linux-kernel - Re: [RFC v5 0/8] Support volatile for anonymous range

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Date:	Thu, 3 Jan 2013 09:19:08 -0800
From:	Sanjay Ghemawat <sanjay@...gle.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Arun Sharma <asharma@...com>, Paul Turner <pjt@...gle.com>,
	David Rientjes <rientjes@...gle.com>,
	John Stultz <john.stultz@...aro.org>,
	Christoph Lameter <cl@...ux.com>,
	Android Kernel Team <kernel-team@...roid.com>,
	Robert Love <rlove@...gle.com>, Mel Gorman <mel@....ul.ie>,
	Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Dave Chinner <david@...morbit.com>, Neil Brown <neilb@...e.de>,
	Mike Hommey <mh@...ndium.org>, Taras Glek <tglek@...illa.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [RFC v5 0/8] Support volatile for anonymous range

On Wed, Jan 2, 2013 at 8:27 PM, Minchan Kim <minchan@...nel.org> wrote:
> This is still RFC because we need more input from user-space
> people, more stress test, design discussion about interface/reclaim

Speaking as one of the authors of tcmalloc, I don't see any particular
need for this new system call for tcmalloc.  We are fine using
madvise(MADV_DONTNEED) and don't notice any significant
performance issues caused by it.  Background: we throttle how
quickly we release memory back to the system (1-10MB/s), so
we do not call madvise() very much, and we don't end up reusing
madvise-ed away pages at a fast rate. My guess is that we won't
see large enough application-level performance improvements to
cause us to change tcmalloc to use this system call.

> - What's different with madvise(DONTNEED)?
>
>   System call semantic
>
>   DONTNEED makes sure user always can see zero-fill pages after
>   he calls madvise while mvolatile can see old data or encounter
>   SIGBUS.

Do you need a new system call for this?  Why not just a new flag to madvise
with weaker guarantees than zero-filling?  All of the implementation changes
you point out below could be triggered from that flag.
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