Messages in this thread | | | Date | Mon, 5 Dec 2016 12:10:14 +0900 | From | Joonsoo Kim <> | Subject | Re: [PATCH 2/2] mm: page_alloc: High-order per-cpu page allocator v5 |
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On Fri, Dec 02, 2016 at 09:21:08AM +0100, Michal Hocko wrote: > 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...
It doesn't perfectly clarify my concern. This is a different problem.
> > > 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.
Yes, it is mentioned simply but we cannot easily notice that the above penalty for high order page is there.
Thanks.
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