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Message-ID: <87pm20p9ra.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Sat, 30 Sep 2023 12:26:01 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Vlastimil Babka <vbabka@...e.cz>,
Mel Gorman <mgorman@...hsingularity.net>,
Miaohe Lin <linmiaohe@...wei.com>,
Kefeng Wang <wangkefeng.wang@...wei.com>,
Zi Yan <ziy@...dia.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] mm: page_alloc: remove pcppage migratetype caching
Johannes Weiner <hannes@...xchg.org> writes:
> On Wed, Sep 27, 2023 at 01:42:25PM +0800, Huang, Ying wrote:
>> Johannes Weiner <hannes@...xchg.org> writes:
>>
>> > The idea behind the cache is to save get_pageblock_migratetype()
>> > lookups during bulk freeing. A microbenchmark suggests this isn't
>> > helping, though. The pcp migratetype can get stale, which means that
>> > bulk freeing has an extra branch to check if the pageblock was
>> > isolated while on the pcp.
>> >
>> > While the variance overlaps, the cache write and the branch seem to
>> > make this a net negative. The following test allocates and frees
>> > batches of 10,000 pages (~3x the pcp high marks to trigger flushing):
>> >
>> > Before:
>> > 8,668.48 msec task-clock # 99.735 CPUs utilized ( +- 2.90% )
>> > 19 context-switches # 4.341 /sec ( +- 3.24% )
>> > 0 cpu-migrations # 0.000 /sec
>> > 17,440 page-faults # 3.984 K/sec ( +- 2.90% )
>> > 41,758,692,473 cycles # 9.541 GHz ( +- 2.90% )
>> > 126,201,294,231 instructions # 5.98 insn per cycle ( +- 2.90% )
>> > 25,348,098,335 branches # 5.791 G/sec ( +- 2.90% )
>> > 33,436,921 branch-misses # 0.26% of all branches ( +- 2.90% )
>> >
>> > 0.0869148 +- 0.0000302 seconds time elapsed ( +- 0.03% )
>> >
>> > After:
>> > 8,444.81 msec task-clock # 99.726 CPUs utilized ( +- 2.90% )
>> > 22 context-switches # 5.160 /sec ( +- 3.23% )
>> > 0 cpu-migrations # 0.000 /sec
>> > 17,443 page-faults # 4.091 K/sec ( +- 2.90% )
>> > 40,616,738,355 cycles # 9.527 GHz ( +- 2.90% )
>> > 126,383,351,792 instructions # 6.16 insn per cycle ( +- 2.90% )
>> > 25,224,985,153 branches # 5.917 G/sec ( +- 2.90% )
>> > 32,236,793 branch-misses # 0.25% of all branches ( +- 2.90% )
>> >
>> > 0.0846799 +- 0.0000412 seconds time elapsed ( +- 0.05% )
>> >
>> > A side effect is that this also ensures that pages whose pageblock
>> > gets stolen while on the pcplist end up on the right freelist and we
>> > don't perform potentially type-incompatible buddy merges (or skip
>> > merges when we shouldn't), whis is likely beneficial to long-term
>> > fragmentation management, although the effects would be harder to
>> > measure. Settle for simpler and faster code as justification here.
>>
>> I suspected the PCP allocating/freeing path may be influenced (that is,
>> allocating/freeing batch is less than PCP high). So I tested
>> one-process will-it-scale/page_fault1 with sysctl
>> percpu_pagelist_high_fraction=8. So pages will be allocated/freed
>> from/to PCP only. The test results are as follows,
>>
>> Before:
>> will-it-scale.1.processes 618364.3 (+- 0.075%)
>> perf-profile.children.get_pfnblock_flags_mask 0.13 (+- 9.350%)
>>
>> After:
>> will-it-scale.1.processes 616512.0 (+- 0.057%)
>> perf-profile.children.get_pfnblock_flags_mask 0.41 (+- 22.44%)
>>
>> The change isn't large: -0.3%. Perf profiling shows the cycles% of
>> get_pfnblock_flags_mask() increases.
>
> Ah, this is going through the free_unref_page_list() path that
> Vlastimil had pointed out as well. I made another change on top that
> eliminates the second lookup. After that, both pcp fast paths have the
> same number of lookups as before: 1. This fixes the regression for me.
>
> Would you mind confirming this as well?
I have done more test for the series and addon patches. The test
results are as follows,
base
perf-profile.children.get_pfnblock_flags_mask 0.15 (+- 32.62%)
will-it-scale.1.processes 618621.7 (+- 0.18%)
mm: page_alloc: remove pcppage migratetype caching
perf-profile.children.get_pfnblock_flags_mask 0.40 (+- 21.55%)
will-it-scale.1.processes 616350.3 (+- 0.27%)
mm: page_alloc: fix up block types when merging compatible blocks
perf-profile.children.get_pfnblock_flags_mask 0.36 (+- 8.36%)
will-it-scale.1.processes 617121.0 (+- 0.17%)
mm: page_alloc: move free pages when converting block during isolation
perf-profile.children.get_pfnblock_flags_mask 0.36 (+- 15.10%)
will-it-scale.1.processes 615578.0 (+- 0.18%)
mm: page_alloc: fix move_freepages_block() range error
perf-profile.children.get_pfnblock_flags_mask 0.36 (+- 12.78%)
will-it-scale.1.processes 615364.7 (+- 0.27%)
mm: page_alloc: fix freelist movement during block conversion
perf-profile.children.get_pfnblock_flags_mask 0.36 (+- 10.52%)
will-it-scale.1.processes 617834.8 (+- 0.52%)
mm: page_alloc: consolidate free page accounting
perf-profile.children.get_pfnblock_flags_mask 0.39 (+- 8.27%)
will-it-scale.1.processes 621000.0 (+- 0.13%)
mm: page_alloc: close migratetype race between freeing and stealing
perf-profile.children.get_pfnblock_flags_mask 0.37 (+- 5.87%)
will-it-scale.1.processes 618378.8 (+- 0.17%)
mm: page_alloc: optimize free_unref_page_list()
perf-profile.children.get_pfnblock_flags_mask 0.20 (+- 14.96%)
will-it-scale.1.processes 618136.3 (+- 0.16%)
It seems that the will-it-scale score is influenced by some other
factors too. But anyway, the series + addon patches restores the score
of will-it-scale. And the cycles% of get_pfnblock_flags_mask() is
almost restored by the final patch (mm: page_alloc: optimize
free_unref_page_list()).
Feel free to add my "Tested-by" for these patches.
--
Best Regards,
Huang, Ying
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