linux-kernel - Re: [PATCH] ksm: count allocated ksm rmap

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Date:   Mon, 22 Aug 2022 14:08:44 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     xu xin <cgel.zte@...il.com>
Cc:     adobriyan@...il.com, willy@...radead.org, hughd@...gle.com,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, xu xin <xu.xin16@....com.cn>,
        Xiaokai Ran <ran.xiaokai@....com.cn>,
        Yang Yang <yang.yang29@....com.cn>
Subject: Re: [PATCH] ksm: count allocated ksm rmap_items for each process

On Mon, 22 Aug 2022 05:36:53 +0000 xu xin <cgel.zte@...il.com> wrote:

> KSM can save memory by merging identical pages, but also can consume
> additional memory, because it needs to generate 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.
> 
> The information about whether KSM save memory or consume memory in
> system-wide range can be determined by the comprehensive calculation
> of pages_sharing, pages_shared, pages_unshared and pages_volatile.
> A simple approximate calculation:
> 
> 	profit ≈ pages_sharing * sizeof(page) - (all_rmap_items) *
> 	         sizeof(rmap_item);
> 
> where all_rmap_items equals to the sum of pages_sharing, pages_shared,
> pages_unshared and pages_volatile.
> 
> But we cannot calculate this kind of ksm profit inner single-process wide
> because the information of ksm rmap_item's number of a process is lacked.
> For user applications, if this kind of information could be obtained,
> it helps upper users know how beneficial the ksm-policy (like madvise)
> they are using brings, and then optimize their app code. For example,
> one application madvise 1000 pages as MERGEABLE, while only a few pages
> are really merged, then it's not cost-efficient.
> 
> So we add a new interface /proc/<pid>/ksm_alloced_items for each
> process to indicate the total allocated ksm rmap_items of this process.

Please add documentation for this profcs item in the appropriate place
under Documentation/.  And please ensure that the documentation
provides readers with a decent amount of information about how to use
this information to improve their system's operation.