linux-kernel - Re: MADV_DONTNEED semantics? Was: [RFC PATCH] mm: madvise: Ignore repeated MADV

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Message-ID: <54D0F664.3070606@gmail.com>
Date:	Tue, 03 Feb 2015 17:25:08 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Michal Hocko <mhocko@...e.cz>, Mel Gorman <mgorman@...e.de>
CC:	mtk.manpages@...il.com, minchan Kim <minchan@...nel.org>,
	Dave Hansen <dave.hansen@...el.com>, linux-mm@...ck.org,
	Minchan Kim <minchan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	linux-man@...r.kernel.org
Subject: Re: MADV_DONTNEED semantics? Was: [RFC PATCH] mm: madvise: Ignore
 repeated MADV_DONTNEED hints

On 02/03/2015 04:21 PM, Michal Hocko wrote:
> On Tue 03-02-15 11:16:00, Mel Gorman wrote:
>> On Tue, Feb 03, 2015 at 09:19:15AM +0100, Vlastimil Babka wrote:
> [...]
>>> And if we agree that there is indeed no guarantee, what's the actual semantic
>>> difference from MADV_FREE? I guess none? So there's only a possible perfomance
>>> difference?
>>>
>>
>> Timing. MADV_DONTNEED if it has an effect is immediate, is a heavier
>> operations and RSS is reduced. MADV_FREE only has an impact in the future
>> if there is memory pressure.
> 
> JFTR. the man page for MADV_FREE has been proposed already
> (https://lkml.org/lkml/2014/12/5/63 should be the last version AFAIR). I
> do not see it in the man-pages git tree but the patch was not in time
> for 3.19 so I guess it will only appear in 3.20.
> 

Yikes! That patch was buried in the bottom of a locked filing cabinet
in a disused lavatory. I unfortunately don't read every thread that comes
my way, especially if it doesn't look like a man-pages patch (i.e., falls
in the middle of an LKML thread that starts on another topic, and doesn't 
see linux-man@). I'll respond to that patch soon. (There are some problems
that mean I could not accept it, AFAICT.)

Thanks,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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