linux-kernel - Re: [PATCH] mm: show proportional swap share of the mapping

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Message-Id: <20150609143548.870150b59d78752680c172db@linux-foundation.org>
Date:	Tue, 9 Jun 2015 14:35:48 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Bongkyu Kim <bongkyu.kim@....com>
Subject: Re: [PATCH] mm: show proportional swap share of the mapping

On Tue,  9 Jun 2015 23:43:51 +0900 Minchan Kim <minchan@...nel.org> wrote:

> For system uses swap heavily and has lots of shared anonymous page,
> it's very trouble to find swap set size per process because currently
> smaps doesn't report proportional set size of swap.
> It ends up that sum of the number of swap for all processes is greater
> than swap device size.
> 
> This patch introduces SwapPss field on /proc/<pid>/smaps.
> 

We should be told quite a bit more about the value of this change,
please.  Use cases, what problems it solves, etc.  Enough to justify
adding new code to the kernel, enough to justify adding yet another
userspace interface which must be maintained for ever.

> --- a/Documentation/filesystems/proc.txt
> +++ b/Documentation/filesystems/proc.txt
>
> ...
>
> @@ -441,7 +442,7 @@ indicates the amount of memory currently marked as referenced or accessed.
>  a mapping associated with a file may contain anonymous pages: when MAP_PRIVATE
>  and a page is modified, the file page is replaced by a private anonymous copy.
>  "Swap" shows how much would-be-anonymous memory is also used, but out on
> -swap.
> +swap. "SwapPss" shows process' proportional swap share of this mapping.
>  
>  "VmFlags" field deserves a separate description. This member represents the kernel
>  flags associated with the particular virtual memory area in two letter encoded

Documentation/filesystems/proc.txt doesn't actually explain what
"proportional share" means.  A patient reader will hopefully find the
comment over PSS_SHIFT in fs/proc/task_mmu.c, but that isn't very
user-friendly.


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