Re: [PATCH 3/5] page allocator: Wait on both sync and asynccongestion after direct reclaim

[Jens Axboe]

On Fri, Nov 13 2009, Mel Gorman wrote:
> On Fri, Nov 13, 2009 at 12:55:58PM +0100, Jens Axboe wrote:
> > On Fri, Nov 13 2009, KOSAKI Motohiro wrote:
> > > (cc to Jens)
> > > 
> > > > Testing by Frans Pop indicated that in the 2.6.30..2.6.31 window at least
> > > > that the commits 373c0a7e 8aa7e847 dramatically increased the number of
> > > > GFP_ATOMIC failures that were occuring within a wireless driver. Reverting
> > > > this patch seemed to help a lot even though it was pointed out that the
> > > > congestion changes were very far away from high-order atomic allocations.
> > > > 
> > > > The key to why the revert makes such a big difference is down to timing and
> > > > how long direct reclaimers wait versus kswapd. With the patch reverted,
> > > > the congestion_wait() is on the SYNC queue instead of the ASYNC. As a
> > > > significant part of the workload involved reads, it makes sense that the
> > > > SYNC list is what was truely congested and with the revert processes were
> > > > waiting on congestion as expected. Hence, direct reclaimers stalled
> > > > properly and kswapd was able to do its job with fewer stalls.
> > > > 
> > > > This patch aims to fix the congestion_wait() behaviour for SYNC and ASYNC
> > > > for direct reclaimers. Instead of making the congestion_wait() on the SYNC
> > > > queue which would only fix a particular type of workload, this patch adds a
> > > > third type of congestion_wait - BLK_RW_BOTH which first waits on the ASYNC
> > > > and then the SYNC queue if the timeout has not been reached.  In tests, this
> > > > counter-intuitively results in kswapd stalling less and freeing up pages
> > > > resulting in fewer allocation failures and fewer direct-reclaim-orientated
> > > > stalls.
> > > 
> > > Honestly, I don't like this patch. page allocator is not related to
> > > sync block queue. vmscan doesn't make read operation.
> > > This patch makes nearly same effect of s/congestion_wait/io_schedule_timeout/.
> > > 
> > > Please don't make mysterious heuristic code.
> > > 
> > > 
> > > Sidenode: I doubt this regression was caused from page allocator.
> 
> Probably not. As noted, the major change is really in how long callers
> are waiting on congestion_wait. The tarball includes graphs from an
> instrumented kernel that shows how long callers are waiting due to
> congestion_wait(). This has changed significantly.
> 
> I'll queue up tests over the weekend that test without dm-crypt being involved.
> 
> > > Probably we need to confirm caller change....
> > 
> > See the email from Chris from yesterday, he nicely explains why this
> > change made a difference with dm-crypt.
> 
> Indeed.
> 
> But bear in mind that it also possible that direct reclaimers are also
> congesting the queue due to swap-in.

Are you speculating, or has this been observed? While I don't contest
that that could happen, it's also not a new thing. And it should be an
unlikely event.

> > dm-crypt needs fixing, not a hack like this added.
> > 
> 
> As noted by Chris in the same mail, dm-crypt has not changed. What has
> changed is how long callers wait in congestion_wait.

Right dm-crypt didn't change, it WAS ALREADY BUGGY.

> > The vm needs to drop congestion hints and usage, not increase it. The
> > above changelog is mostly hand-wavy nonsense, imho.
> > 
> 
> Suggest an alternative that brings congestion_wait() more in line with
> 2.6.30 behaviour then.

I don't have a good explanation as to why the delays have changed,
unfortunately. Are we sure that they have between .30 and .31? The
dm-crypt case is overly complex and lots of changes could have broken
that house of cards.

-- 
Jens Axboe

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