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

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Andrew Morton wrote:
Nick Piggin <[email protected]> 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? pdflush
does write them out in file offset order doesn't it?

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.

What the app does lose some control of is when IO actually should get started,
(MS_SYNC still allows it to control when IO *finishes*).

--
SUSE Labs, Novell Inc.
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