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Message-ID: <43EC1BFF.1080808@yahoo.com.au>
Date: Fri, 10 Feb 2006 15:52:15 +1100
From: Nick Piggin <nickpiggin@yahoo.com.au>
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To: Andrew Morton <akpm@osdl.org>
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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Andrew Morton wrote:
> Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> 
>>>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.
>>
>> Well it remains the same conceptual operation (asynchronously "schedule"
>> dirty pages for writeout). However it simply becomes more useful to start
>> the writeout immediately, given that's the (pretty explicit) hint that is
>> given to us.
> 
> 
> If you want to start the I/O now, fine, start the I/O now.
> 
> If you don't want to start I/O now, fine, don't start I/O now.
> 
> If msync() were to unconditionally start I/O, you don't get that option.
> 

Huh? Sure you do.

If you want to start the IO *now* without waiting on it, call msync(MS_ASYNC)
If you don't want to start the IO now, that's really easy, do nothing.
If you want to start the IO now and also wait for it to finish, call msync(MS_SYNC)

Presently, the first option is unavailable.

> It's pretty simple, isn't it?
> 

Yes.

-- 
SUSE Labs, Novell Inc.
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