Re: [PATCH 2/3] page-allocator: Check zone pressure when batch of pages are freed

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

On Tue, Mar 09, 2010 at 09:23:45PM +1100, Nick Piggin wrote:
> On Tue, Mar 09, 2010 at 10:08:35AM +0000, Mel Gorman wrote:
> > On Tue, Mar 09, 2010 at 08:53:42PM +1100, Nick Piggin wrote:
> > > Cool, you found this doesn't hurt performance too much?
> > > 
> > 
> > Nothing outside the noise was measured. I didn't profile it to be
> > absolutly sure but I expect it's ok.
> 
> OK. Moving the waitqueue cacheline out of the fastpath footprint
> and doing the flag thing might be a good idea?
> 

Probably, I'll do it as a separate micro-optimisation patch so it's
clear what I'm doing.

> > > Can't you remove the check from the reclaim code now? (The check
> > > here should give a more timely wait anyway)
> > > 
> > 
> > I'll try and see what the timing and total IO figures look like.
> 
> Well reclaim goes through free_pages_bulk anyway, doesn't it? So
> I don't see why you would have to run any test.
>  

It should be fine but no harm in double checking. The tests I'm doing
are not great anyway. I'm somewhat depending on people familar with
IO-related performance testing to give this a whirl or tell me how they
typically benchmark low-memory situations.

> > > This is good because it should eliminate most all cases of extra
> > > waiting. I wonder if you've also thought of doing the check in the
> > > allocation path too as we were discussing? (this would give a better
> > > FIFO behaviour under memory pressure but I could easily agree it is not
> > > worth the cost)
> > > 
> > 
> > I *could* make the check but as I noted in the leader, there isn't
> > really a good test case that determines if these changes are "good" or
> > "bad". Removing congestion_wait() seems like an obvious win but other
> > modifications that alter how and when processes wait are less obvious.
> 
> Fair enough. But we could be sure it increases fairness, which is a
> good thing. So then we'd just have to check it against performance.
> 

Ordinarily, I'd agree but we've seen bug reports before from applications
that depended on unfairness for good performance. dbench figures depended
at one point in unfair behaviour (specifically being allowed to dirty the
whole system). volanomark was one that suffered when the scheduler became
more fair (think sched_yield was also a biggie). The new behaviour was
better and arguably the applications were doing the wrong thing but I'd
still like to treat "increase fairness in the page allocator" as a
separate patch as a result.

> Your patches seem like a good idea regardless of this issue, don't get
> me wrong.
> 

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

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