Date: Thu, 9 Feb 2006 00:42:08 -0800
From: Andrew Morton <akpm@osdl.org>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: linux@horizon.com, linux-kernel@vger.kernel.org, sct@redhat.com
Subject: Re: msync() behaviour broken for MS_ASYNC, revert patch?
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Nick Piggin <nickpiggin@yahoo.com.au> wrote:
>
> Andrew Morton wrote:
> 
> > 
> > 2.4:
> > 
> > 	MS_ASYNC: dirty the pagecache pages, start I/O
> > 	MS_SYNC: dirty the pagecache pages, start I/O, wait on I/O
> > 
> > 2.6:
> > 
> > 	MS_ASYNC: dirty the pagecache pages
> > 	MS_SYNC: dirty the pagecache pages, start I/O, wait on I/O.
> > 
> > So you're saying that doing the I/O in that 25-100msec window allowed your
> > app to do more pipelining.
> > 
> > I think for most scenarios, what we have in 2.6 is better: it gives the app
> > more control over when the I/O should be started. 
> 
> How so?
> 

Well, for example you might want to msync a number of disjoint parts of the
mapping, then write them all out in one hit.

Or you may not actually _want_ to start the I/O now - you just want pdflush
to write things back in a reasonable time period, so you don't have unsynced
data floating about in memory for eight hours.  That's a quite reasonable
application of msync(MS_ASYNC).

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