Re: [PATCH] make crypto unplug fix V3

[Mel Gorman <mel@csn.ul.ie>]

On Fri, Nov 13, 2009 at 01:40:04PM -0500, Chris Mason wrote:
> On Fri, Nov 13, 2009 at 05:34:46PM +0000, Mel Gorman wrote:
> > On Fri, Nov 13, 2009 at 07:58:12AM -0500, Chris Mason wrote:
> > > This is still likely to set your dm data on fire.  It is only meant for
> > > testers that start with mkfs and don't have any valuable dm data.
> > > 
> > 
> > The good news is that my room remains fire-free. Despite swap also
> > running from dm-crypt, I had no corruption or instability issues.
> 
> Ok, definitely not so convincing I'd try and shove it into a late rc.
> 
> > 
> > Here is an updated set of results for fake-gitk running.
> > 
> > X86
> > 2.6.30-0000000-force-highorder           Elapsed:12:08.908    Failures:0
> > 2.6.31-0000000-force-highorder           Elapsed:10:56.283    Failures:0
> > 2.6.31-0000006-dm-crypt-unplug           Elapsed:11:51.653    Failures:0
> > 2.6.31-0000012-pgalloc-2.6.30            Elapsed:12:26.587    Failures:0
> > 2.6.31-0000123-congestion-both           Elapsed:10:55.298    Failures:0
> > 2.6.31-0001234-kswapd-quick-recheck      Elapsed:18:01.523    Failures:0
> > 2.6.31-0123456-dm-crypt-unplug           Elapsed:10:45.720    Failures:0
> > 2.6.31-revert-8aa7e847                   Elapsed:15:08.020    Failures:0
> > 2.6.32-rc6-0000000-force-highorder       Elapsed:16:20.765    Failures:4
> > 2.6.32-rc6-0000006-dm-crypt-unplug       Elapsed:13:42.920    Failures:0
> > 2.6.32-rc6-0000012-pgalloc-2.6.30        Elapsed:16:13.380    Failures:1
> > 2.6.32-rc6-0000123-congestion-both       Elapsed:18:39.118    Failures:0
> > 2.6.32-rc6-0001234-kswapd-quick-recheck  Elapsed:15:04.398    Failures:0
> > 2.6.32-rc6-0123456-dm-crypt-unplug       Elapsed:12:50.438    Failures:0
> > 2.6.32-rc6-revert-8aa7e847               Elapsed:20:50.888    Failures:0
> > 
> > X86-64
> > 2.6.30-0000000-force-highorder           Elapsed:10:37.300    Failures:0
> > 2.6.31-0000000-force-highorder           Elapsed:08:49.338    Failures:0
> > 2.6.31-0000006-dm-crypt-unplug           Elapsed:09:37.840    Failures:0
> > 2.6.31-0000012-pgalloc-2.6.30            Elapsed:15:49.690    Failures:0
> > 2.6.31-0000123-congestion-both           Elapsed:09:18.790    Failures:0
> > 2.6.31-0001234-kswapd-quick-recheck      Elapsed:08:39.268    Failures:0
> > 2.6.31-0123456-dm-crypt-unplug           Elapsed:08:20.965    Failures:0
> > 2.6.31-revert-8aa7e847                   Elapsed:08:07.457    Failures:0
> > 2.6.32-rc6-0000000-force-highorder       Elapsed:18:29.103    Failures:1
> > 2.6.32-rc6-0000006-dm-crypt-unplug       Elapsed:25:53.515    Failures:3
> > 2.6.32-rc6-0000012-pgalloc-2.6.30        Elapsed:19:55.570    Failures:6
> > 2.6.32-rc6-0000123-congestion-both       Elapsed:17:29.255    Failures:2
> > 2.6.32-rc6-0001234-kswapd-quick-recheck  Elapsed:14:41.068    Failures:0
> > 2.6.32-rc6-0123456-dm-crypt-unplug       Elapsed:15:48.028    Failures:1
> > 2.6.32-rc6-revert-8aa7e847               Elapsed:14:48.647    Failures:0
> > 
> > The numbering in the kernel indicates what patches are applied. I tested
> > the dm-crypt patch both in isolation and in combination with the patches
> > in this series.
> > 
> > Basically, the dm-crypt-unplug makes a small difference in performance
> > overall, mostly slight gains and losses. There was one massive regression
> > with the dm-crypt patch applied to 2.6.32-rc6 but at the moment, I don't
> > know what that is.
> 
> How consistent are your numbers between runs?  I was trying to match
> this up with your last email and things were pretty different.
> 

The figures from the first mail were based on kernels that were not
instrumented. It so happened that this run was based on an instrumented
kernel to get the congestion_wait figures so the results are different.

However, the results vary a lot. In some cases, it will spike just as
2.6.32-rc6-0000006-dm-crypt-unplug did. The reported figure is the
average of 4 fake-gitk runs. I don't have the standard deviation handy
but it's high.

> > 
> > In general, the patch reduces the amount of time direct reclaimers are
> > spending on congestion_wait.
> > 
> > > It includes my patch from last night, along with changes to force dm to
> > > unplug when its IO queues empty.
> > > 
> > > The problem goes like this:
> > > 
> > > Process: submit read bio
> > > dm: put bio onto work queue
> > > process: unplug
> > > dm: work queue finds bio, does a generic_make_request
> > > 
> > > The end result is that we miss the unplug completely.  dm-crypt needs to
> > > unplug for sync bios.  This patch also changes it to unplug whenever the
> > > queue is empty, which is far from ideal but better than missing the
> > > unplugs.
> > > 
> > > This doesn't completely fix io stalls I'm seeing with dm-crypt, but its
> > > my best guess.  If it works, I'll break it up and submit for real to
> > > the dm people.
> > > 
> > 
> > Out of curiousity, how are you measuring IO stalls? In the tests I'm doing,
> > the worker processes output their progress and it should be at a steady
> > rate. I considered a stall to be an excessive delay between updates which
> > is a pretty indirect measure.
> 
> I just setup a crypto disk and did dd if=/dev/zero of=/mnt/foo bs=1M
> 
> If you watch vmstat 1, there's supposed to be a constant steam of IO to
> the disk.  If a whole second goes by with zero IO, we're doing something
> wrong, I get a number of multi-second stalls where we are just waiting
> for IO to happen.
> 

Ok, cool.

> Most of the time I was able to catch a sysrq-w for it, someone was
> waiting on a read to finish.   It isn't completely clear to me if the
> unplugging is working properly.
> 

The machines I'm using are now tied up doing the same tests with dm-crypt
and then I need to rerun with dm-crypt with the changed patches. It'll be
early next week before the machines free up again for me to investigate your
patch more but initial results look promising at least.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab