linux-kernel - Re: [RFC PATCH] mm: madvise: Ignore repeated MADV

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Date:	Mon, 02 Feb 2015 14:22:36 -0800
From:	Dave Hansen <dave.hansen@...el.com>
To:	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org
CC:	Minchan Kim <minchan@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mm: madvise: Ignore repeated MADV_DONTNEED hints

On 02/02/2015 08:55 AM, Mel Gorman wrote:
> This patch identifies when a thread is frequently calling MADV_DONTNEED
> on the same region of memory and starts ignoring the hint. On an 8-core
> single-socket machine this was the impact on ebizzy using glibc 2.19.

The manpage, at least, claims that we zero-fill after MADV_DONTNEED is
called:

>      MADV_DONTNEED
>               Do  not  expect  access in the near future.  (For the time being, the application is finished with the given range, so the kernel can free resources
>               associated with it.)  Subsequent accesses of pages in this range will succeed, but will result either in reloading of the memory contents  from  the
>               underlying mapped file (see mmap(2)) or zero-fill-on-demand pages for mappings without an underlying file.

So if we have anything depending on the behavior that it's _always_
zero-filled after an MADV_DONTNEED, this will break it.
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