On Tue, 26 Mar 2013 13:38:43 +0800 Shaohua Li <shli@xxxxxxxxxx> wrote: > swap cluster allocation is to get better request merge to improve performance. > But the cluster is shared globally, if multiple tasks are doing swap, this will > cause interleave disk access. While multiple tasks swap is quite common, for > example, each numa node has a kswapd thread doing swap or multiple > threads/processes do direct page reclaim. > > We makes the cluster allocation per-cpu here. The interleave disk access issue > goes away. All tasks will do sequential swap. Why per-cpu rather than, say, per-mm or per-task? > If one CPU can't get its per-cpu cluster, it will fallback to scan swap_map. Under what circumstances can a cpu "not get its per-cpu cluster"? A cpu can always "get" its per-cpu data, by definition (unless perhaps interrupts are involved). Perhaps this description needs some expanding upon. > The CPU can still continue swap. We don't need recycle free swap entries of > other CPUs. > > In my test (swap to a 2-disk raid0 partition), this improves around 10% > swapout throughput, and request size is increased significantly. > > How does this impact swap readahead is uncertain though. On one side, page > reclaim always isolates and swaps several adjancent pages, this will make page > reclaim write the pages sequentially and benefit readahead. On the other side, > several CPU write pages interleave means the pages don't live _sequentially_ > but relatively _near_. In the per-cpu allocation case, if adjancent pages are > written by different cpus, they will live relatively _far_. So how this > impacts swap readahead depends on how many pages page reclaim isolates and > swaps one time. If the number is big, this patch will benefit swap readahead. > Of course, this is about sequential access pattern. The patch has no impact for > random access pattern, because the new cluster allocation algorithm is just for > SSD. > > > ... > -- 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>