Re: [patch] mm: skip rebalance of hopeless zones

[Mel Gorman <mel@csn.ul.ie>]

On Wed, Dec 08, 2010 at 05:23:24PM -0800, Andrew Morton wrote:
> On Wed, 8 Dec 2010 16:36:21 -0800 Simon Kirby <sim@hostway.ca> wrote:
> 
> > On Wed, Dec 08, 2010 at 04:16:59PM +0100, Johannes Weiner wrote:
> > 
> > > Kswapd tries to rebalance zones persistently until their high
> > > watermarks are restored.
> > > 
> > > If the amount of unreclaimable pages in a zone makes this impossible
> > > for reclaim, though, kswapd will end up in a busy loop without a
> > > chance of reaching its goal.
> > > 
> > > This behaviour was observed on a virtual machine with a tiny
> > > Normal-zone that filled up with unreclaimable slab objects.
> > > 
> > > This patch makes kswapd skip rebalancing on such 'hopeless' zones and
> > > leaves them to direct reclaim.
> > 
> > Hi!
> > 
> > We are experiencing a similar issue, though with a 757 MB Normal zone,
> > where kswapd tries to rebalance Normal after an order-3 allocation while
> > page cache allocations (order-0) keep splitting it back up again.  It can
> > run the whole day like this (SSD storage) without sleeping.
> 
> People at google have told me they've seen the same thing.  A fork is
> taking 15 minutes when someone else is doing a dd, because the fork
> enters direct-reclaim trying for an order-one page.  It successfully
> frees some order-one pages but before it gets back to allocate one, dd
> has gone and stolen them, or split them apart.
> 

Is there a known test case for this or should I look at doing a
streaming-IO test with a basic workload constantly forking in the
background to measure the fork latency?

> This problem would have got worse when slub came along doing its stupid
> unnecessary high-order allocations.
> 
> Billions of years ago a direct-reclaimer had a one-deep cache in the
> task_struct into which it freed the page to prevent it from getting
> stolen.
> 
> Later, we took that out because pages were being freed into the
> per-cpu-pages magazine, which is effectively task-local anyway.  But
> per-cpu-pages are only for order-0 pages.  See slub stupidity, above.
> 
> I expect that this is happening so repeatably because the
> direct-reclaimer is dong a sleep somewhere after freeing the pages it
> needs - if it wasn't doing that then surely the window wouldn't be wide
> enough for it to happen so often.  But I didn't look.
> 
> Suitable fixes might be
> 
> a) don't go to sleep after the successful direct-reclaim.
> 

I submitted a patch for this a long time ago but at the time we didn't
have a test case that made a difference to it. Might be worth
revisiting. I can't find the related patch any more but it was fairly
trivial.

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

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