linux-kernel - Re: [PATCH 1/2] mm: swap: make page

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Date:   Fri, 13 Mar 2020 12:50:04 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm: swap: make page_evictable() inline

On Fri, Mar 13, 2020 at 12:46 PM Yang Shi <yang.shi@...ux.alibaba.com> wrote:
>
>
>
> On 3/13/20 12:33 PM, Shakeel Butt wrote:
> > On Fri, Mar 13, 2020 at 11:34 AM Yang Shi <yang.shi@...ux.alibaba.com> wrote:
> >> When backporting commit 9c4e6b1a7027 ("mm, mlock, vmscan: no more
> >> skipping pagevecs") to our 4.9 kernel, our test bench noticed around 10%
> >> down with a couple of vm-scalability's test cases (lru-file-readonce,
> >> lru-file-readtwice and lru-file-mmap-read).  I didn't see that much down
> >> on my VM (32c-64g-2nodes).  It might be caused by the test configuration,
> >> which is 32c-256g with NUMA disabled and the tests were run in root memcg,
> >> so the tests actually stress only one inactive and active lru.  It
> >> sounds not very usual in mordern production environment.
> >>
> >> That commit did two major changes:
> >> 1. Call page_evictable()
> >> 2. Use smp_mb to force the PG_lru set visible
> >>
> >> It looks they contribute the most overhead.  The page_evictable() is a
> >> function which does function prologue and epilogue, and that was used by
> >> page reclaim path only.  However, lru add is a very hot path, so it
> >> sounds better to make it inline.  However, it calls page_mapping() which
> >> is not inlined either, but the disassemble shows it doesn't do push and
> >> pop operations and it sounds not very straightforward to inline it.
> >>
> >> Other than this, it sounds smp_mb() is not necessary for x86 since
> >> SetPageLRU is atomic which enforces memory barrier already, replace it
> >> with smp_mb__after_atomic() in the following patch.
> >>
> >> With the two fixes applied, the tests can get back around 5% on that
> >> test bench and get back normal on my VM.  Since the test bench
> >> configuration is not that usual and I also saw around 6% up on the
> >> latest upstream, so it sounds good enough IMHO.
> >>
> >> The below is test data (lru-file-readtwice throughput) against the v5.6-rc4:
> >>          mainline        w/ inline fix
> >>            150MB            154MB
> >>
> > What is the test setup for the above experiment? I would like to get a repro.
>
> Just startup a VM with two nodes, then run case-lru-file-readtwice or
> case-lru-file-readonce in vm-scalability in root memcg or with memcg
> disabled.  Then get the average throughput (dd result) from the test.
> Our test bench uses the script from lkp, but I just ran it manually.
> Single node VM should be more obvious showed in my test.
>

Thanks, I will try this on a real machine.