On Mon, Aug 07, 2017 at 01:40:36PM +0800, Huang, Ying wrote: > From: Huang Ying <ying.huang@xxxxxxxxx> > > The swap readahead is an important mechanism to reduce the swap in > latency. Although pure sequential memory access pattern isn't very > popular for anonymous memory, the space locality is still considered > valid. > > In the original swap readahead implementation, the consecutive blocks > in swap device are readahead based on the global space locality > estimation. But the consecutive blocks in swap device just reflect > the order of page reclaiming, don't necessarily reflect the access > pattern in virtual memory. And the different tasks in the system may > have different access patterns, which makes the global space locality > estimation incorrect. > > In this patch, when page fault occurs, the virtual pages near the > fault address will be readahead instead of the swap slots near the > fault swap slot in swap device. This avoid to readahead the unrelated > swap slots. At the same time, the swap readahead is changed to work > on per-VMA from globally. So that the different access patterns of > the different VMAs could be distinguished, and the different readahead > policy could be applied accordingly. The original core readahead > detection and scaling algorithm is reused, because it is an effect > algorithm to detect the space locality. Andrew, Every zram users like low-end android device has used 0 page-cluster to disable swap readahead because it has no seek cost and works as synchronous IO operation so if we do readahead multiple pages, swap falut latency would be (4K * readahead window size). IOW, readahead is meaningful only if it doesn't bother faulted page's latency. However, this patch introduces additional knob /sys/kernel/mm/swap/ vma_ra_max_order as well as page-cluster. It means existing users has used disabled swap readahead doesn't work until they should be aware of new knob and modification of their script/code to disable vma_ra_max_order as well as page-cluster. I say it's a *regression* and wanted to fix it but Huang's opinion is that it's not a functional regression so userspace should be fixed by themselves. Please look into detail of discussion in http://lkml.kernel.org/r/%3C1505183833-4739-4-git-send-email-minchan@xxxxxxxxxx%3E The discussion is never productive so it's time to follow maintainer's opinion. Could you share your opinion? 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>
