Re: [PATCH 3/3] vmscan: Force kswapd to take notice faster whenhigh-order watermarks are being hit

From: Mel Gorman
Date: Wed Nov 04 2009 - 10:49:04 EST

On Wed, Nov 04, 2009 at 03:05:55AM +0100, Frans Pop wrote:
> On Wednesday 04 November 2009, Mel Gorman wrote:
> > > If you'd like me to test with the congestion_wait() revert on top of
> > > this for comparison, please let me know.
> >
> > No, there is resistance to rolling back the congestion_wait() changes
> I've never promoted the revert as a solution. It just shows the cause of a
> regression.

Yeah, I still haven't managed to figure out what exactly is wrong in there
other than "something changed with timing" and writeback behaves differently. I
still don't know the why of it because I haven't digged into that area in
depth in the past and failed at reproducing this. "My desktop is fine" :/

> > from what I gather because they were introduced for sane reasons. The
> > consequence is just that the reliability of high-order atomics are
> > impacted because more processes are making forward progress where
> > previously they would have waited until kswapd had done work. Your
> > driver has already been fixed in this regard and maybe it's a case that
> > the other atomic users simply have to be fixed to "not do that".
> The problem is that although my driver has been fixed so that it no longer
> causes the SKB allocation errors, the also rather serious behavior change
> where due to swapping my 3rd gitk takes up to twice as long to load with
> desktop freezes of up 45 seconds or so is still there.
> Although that's somewhat separate from the issue that started this whole
> investigation, I still feel that should be sorted out as well.

You're right. That behaviour sucks.

> The congestion_wait() change, even if theoretically valid, introduced a
> very real regression IMO. Such long desktop freezes during swapping should
> be avoided; .30 and earlier simply behaved a whole lot better in the same
> situation.

Agreed. I'll start from scratch again trying to reproduce what you're seeing
locally. I'll try breaking my network card so that it's making high-order
atomics and see where I get. Machines that were previously tied up are now
free so I might have a better chance.

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