Date:	Mon, 30 Aug 2010 15:19:29 +0200
From:	Johannes Weiner <hannes@...xchg.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] writeback: Record if the congestion was unnecessary

On Fri, Aug 27, 2010 at 10:24:16AM +0100, Mel Gorman wrote:
> On Fri, Aug 27, 2010 at 10:16:48AM +0200, Johannes Weiner wrote:
> > On Thu, Aug 26, 2010 at 09:31:30PM +0100, Mel Gorman wrote:
> > > On Thu, Aug 26, 2010 at 08:29:04PM +0200, Johannes Weiner wrote:
> > > > On Thu, Aug 26, 2010 at 04:14:15PM +0100, Mel Gorman wrote:
> > > > > If congestion_wait() is called when there is no congestion, the caller
> > > > > will wait for the full timeout. This can cause unreasonable and
> > > > > unnecessary stalls. There are a number of potential modifications that
> > > > > could be made to wake sleepers but this patch measures how serious the
> > > > > problem is. It keeps count of how many congested BDIs there are. If
> > > > > congestion_wait() is called with no BDIs congested, the tracepoint will
> > > > > record that the wait was unnecessary.
> > > > 
> > > > I am not convinced that unnecessary is the right word.  On a workload
> > > > without any IO (i.e. no congestion_wait() necessary, ever), I noticed
> > > > the VM regressing both in time and in reclaiming the right pages when
> > > > simply removing congestion_wait() from the direct reclaim paths (the
> > > > one in __alloc_pages_slowpath and the other one in
> > > > do_try_to_free_pages).
> > > > 
> > > > So just being stupid and waiting for the timeout in direct reclaim
> > > > while kswapd can make progress seemed to do a better job for that
> > > > load.
> > > > 
> > > > I can not exactly pinpoint the reason for that behaviour, it would be
> > > > nice if somebody had an idea.
> > > > 
> > > 
> > > There is a possibility that the behaviour in that case was due to flusher
> > > threads doing the writes rather than direct reclaim queueing pages for IO
> > > in an inefficient manner. So the stall is stupid but happens to work out
> > > well because flusher threads get the chance to do work.
> > 
> > The workload was accessing a large sparse-file through mmap, so there
> > wasn't much IO in the first place.
> > 
> 
> Then waiting on congestion was the totally wrong thing to do. We were
> effectively calling sleep(HZ/10) and magically this was helping in some
> undefined manner. Do you know *which* called of congestion_wait() was
> the most important to you?

Removing congestion_wait() in do_try_to_free_pages() definitely
worsens reclaim behaviour for this workload:

1. wallclock time of the testrun increases by 11%

2. the scanners do a worse job and go for the wrong zone:

-pgalloc_dma 79597
-pgalloc_dma32 134465902
+pgalloc_dma 297089
+pgalloc_dma32 134247237

-pgsteal_dma 77501
-pgsteal_dma32 133939446
+pgsteal_dma 294998
+pgsteal_dma32 133722312

-pgscan_kswapd_dma 145897
-pgscan_kswapd_dma32 266141381
+pgscan_kswapd_dma 287981
+pgscan_kswapd_dma32 186647637

-pgscan_direct_dma 9666
-pgscan_direct_dma32 1758655
+pgscan_direct_dma 302495
+pgscan_direct_dma32 80947179

-pageoutrun 1768531
-allocstall 614
+pageoutrun 1927451
+allocstall 8566

I attached the full vmstat contents below.  Also the test program,
which I ran in this case as: ./mapped-file-stream 1 $((512 << 30))

> > > > So personally I think it's a good idea to get an insight on the use of
> > > > congestion_wait() [patch 1] but I don't agree with changing its
> > > > behaviour just yet, or judging its usefulness solely on whether it
> > > > correctly waits for bdi congestion.
> > > > 
> > > 
> > > Unfortunately, I strongly suspect that some of the desktop stalls seen during
> > > IO (one of which involved no writes) were due to calling congestion_wait
> > > and waiting the full timeout where no writes are going on.
> > 
> > Oh, I am in full agreement here!  Removing those congestion_wait() as
> > described above showed a reduction in peak latency.  The dilemma is
> > only that it increased the overall walltime of the load.
> > 
> 
> Do you know why because leaving in random sleeps() hardly seems to be
> the right approach?

I am still trying to find out what's going wrong.

> > And the scanning behaviour deteriorated, as in having increased
> > scanning pressure on other zones than the unpatched kernel did.
> > 
> 
> Probably because it was scanning more but not finding what it needed.
> There is a condition other than congestion it is having trouble with. In
> some respects, I think if we change congestion_wait() as I propose,
> we may see a case where CPU usage is higher because it's now
> encountering the unspecified reclaim problem we have.

Exactly.

> > So I think very much that we need a fix.  congestion_wait() causes
> > stalls and relying on random sleeps for the current reclaim behaviour
> > can not be the solution, at all.
> > 
> > I just don't think we can remove it based on the argument that it
> > doesn't do what it is supposed to do, when it does other things right
> > that it is not supposed to do ;-)
> > 
> 
> We are not removing it, we are just stopping it going to sleep for
> stupid reasons. If we find that wall time is increasing as a result, we
> have a path to figuring out what the real underlying problem is instead
> of sweeping it under the rug.

Well, for that testcase it is in effect the same as a removal as
there's never congestion.

But again: I agree with your changes per-se, I just don't think they
should get merged as long as they knowingly catalyze a problem that
has yet to be identified.

View attachment "mapped-file-stream.c" of type "text/plain" (1885 bytes)

Download attachment "vmstat.a.2" of type "application/x-troff-man" (1709 bytes)

Download attachment "vmstat.b.2" of type "application/x-troff-man" (1719 bytes)

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