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:

quoted

On Tue, Feb 03, 2015 at 09:19:15AM +0100, Vlastimil Babka wrote:

[...]

quoted

quoted

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/