Re: [Bug #14141] order 2 page allocation failures in iwlagn

From: Frans Pop
Date: Thu Nov 05 2009 - 15:14:44 EST

On Monday 26 October 2009, Frans Pop wrote:
> On Tuesday 20 October 2009, Mel Gorman wrote:
> > I've attached a patch below that should allow us to cheat. When it's
> > applied, it outputs who called congestion_wait(), how long the timeout
> > was and how long it waited for. By comparing before and after sleep
> > times, we should be able to see which of the callers has significantly
> > changed and if it's something easily addressable.
> The results from this look fairly interesting (although I may be a bad
> judge as I don't really know what I'm looking at ;-).
> I've tested with two kernels:
> 1) 1 test run
> 2) + congestion_wait() reverts: 2 test runs

I've taken another look at the data from this debug patch, resulting in
these graphs:

I think the graph may show the reason for the congestion_wait() regression.
Horizontal axis shows time, vertical axis shows number of logged
congestion_wait calls per type.

The top chart is without the revert, the bottom one after the revert.

Note how before the revert the graph shows distinct steps: first you get
almost exclusively kwapd, followed by almost exclusively alloc_pages and
try_to_free. I suspect the periods where kswapd is almost horizontal
correspond to the freezes.

With the revert the lines for the different functions are almost straight
and everything happens much better interspersed.

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