On Wed, Mar 29, 2017 at 06:06:41PM +0200, Vlastimil Babka wrote: > On 03/16/2017 03:14 AM, Joonsoo Kim wrote: > > On Tue, Mar 07, 2017 at 02:15:44PM +0100, Vlastimil Babka wrote: > >> The migrate scanner in async compaction is currently limited to MIGRATE_MOVABLE > >> pageblocks. This is a heuristic intended to reduce latency, based on the > >> assumption that non-MOVABLE pageblocks are unlikely to contain movable pages. > >> > >> However, with the exception of THP's, most high-order allocations are not > >> movable. Should the async compaction succeed, this increases the chance that > >> the non-MOVABLE allocations will fallback to a MOVABLE pageblock, making the > >> long-term fragmentation worse. > > > > I agree with this idea but have some concerns on this change. > > > > *ASYNC* compaction is designed for reducing latency and this change > > doesn't fit it. If everything works fine, there is a few movable pages > > in non-MOVABLE pageblocks as you noted above. Moreover, there is quite > > less the number of non-MOVABLE pageblock than MOVABLE one so finding > > non-MOVABLE pageblock takes long time. These two factors will increase > > the latency of *ASYNC* compaction. > > Right. I lately started to doubt the whole idea of async compaction (for > non-movable allocations). Seems it's one of the compaction heuristics tuned > towards the THP usecase. But for non-movable allocations, we just can't have > both the low latency and long-term fragmentation avoidance. I see now even my > own skip_on_failure mode in isolate_migratepages_block() as a mistake for > non-movable allocations. Why do you think that skip_on_failure mode is a mistake? I think that it would lead to reduce the latency and it fits the goal of async compaction. > > Ideally I'd like to make async compaction redundant by kcompactd, and direct > compaction would mean a serious situation which should warrant sync compaction. > Meanwhile I see several options to modify this patch > - async compaction for non-movable allocations will stop doing the > skip_on_failure mode, and won't restrict the pageblock at all. patch 8/8 will > make sure that also this kind of compaction finishes the whole pageblock > - non-movable allocations will skip async compaction completely and go for sync > compaction immediately IMO, concept of async compaction is also important for non-movable allocation. Non-movable allocation is essential for some workload and they hope the low latency. 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>
