Re: [RFC PATCH 0/3] Avoid the use of congestion_wait under zone pressure

[Christian Ehrhardt]

Mel Gorman wrote:
> On Fri, Mar 12, 2010 at 09:37:55AM -0500, Andrew Morton wrote:
> > On Fri, 12 Mar 2010 13:15:05 +0100 Christian Ehrhardt <ehrhardt@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > > > It still feels a bit unnatural though that the page allocator waits on
> > > > congestion when what it really cares about is watermarks. Even if this
> > > > patch works for Christian, I think it still has merit so will kick it a
> > > > few more times.
> > > In whatever way I can look at it watermark_wait should be supperior to 
> > > congestion_wait. Because as Mel points out waiting for watermarks is 
> > > what is semantically correct there.
> > If a direct-reclaimer waits for some thresholds to be achieved then what
> > task is doing reclaim?
> >
> > Ultimately, kswapd. 
>
> Well, not quite. The direct reclaimer will still wake up after a timeout
> and try again regardless of whether watermarks have been met or not. The
> intention is to back after after direct reclaim has failed. Granted, the
> window during which a direct reclaim finishes and an allocation attempt
> occurs is unnecessarily large. This may be addressed by the patch that
> changes where cond_resched() is called.
>
> > This will introduce a hard dependency upon kswapd
> > activity.  This might introduce scalability problems.  And latency
> > problems if kswapd if off doodling with a slow device (say), or doing a
> > journal commit.  And perhaps deadlocks if kswapd tries to take a lock
> > which one of the waiting-for-watermark direct relcaimers holds.
>
> What lock could they be holding? Even if that is the case, the direct
> reclaimers do not wait indefinitily.
>
> > Generally, kswapd is an optional, best-effort latency optimisation
> > thing and we haven't designed for it to be a critical service. 
> > Probably stuff would break were we to do so.
>
> No disagreements there.
>
> > This is one of the reasons why we avoided creating such dependencies in
> > reclaim.  Instead, what we do when a reclaimer is encountering lots of
> > dirty or in-flight pages is
> >
> > 	msleep(100);
> >
> > then try again.  We're waiting for the disks, not kswapd.
> >
> > Only the hard-wired 100 is a bit silly, so we made the "100" variable,
> > inversely dependent upon the number of disks and their speed.  If you
> > have more and faster disks then you sleep for less time.
> >
> > And that's what congestion_wait() does, in a very simplistic fashion. 
> > It's a facility which direct-reclaimers use to ratelimit themselves in
> > inverse proportion to the speed with which the system can retire writes.
>
> The problem being hit is when a direct reclaimer goes to sleep waiting
> on congestion when in reality there were not lots of dirty or in-flight
> pages. It goes to sleep for the wrong reasons and doesn't get woken up
> again until the timeout expires.
>
> Bear in mind that even if congestion clears, it just means that dirty
> pages are now clean although I admit that the next direct reclaim it
> does is going to encounter clean pages and should succeed.
>
> Lets see how the other patch that changes when cond_reched() gets called
> gets on. If it also works out, then it's harder to justify this patch.
> If it doesn't work out then it'll need to be kicked another few times.

Unfortunately "page-allocator: Attempt page allocation immediately after 
direct reclaim" don't help. No improvement in the regression we had 
fixed with the watermark wait patch.

-> *kick*^^


--

Grüsse / regards, Christian Ehrhardt
IBM Linux Technology Center, System z Linux Performance
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