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From:   Nhat Pham <nphamcs@gmail.com>
Date:   Thu, 7 Dec 2023 17:14:22 -0800
Message-ID: <CAKEwX=O7uovg611oyXFKTJdQ3y+Fi42RAXVheT904RcDOXwtjg@mail.gmail.com>
Subject: Re: [PATCH v6] zswap: memcontrol: implement zswap writeback disabling
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     tj@kernel.org, lizefan.x@bytedance.com, hannes@cmpxchg.org,
        cerasuolodomenico@gmail.com, yosryahmed@google.com,
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        mhocko@kernel.org, roman.gushchin@linux.dev, shakeelb@google.com,
        muchun.song@linux.dev, hughd@google.com, corbet@lwn.net,
        konrad.wilk@oracle.com, senozhatsky@chromium.org, rppt@kernel.org,
        linux-mm@kvack.org, kernel-team@meta.com,
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        david@ixit.cz, chrisl@kernel.org
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On Thu, Dec 7, 2023 at 4:42=E2=80=AFPM Nhat Pham <nphamcs@gmail.com> wrote:
>
[..]
>
> I don't have any concrete numbers though - any numbers I can pull out
> are from highly artificial tasks that only serve to test the
> correctness aspect of the implementation. zswap.writeback disablement
> would of course be faster in these situations (up to 33%!!!!) - but
> that's basically just saying HDD is slow. Which is not very
> informative or surprising, so I did not include it in the changelog.

For instance, on a server with HDD, I allocate memories and populate
them with random values (so that zswap store will always fail), and
specify memory.high low enough to trigger reclaim. The time it takes
to allocate the memories and just read through it a couple of times
(doing silly things like computing the values' average etc.):

zswap.writeback disabled:
real 0m30.537s
user 0m23.687s
sys 0m6.637s
0 pages swapped in
0 pages swapped out

zswap.writeback enabled:
real 0m45.061s
user 0m24.310s
sys 0m8.892s
712686 pages swapped in
461093 pages swapped out

(the last two lines are from vmstat -s).