On Thu, Jul 14, 2016 at 10:05:00AM +0100, Mel Gorman wrote: > On Thu, Jul 14, 2016 at 02:23:32PM +0900, Joonsoo Kim wrote: > > > > > > > > > And, I'd like to know why max() is used for classzone_idx rather than > > > > > > min()? I think that kswapd should balance the lowest zone requested. > > > > > > > > > > > > > > > > If there are two allocation requests -- one zone-constraned and the other > > > > > zone-unconstrained, it does not make sense to have kswapd skip the pages > > > > > usable for the zone-unconstrained and waste a load of CPU. You could > > > > > > > > I agree that, in this case, it's not good to skip the pages usable > > > > for the zone-unconstrained request. But, what I am concerned is that > > > > kswapd stop reclaim prematurely in the view of zone-constrained > > > > requestor. > > > > > > It doesn't stop reclaiming for the lower zones. It's reclaiming the LRU > > > for the whole node that may or may not have lower zone pages at the end > > > of the LRU. If it does, then the allocation request will be satisfied. > > > If it does not, then kswapd will think the node is balanced and get > > > rewoken to do a zone-constrained reclaim pass. > > > > If zone-constrained request could go direct reclaim pass, there would > > be no problem. But, please assume that request is zone-constrained > > without __GFP_DIRECT_RECLAIM which is common for some device driver > > implementation. > > Then it's likely GFP_ATOMIC and it'll wake kswapd on each failure. If > kswapd is containtly awake for highmem requests then we're reclaiming > everything anyway. Remember that if kswapd is reclaiming for higher zones, > it'll still cover the lower zones eventually. There is no guarantee that > skipping the highmem pages will satisfy the atomic allocations any faster > but consuming the CPU to skip the pages is a definite cost. Okay. > > Even worse, skipping highmem pages when a highmem pages are required may > ake lowmem pressure worse because those pages are freed faster and can > be consumed by zone-unconstrained requests. Okay. > > If this really is a problem in practice then we can consider having > allocation requests that are zone-constrained and !__GFP_DIRECT_RECLAIM > set a flag and use the min classzone for the wakeup. That flag remains > set until kswapd takes at least one pass using the lower classzone and > clears it. The classzone will not be adjusted higher until that flag is It would work. > cleared. I don't think we should do it without evidence that it's a real > problem because kswapd potentially uses useless CPU and the potential for > higher lowmem pressure. Hmmm... I think differently. Your patch changes current behaviour without any evidence. Code simplification cannot compensate potential stability issue. Before your patch, kswapd try to balance for minimum classzone so until dis-advantage of this approach is proved, it's better to keep original logic. Thanks. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>