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

Mel Gorman
2009-11-13T12:28:28Z

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.

> 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.

> 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.

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