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Message-ID: <43ECE1FE.1050704@yahoo.com.au>
Date: Sat, 11 Feb 2006 05:57:02 +1100
From: Nick Piggin <nickpiggin@yahoo.com.au>
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To: Linus Torvalds <torvalds@osdl.org>
CC: linux@horizon.com, akpm@osdl.org, linux-kernel@vger.kernel.org,
       sct@redhat.com
Subject: Re: msync() behaviour broken for MS_ASYNC, revert patch?
References: <20060210172929.27423.qmail@science.horizon.com> <Pine.LNX.4.64.0602100935350.19172@g5.osdl.org>
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Linus Torvalds wrote:
> 
> On Fri, 10 Feb 2006, linux@horizon.com wrote:
> 
>>No.  MS_ASYNC says "I need the data written now.".
> 
> 
> Says you.
> 
> I say (and I have a decade of Linux historical behaviour to back it up) 
> that is says "I'm done, start flushing this out asynchronously like all 
> the other data I have written".
> 
> And yes, there are performance implications. But your claim that "start IO 
> now" performs better is bogus. It _sometimes_ performs better, but 
> sometimes performs much worse.
> 
> Take an example. You have a 200MB dirty area in a 1GB machine. You do 
> MS_ASYNC. What do you want to happen?
> 

It quite obviously depends on the context in which one is using it,
which will depend on what one expects it to do (unless one is an idiot).

If linux@horizon.com's[1] database has dirtied 200MB of data and
knows it will not dirty it again and has several hundred ms of useful
work to do before it must call MS_SYNC, then...

> Do you want IO to be started on all of it?

... yes.

[1] Come on, linux, can you at least make up a name for me, or are
     you really called Linux? (in which case you'd better make up a
     new name anyway when arguing with Linus about Linux, for the
     sake of everyone's sanity)

-- 
SUSE Labs, Novell Inc.
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