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

[Nick Piggin]

On Tue, Mar 09, 2010 at 10:36:08AM +0000, Mel Gorman wrote:
> 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.

Fair enough.

 
> > > > 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.

I don't really like that logic. It makes things harder to understand
down the road if you have double checks.

 
> > > > 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.

Yeah sure it would be done as another patch. I don't think there is much
question that making things fairer is better. Especially if the
alternative is a theoretical starvation.

That's not to say that batching shouldn't then be used to help improve
performance of fairly scheduled resources. But it should be done in a
carefully designed and controlled way, so that neither the fairness /
starvation, nor the good performance from batching, depend on timing
and behaviours of the hardware interconnect etc.


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