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

[Nick Piggin <nickpiggin@yahoo.com.au>]

Andrew Morton wrote:
> Nick Piggin <nickpiggin@yahoo.com.au> wrote:

> > 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);

There really is if you're expecting a short time later to do

	*p = <expr2>

and had no need for a MS_SYNC anywhere in the meantime.
If you did have the need for MS_SYNC, then kicking off the IO
ASAP is going to be more efficient.

> > Is a more efficient implementation know-problematic?
>
>
> It's less efficient for some things.  A lot.

But only for stupid things, right?

> > 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"...)

Can he remember what it is? It sounds like it is broken.

OTOH, it could have been blocking on pages already under writeout
but a smarter implementation could ignore those (at the cost of
worse IO efficiency in these rare cases).

--
SUSE Labs, Novell Inc.
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