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Date: Thu, 9 Feb 2006 21:50:40 -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:
> > Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> > 
> >>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)
> > 
> > 
> > I've already explained the problems with the start-io-in-MS_ASYNC approach.
> > 
> 
> But I've explained that they only matter for people using it in stupid ways.
> fsync also poses a performance problem for programs that call it after every
> write(2).

There's absolutely nothing stupid about

	*p = <expr>
	msync(p, sizeof(*p), MS_ASYNC);

> > 
> >> Presently, the first option is unavailable.
> > 
> > 
> > We need to patch the kernel either way.  There's no point in going back to
> > either the known-problematic approach or to something half-assed.
> > 
> 
> The system call indicates to the kernel that IO submission should be started.
> The earlier the kernel does that, the better (because it is likely that an
> MS_SYNC is coming soon).
> 
> I think the current way of just moving the dirty bits is half-assed.
> 
> Is a more efficient implementation know-problematic?

It's less efficient for some things.  A lot.

> What applications did
> you observe problems with, can you remember?

Linus has some application which was doing the above.  It ran extremely
slowly, so we changed MS_ASYNC (ie: made it "more efficient"...)
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