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:
> 
>>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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