linux-kernel - Re: [PATCH v4 2/2] ksm: add profit monitoring documentation

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Message-ID: <bd50d4d2-f3dc-4e66-1d82-5221dab8b456@gmail.com>
Date:   Wed, 24 Aug 2022 20:02:40 +0700
From:   Bagas Sanjaya <bagasdotme@...il.com>
To:     xu xin <cgel.zte@...il.com>, akpm@...ux-foundation.org,
        corbet@....net
Cc:     adobriyan@...il.com, willy@...radead.org, hughd@...gle.com,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-doc@...r.kernel.org,
        xu xin <xu.xin16@....com.cn>,
        Xiaokai Ran <ran.xiaokai@....com.cn>,
        Yang Yang <yang.yang29@....com.cn>
Subject: Re: [PATCH v4 2/2] ksm: add profit monitoring documentation

On 8/24/22 19:48, xu xin wrote:
> +Monitoring KSM profit
> +=====================
> +
> +KSM can save memory by merging identical pages, but also can consume
> +additional memory, because it needs to generate a number of rmap_items to
> +save each scanned page's brief rmap information. Some of these pages may
> +be merged, but some may not be abled to be merged after being checked
> +several times, which are unprofitable memory consumed.
> +
> +1) How to determine whether KSM save memory or consume memory in system-wide
> +   range? Here is a simple approximate calculation for reference::
> +
> +	general_profit =~ pages_sharing * sizeof(page) - (all_rmap_items) *
> +			  sizeof(rmap_item);
> +
> +   where all_rmap_items can be easily obtained by summing ``pages_sharing``,
> +   ``pages_shared``, ``pages_unshared`` and ``pages_volatile``.
> +
> +2) The KSM profit inner a single process can be similarly obtained by the
> +   following approximate calculation::
> +
> +	process_profit =~ ksm_merging_pages * sizeof(page) -
> +			  ksm_rmp_items * sizeof(rmap_item).
> +
> +   where both ksm_merging_pages and ksm_rmp_items are shown under the
> +   directory ``/proc/<pid>/``.
> +
> +From the perspective of application, a high ratio of ``ksm_rmp_items`` to
> +``ksm_merging_pages`` means a bad madvise-applied policy, so developers or
> +administrators have to rethink how to change madvise policy. Giving an example
> +for reference, a page's size is usually 4K, and the rmap_item's size is
> +separately 32B on 32-bit CPU architecture and 64B on 64-bit CPU architecture.
> +so if the ``ksm_rmp_items/ksm_merging_pages`` ratio exceeds 64 on 64-bit CPU
> +or exceeds 128 on 32-bit CPU, then the app's madvise policy should be dropped,
> +because the ksm profit is approximately zero or negative.
> +
>  Monitoring KSM events
>  =====================
>  

LGTM.

Reviewed-by: Bagas Sanjaya <bagasdotme@...il.com>

-- 
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