Re: sync_file_range(SYNC_FILE_RANGE_WRITE) blocks?

[Hugh Dickins]

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 ...

Hugh
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