Re: sync_file_range(SYNC_FILE_RANGE_WRITE) blocks?

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 31 May 2008 19:44:49 +0100 (BST) Hugh Dickins <hugh@veritas.com> wrote:

> On Fri, 30 May 2008, Pavel Machek wrote:
> > > > sync_file_range(SYNC_FILE_RANGE_WRITE) blocks ... which makes problem
> > > > for s2disk: there we want to start writeout as early as possible
> > > > (system is going to shut down after write, and we need the data on
> > > > disk).
> > > > 
> > > > Unfortuantely, sync_file_range(SYNC_FILE_RANGE_WRITE) blocks, which
> > > > does not work for us. Is there non-blocking variant? "Start writeout
> > > > on this fd, but don't block me"?
> > > 
> > > I guess there are lots of reasons why it may block (get rescheduled)
> > > briefly, but why would that matter to you?  Are you saying that its
> > > whole design has got broken somehow, and now SYNC_FILE_RANGE_WRITE
> > > is behaving as if SYNC_FILE_RANGE_WAIT_AFTER had been supplied too?
> > 
> > It appears to me like it includes WAIT_AFTER, yes.
> > 
> > I was not sure what the expected behaviour was... lets say we have a
> > lot of dirty data (like 40MB) and system with enough free memory. Is
> > it reasonable to expect SYNC_FILE_RANGE_WRITE to return pretty much
> > immediately? (like in less than 10msec)? Because it seems to take more
> > like a second here...
> > 
> > (Underlying 'file' is actually /dev/sda1 -- aka my swap partition, but
> > that should not matter --right?)
> 
> Right (so long as you're not swapping to it at the same time!).
> And it seems to be behaving the same way on a regular file.
> 
> All I can say so far is that I find the same as you do:
> SYNC_FILE_RANGE_WRITE (after writing) takes a significant amount of time,
> more than half as long as when you add in SYNC_FILE_RANGE_WAIT_AFTER too.
> 
> Which make the sync_file_range call pretty pointless: your usage seems
> perfectly reasonable to me, but somehow we've broken its behaviour.
> I'll be investigating ...
> 

It will block on disk queue fullness - sysrq-W will tell.