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Re: xfs_iomap_write_unwritten stuck in congestion_wait?

To: Peter Watkins <treestem@xxxxxxxxx>
Subject: Re: xfs_iomap_write_unwritten stuck in congestion_wait?
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Fri, 5 Apr 2013 07:25:50 +1100
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <CAH4wwdHZfs3DFBdiQbbni4yw0HE6LzYaa2MCxuf7=AQvCsuVsA@xxxxxxxxxxxxxx>
References: <CAH4wwdF+gQTnbUR89e2KCUUJfS_cT_09wzmS55vys5rfbStW7Q@xxxxxxxxxxxxxx> <20130404040041.GB12011@dastard> <CAH4wwdHZfs3DFBdiQbbni4yw0HE6LzYaa2MCxuf7=AQvCsuVsA@xxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Thu, Apr 04, 2013 at 11:50:15AM -0400, Peter Watkins wrote:
> On Thu, Apr 4, 2013 at 12:00 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> > On Wed, Apr 03, 2013 at 03:33:11PM -0400, Peter Watkins wrote:
> >> Hello,
> >>
> >> Wondering if anyone has a suggestion for when
> >> xfs_iomap_write_unwritten gets into congestion_wait.
> >
> > Do less IO?
> >
> >> In this case the system has almost half of normal zone pages in
> >> NR_WRITEBACK with pretty much everybody held up in either
> >> congestion_wait or balance_dirty_pages.
> >
> > Which is excessive - how are you getting to the point of having that
> > many pages under IO at once? Writeback depth is limited by the IO
> > elevator queue depths, so this shouldn't happen unless you've been
> > tweaking block device parameters (i.e. nr_requests/max_sectors_kb)...
> >
> >> Since there are some free pages, seems like we'd be better off just
> >> using a little more memory to finish this IO and in turn reduce pages
> >> under write-back and add to free memory, rather than holding up here.
> >> So maybe PF_MEMALLOC?
> >
> > Definitely not. Unwritten extent conversion can require hundreds of
> > kilobytes of memory to complete, so all this will do is trigger even
> > further exhaustion of memory reserves before we block on IO.
> >
> >> It also looks like this path allocates log vectors with KM_SLEEP but
> >> lv_buf's with KM_SLEEP|KM_NOFS. Why is that?
> >
> > The transaction commit is copying the changes made into separate
> > buffers to insert into the CIL for a later checkpoint to write to
> > disk. This is normal behaviour - we can sleep there, but we cannot
> > allow memory reclaim to recurse into the filesystem (for obvious
> > reasons).
> >
> > Cheers,
> >
> > Dave.
> > --
> > Dave Chinner
> > david@xxxxxxxxxxxxx
> 
> Thanks for the help.
> 
> There are other clues the VM system was rather quickly overwhelmed,
> i.e. it couldn't even get bdi flush threads started without sending
> threadd into congestion_wait.

Yeah, that's a sure sign that you'ev overloaded the system with
dirty pages.

> So indeed there is a big multi-threaded writer which starts all at
> once, and that can be smoothed out.
> 
> And nr_requests is dialed up from 128 to 1024. Is anyone really able
> to resist that temptation?

I haven't had to do this on a system to get decent write performance
for years. And in general, the deepest IO parallelism you can get
from SAS/SCSI/FC hardware devices is around 240 IOs, so going depper
than that doesn't buy you a whole lot except for queuing up lots of
IO and causing high IO latencie.

FWIW, on HW RAID the BBWC is where all the significant IO
aggregation and reordering takes place, not the IO elevator. The
BBWC has a much bigger window for reordering than the elevator, and
doesn't cause any nasty interactions with the VM by being large...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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