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Message-ID: <ae2be79c-a030-a4d2-0d89-15efa17fe77c@redhat.com>
Date:   Tue, 26 Sep 2023 09:41:33 +0200
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Subject: Re: [PATCH v3 1/4] mm/ksm: add "smart" page scanning mode
To:     Stefan Roesch <shr@devkernel.io>, kernel-team@fb.com
Cc:     akpm@linux-foundation.org, hannes@cmpxchg.org, riel@surriel.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
References: <20230926040939.516161-1-shr@devkernel.io>
 <20230926040939.516161-2-shr@devkernel.io>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20230926040939.516161-2-shr@devkernel.io>
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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.

-- 
Cheers,

David / dhildenb