Re: long hangs when deleting large directories (3.0-rc3)

Dave Chinner
2011-06-29T23:53:31Z

On Wed, Jun 29, 2011 at 02:48:14PM +0200, Markus Trippelsdorf wrote:
> On 2011.06.29 at 22:10 +1000, Dave Chinner wrote:
> > On Wed, Jun 29, 2011 at 09:41:27AM +0200, Markus Trippelsdorf wrote:
> > > On 2011.06.29 at 17:24 +1000, Dave Chinner wrote:
> > > > On Wed, Jun 29, 2011 at 08:19:54AM +0200, Markus Trippelsdorf wrote:
> > > > > On 2011.06.29 at 14:31 +1000, Dave Chinner wrote:
> > > > > > On Wed, Jun 22, 2011 at 05:30:47PM +1000, Dave Chinner wrote:
> > > > > > > Jun 22 08:53:09 x4 kernel: XFS (sdb1): ail: ooo splice, tail 0x12000156e7, item 0x12000156e6
> > > > > > > Jun 22 08:53:09 x4 kernel: XFS (sdb1): ail: ooo splice, walked 15503 items
> > > > > > > .....
> > > > > > > Jun 22 08:53:12 x4 kernel: XFS (sdb1): ail: ooo splice, tail 0x12000156e7, item 0x12000156e6
> > > > > > > Jun 22 08:53:12 x4 kernel: XFS (sdb1): ail: ooo splice, walked 16945 items
> > > > > > >
> > > > > > > Interesting is the LSN of the tail - it's only one sector further on
> > > > > > > than the items being inserted. That's what I'd expect from a commit
> > > > > > > record write race between two checkpoints. I'll have a deeper look
> > > > > > > into whether this can be avoided later tonight and also whether I
> > > > > > > can easily implement a "last insert cursor" easily so subsequent
> > > > > > > inserts at the same LSN avoid the walk....
> > > > > >
> > > > > > Ok, so here's a patch that does just this. I should probably also do
> > > > > > a little bit of cleanup on the cursor code as well, but this avoids
> > > > > > the repeated walks of the AIL to find the insert position.
> > > > > >
> > > > > > Can you try it without the WQ changes you made, Marcus, and see if
> > > > > > the interactivity problems go away?
> > > > >
> > > > > Sorry to be the bringer of bad news, but this made things much worse:
....
> > > > > As you can see in the table above (resolution 1sec) the hang is now
> > > > > 5-6 seconds long, instead of the 1-3 seconds seen before.
> > > >
> > > > Interesting. I checked that the ordering was correct in each case
> > > > adn that it was behaving correctly here.
> > > >
> > > > Can you add the following patch and send me the dmesg output over a
> > > > hang? It will tell me where the cursor is being initialised and when
> > > > it is being dropped, so should indicate if a specific insert chain
> > > > is getting stuck or doing something stoopid.
> > >
> > > The kernel log is attached.
> > > rm -fr && sync starts at Jun 29 09:32:24.
> >
> > Add this patch on top of the first one I sent. If it doesn't fix the
> > problem, can you readd the debug patch and send the log again?
>
> This completely fixes the issue. As a bonus "rm -fr && sync" completes
> much quicker now.

Great to hear the hang has gone away.

I'm also seeing performance improvements on unlink workloads with
these two patches - quite significant, too. Cold cache parallel rm
-rf tests over tens of millions of inodes are finishing 15-20%
faster. Hot cache parallel rm -rf now go to being CPU bound on a
8p system with the unlink rate improving by about 50%....

As I always say, the hardest part of fixing a bug is getting a
reproducable test case to analyse and test. Thank's for providing
the test case and the testing, Markus!

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com