Date: Thu, 9 Feb 2006 20:13:33 -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:
>
>  I don't think anyone would use MS_ASYNC for anything other than
>  performance improvement, so it is not like we need super well
>  defined behaviour... the earlier it will start IO AFAIKS the better.

Well, no.  Consider a continuously-running application which modifies its
data store via MAP_SHARED+msync(MS_ASYNC).  If the msync() immediately
started I/O, the disk would be seeking all over the place all the time.  The
queue merging and timer-based unplugging would help here, but it won't be
as good as a big, infrequent ascending-file-offset pdflush pass.

Secondly, consider the behaviour of the above application if it is modifying
the same page relatively frequently (quite likely).  If MS_ASYNC starts I/O
immediately, that page will get written 10, 100 or 1000 times per second. 
If MS_ASYNC leaves it to pdflush, that page gets written once per 30
seconds, so we do far much less I/O.

We just don't know.  It's better to leave it up to the application designer
rather than lumping too many operations into the one syscall.
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