On Tue, Mar 09, 2010 at 10:11:18PM +1100, Nick Piggin wrote: > 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. There *should* be no difference and that is my expectation. If there is, it means I'm missing something important. Hence, the double check. > > > > > 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. > Agreed. > 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. > Indeed. Batching is less clear-cut in this context. We are already batching on a per-CPU basis but not on a per-process basis. My feeling is that the problem to watch out for with queueing in the allocation path is 2+ processes waiting on the queue and then allocating too much on the per-cpu lists. Easy enough to handle that one but there are probably a few more gotchas in there somewhere. Will revisit for sure though. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>