linux-kernel - Re: [PATCH v2 9/9] mm/swap, shmem: use new swapin helper to skip readahead conditionally

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Date: Wed, 10 Jan 2024 11:35:32 +0800
From: Kairui Song <ryncsn@...il.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, 
	Chris Li <chrisl@...nel.org>, Hugh Dickins <hughd@...gle.com>, 
	Johannes Weiner <hannes@...xchg.org>, Matthew Wilcox <willy@...radead.org>, Michal Hocko <mhocko@...e.com>, 
	Yosry Ahmed <yosryahmed@...gle.com>, David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 9/9] mm/swap, shmem: use new swapin helper to skip
 readahead conditionally

Huang, Ying <ying.huang@...el.com> 于2024年1月9日周二 10:05写道：
>
> Kairui Song <ryncsn@...il.com> writes:
>
> > From: Kairui Song <kasong@...cent.com>
> >
> > Currently, shmem uses cluster readahead for all swap backends. Cluster
> > readahead is not a good solution for ramdisk based device (ZRAM) at all.
> >
> > After switching to the new helper, most benchmarks showed a good result:
> >
> > - Single file sequence read:
> >   perf stat --repeat 20 dd if=/tmpfs/test of=/dev/null bs=1M count=8192
> >   (/tmpfs/test is a zero filled file, using brd as swap, 4G memcg limit)
> >   Before: 22.248 +- 0.549
> >   After:  22.021 +- 0.684 (-1.1%)
> >
> > - Random read stress test:
> >   fio -name=tmpfs --numjobs=16 --directory=/tmpfs \
> >   --size=256m --ioengine=mmap --rw=randread --random_distribution=random \
> >   --time_based --ramp_time=1m --runtime=5m --group_reporting
> >   (using brd as swap, 2G memcg limit)
> >
> >   Before: 1818MiB/s
> >   After:  1888MiB/s (+3.85%)
> >
> > - Zipf biased random read stress test:
> >   fio -name=tmpfs --numjobs=16 --directory=/tmpfs \
> >   --size=256m --ioengine=mmap --rw=randread --random_distribution=zipf:1.2 \
> >   --time_based --ramp_time=1m --runtime=5m --group_reporting
> >   (using brd as swap, 2G memcg limit)
> >
> >   Before: 31.1GiB/s
> >   After:  32.3GiB/s (+3.86%)
> >
> > So cluster readahead doesn't help much even for single sequence read,
> > and for random stress test, the performance is better without it.
> >
> > Considering both memory and swap device will get more fragmented
> > slowly, and commonly used ZRAM consumes much more CPU than plain
> > ramdisk, false readahead could occur more frequently and waste
> > more CPU. Direct SWAP is cheaper, so use the new helper and skip
> > read ahead for SWP_SYNCHRONOUS_IO device.
>
> It's good to take advantage of swap_direct (no readahead).  I also hopes
> we can take advantage of VMA based swapin if shmem is accessed via mmap.
> That appears possible.

Good idea, that should be doable, will update the series.