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

[Andrew Morton <akpm@osdl.org>]

Nick Piggin <nickpiggin@yahoo.com.au> wrote:
>
> Andrew Morton wrote:
> > Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> > 
> >>Andrew Morton wrote:
> >>
> >>
> >>>2.4:
> >>>
> >>>	MS_ASYNC: dirty the pagecache pages, start I/O
> >>>	MS_SYNC: dirty the pagecache pages, start I/O, wait on I/O
> >>>
> >>>2.6:
> >>>
> >>>	MS_ASYNC: dirty the pagecache pages
> >>>	MS_SYNC: dirty the pagecache pages, start I/O, wait on I/O.
> >>>
> >>>So you're saying that doing the I/O in that 25-100msec window allowed your
> >>>app to do more pipelining.
> >>>
> >>>I think for most scenarios, what we have in 2.6 is better: it gives the app
> >>>more control over when the I/O should be started. 
> >>
> >>How so?
> >>
> > 
> > 
> > Well, for example you might want to msync a number of disjoint parts of the
> > mapping, then write them all out in one hit.
> > 
> 
> That should still be pretty efficient with 2.4 like behaviour?

It's a bit of a disaster if you happen to msync(MS_ASYNC) the same page at
any sort of frequency - we have to wait for the previous I/O to complete
before new I/O can be started.  That was the main problem which caused this
change to be made.  You can see that it'd make 100x or 1000x speed improvements
with some sane access patterns.

> pdflush
> does write them out in file offset order doesn't it?

pdflush does, but an msync(MS_ASYNC) which starts I/O puts the IO order
into the application's control.

> > Or you may not actually _want_ to start the I/O now - you just want pdflush
> > to write things back in a reasonable time period, so you don't have unsynced
> > data floating about in memory for eight hours.  That's a quite reasonable
> > application of msync(MS_ASYNC).
> > 
> 
> I think data integrity requirements should be handled by MS_SYNC.

Well that's always been the case.  MS_ASYNC doesn't write metadata.