Re: msync() behaviour broken for MS_ASYNC, revert patch?

[Nick Piggin]

Andrew Morton wrote:
> Nick Piggin <nickpiggin@xxxxxxxxxxxx> 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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