> > Stddev elapsed 4.83 ( 0.00%) 0.00 ( 100.00%) 0.00 ( 100.00%)
> > CoeffVar elapsed 0.89 ( 0.00%) 0.00 ( 100.00%) 0.00 ( 100.00%)
> > Max elapsed 550.00 ( 0.00%) 530.00 ( 3.64%) 530.00 ( 3.64%)
> > BAmean-50 elapsed 540.00 ( 0.00%) 530.00 ( 1.85%) 530.00 ( 1.85%)
> > BAmean-95 elapsed 542.22 ( 0.00%) 530.00 ( 2.25%) 530.00 ( 2.25%)
> > BAmean-99 elapsed 542.22 ( 0.00%) 530.00 ( 2.25%) 530.00 ( 2.25%)
> >
> > 2-socket CascadeLake
> > 5.17.0-rc3 5.17.0-rc3 5.17.0-rc3
> > vanilla mm-reverthighpcp-v1 mm-highpcpopt-v2
> > Min elapsed 510.00 ( 0.00%) 500.00 ( 1.96%) 500.00 ( 1.96%)
> > Amean elapsed 529.00 ( 0.00%) 521.00 ( 1.51%) 510.00 * 3.59%*
> > Stddev elapsed 16.63 ( 0.00%) 12.87 ( 22.64%) 11.55 ( 30.58%)
> > CoeffVar elapsed 3.14 ( 0.00%) 2.47 ( 21.46%) 2.26 ( 27.99%)
> > Max elapsed 550.00 ( 0.00%) 540.00 ( 1.82%) 530.00 ( 3.64%)
> > BAmean-50 elapsed 516.00 ( 0.00%) 512.00 ( 0.78%) 500.00 ( 3.10%)
> > BAmean-95 elapsed 526.67 ( 0.00%) 518.89 ( 1.48%) 507.78 ( 3.59%)
> > BAmean-99 elapsed 526.67 ( 0.00%) 518.89 ( 1.48%) 507.78 ( 3.59%)
> >
> > The original motivation for multi-passes was will-it-scale page_fault1
> > using $nr_cpu processes.
> >
> > 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.
The test on CooperLake has finished and the result is in noise range,
i.e. no obvious performance change due to this patch. Feel free to add
my tested-by tag.
There is a problem with the CascadeLake server, I can't test there. But
I don't expect a surprising result either :-)
BTW, the lock contention on the 4s CooperLake is only 46%, with 14% from
page allocator allocation path and 4% from the free path, others are on
LRU path. I remember the lock contention used to be in the 80% percent
in my previous testing, and I found this old mail where page_fault1 test
on 2s skylake saw 80% lock contention:
https://lore.kernel.org/linux-mm/20180205053139.GC16980@xxxxxxxxx/
So I also did the test on a 2s CascadeLake(failed to find a skylake server)
and found that the lock contention is about 70%, with 12% coming from page
allocator allocation path and 1.6% from free path, others are on LRU path.
I guess something has happend in the page allocator and zone lock contention
is much better now, I don't know what exactly, I'll take a look at this later.
And since free path's lock contention is small, I wonder how much
value of the prefetch_done_on_free_path still has. I'll also give it a
test later.
Regards,
Aaron
>
> 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 kernel for each commit will have to be fetched and built and then
> the job will be run for 3 times for each commit. These servers are also
> busy with other jobs so it may take a while.
>
> Regards,
> Aaron
