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

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Date:	Tue, 3 Feb 2015 12:47:56 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Dave Hansen <dave.hansen@...el.com>, linux-mm@...ck.org,
	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 Tue, Feb 03, 2015 at 09:47:18AM +0000, Mel Gorman wrote:
> On Mon, Feb 02, 2015 at 02:22:36PM -0800, Dave Hansen wrote:
> > 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:
> > 
> 
> It also claims that the kernel is free to ignore the advice.
> 
> > >      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.
> 
> True. I'd be surprised if any application depended on that 

IIUC, jemalloc depends on this[1].

[1] https://github.com/jemalloc/jemalloc/blob/dev/src/chunk_mmap.c#L117

-- 
 Kirill A. Shutemov
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