On Fri 02-12-16 15:03:46, Joonsoo Kim wrote: [...] > > o pcp accounting during free is now confined to free_pcppages_bulk as it's > > impossible for the caller to know exactly how many pages were freed. > > Due to the high-order caches, the number of pages drained for a request > > is no longer precise. > > > > o The high watermark for per-cpu pages is increased to reduce the probability > > that a single refill causes a drain on the next free. [...] > I guess that this patch would cause following problems. > > 1. If pcp->batch is too small, high order page will not be freed > easily and survive longer. Think about following situation. > > Batch count: 7 > MIGRATE_UNMOVABLE -> MIGRATE_MOVABLE -> MIGRATE_RECLAIMABLE -> order 1 > -> order 2... > > free count: 1 + 1 + 1 + 2 + 4 = 9 > so order 3 would not be freed. I guess the second paragraph above in the changelog tries to clarify that... > 2. And, It seems that this logic penalties high order pages. One free > to high order page means 1 << order pages free rather than just > one page free. This logic do round-robin to choose the target page so > amount of freed page will be different by the order. Yes this is indeed possible. The first paragraph above mentions this problem. > I think that it > makes some sense because high order page are less important to cache > in pcp than lower order but I'd like to know if it is intended or not. > If intended, it deserves the comment. > > 3. I guess that order-0 file/anon page alloc/free is dominent in many > workloads. If this case happen, it invalidates effect of high order > cache in pcp since cached high order pages would be also freed to the > buddy when burst order-0 free happens. Yes this is true and I was wondering the same but I believe this can be enahanced later on. E.g. we can check the order when crossing pcp->high mark and only the given order portion of the batch. I just wouldn't over optimize at this stage. -- Michal Hocko SUSE Labs -- 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>