linux-kernel - Re: [PATCH v3 1/4] mm/ksm: add "smart" page scanning mode

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Message-ID: <qvqwmsx83o68.fsf@devbig1114.prn1.facebook.com>
Date:   Tue, 26 Sep 2023 09:13:31 -0700
From:   Stefan Roesch <shr@...kernel.io>
To:     David Hildenbrand <david@...hat.com>
Cc:     kernel-team@...com, akpm@...ux-foundation.org, hannes@...xchg.org,
        riel@...riel.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 1/4] mm/ksm: add "smart" page scanning mode


David Hildenbrand <david@...hat.com> writes:

> On 26.09.23 06:09, Stefan Roesch wrote:
>> This change adds a "smart" page scanning mode for KSM. So far all the
>> candidate pages are continuously scanned to find candidates for
>> de-duplication. There are a considerably number of pages that cannot be
>> de-duplicated. This is costly in terms of CPU. By using smart scanning
>> considerable CPU savings can be achieved.
>> This change takes the history of scanning pages into account and skips
>> the page scanning of certain pages for a while if de-deduplication for
>> this page has not been successful in the past.
>> To do this it introduces two new fields in the ksm_rmap_item structure:
>> age and remaining_skips. age, is the KSM age and remaining_skips
>> determines how often scanning of this page is skipped. The age field is
>> incremented each time the page is scanned and the page cannot be de-
>> duplicated. age updated is capped at U8_MAX.
>> How often a page is skipped is dependent how often de-duplication has
>> been tried so far and the number of skips is currently limited to 8.
>> This value has shown to be effective with different workloads.
>> The feature is currently disable by default and can be enabled with the
>> new smart_scan knob.
>> The feature has shown to be very effective: upt to 25% of the page scans
>> can be eliminated; the pages_to_scan rate can be reduced by 40 - 50% and
>> a similar de-duplication rate can be maintained.
>
> Thinking about it, what are the cons of just enabling this always and not
> exposing new toggles? Alternatively, we could make this a compile-time option.
>
> In general, LGTM, just curious if we really have to make this configurable.
>

The only downside I can see is that it might take a longer time for some
pages to be de-duplicated (a new candidate page is added, but its
duplicate is skipped in this round). So it will take longer to
de-duplicate this page.

I tested with more than one workload, but it might be useful to get some
data with additional workloads. I was thinking of enabling it after one or
two releases.