> On Tue, Jan 17, 2023 at 10:54 PM John Stultz <jstultz@xxxxxxxxxx> wrote: > > > > On Tue, Jan 17, 2023 at 12:31 AM Jaewon Kim <jaewon31.kim@xxxxxxxxxxx> wrote: > > > > Using order 4 pages would be helpful for many IOMMUs, but it could spend > > > > quite much time in page allocation perspective. > > > > > > > > The order 4 allocation with __GFP_RECLAIM may spend much time in > > > > reclaim and compation logic. __GFP_NORETRY also may affect. These cause > > > > unpredictable delay. > > > > > > > > To get reasonable allocation speed from dma-buf system heap, use > > > > HIGH_ORDER_GFP for order 4 to avoid reclaim. > > > > Thanks for sharing this! > > The case where the allocation gets stuck behind reclaim under pressure > > does sound undesirable, but I'd be a bit hesitant to tweak numbers > > that have been used for a long while (going back to ion) without a bit > > more data. > > > > It might be good to also better understand the tradeoff of potential > > on-going impact to performance from using low order pages when the > > buffer is used. Do you have any details like or tests that you could > > share to help ensure this won't impact other users? > > > > TJ: Do you have any additional thoughts on this? > > > I don't have any data on how often we hit reclaim for mid order > allocations. That would be interesting to know. However the 70th > percentile of system-wide buffer sizes while running the camera on my > phone is still only 1 page, so it looks like this change would affect > a subset of use-cases. > > Wouldn't this change make it less likely to get an order 4 allocation > (under memory pressure)? The commit message makes me think the goal of > the change is to get more of them. Hello John Stultz I've been waiting for your next reply. With my commit, we may gather less number of order 4 pages and fill the requested size with more number of order 0 pages. I think, howerver, stable allocation speed is quite important so that corresponding user space context can move on within a specific time. Not only compaction but reclaim also, I think, would be invoked more if the __GFP_RECLAIM is added on order 4. I expect the reclaim could be decreased if we move to order 0. Thank you Jaewon Kim > > Actually with the low order being 0, I don't think __GFP_COMP makes > sense in LOW_ORDER_GFP. But I guess that flag isn't harmful there. > > > thanks > > -john
