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

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Message-ID: <20150203105007.GP2395@suse.de>
Date:	Tue, 3 Feb 2015 10:50:07 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mm@...ck.org, Minchan Kim <minchan@...nel.org>,
	Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mm: madvise: Ignore repeated MADV_DONTNEED hints

On Mon, Feb 02, 2015 at 02:35:41PM -0800, Andrew Morton wrote:
> On Mon, 2 Feb 2015 22:18:24 +0000 Mel Gorman <mgorman@...e.de> wrote:
> 
> > > Is there something
> > > preventing this from being addressed within glibc?
> >  
> > I doubt it other than I expect they'll punt it back and blame either the
> > application for being stupid or the kernel for being slow.
> 
> *Is* the application being stupid?  What is it actually doing? 

Only a little. There is little simulated think time between the allocation
and the subsequent free. It means the cost of alloc/free dominates where
in "real" applications they would either be reusing buffers if they were
constantly needed or the think time would mask the cost of the free.

> Something like
> 
> pthread_routine()
> {
> 	p = malloc(X);
> 	do_some(work);
> 	free(p);
> 	return;
> }
> 

Pretty much. There is a search_mem() function that

alloc(copy_size)
memcpy
search
free(copy)

A real application might try and avoid the copy or reuse buffers if they
encountered this particular problem.

-- 
Mel Gorman
SUSE Labs
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