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. 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 Both options mean we won't clean the unmovable/reclaimable pageblocks as aggressively, but perhaps the tradeoff won't be bad. What do you think? Johannes, would you be able/willing to test such modification? Thanks > And, there is a concern in implementaion side. With this change, there > is much possibilty that compaction scanner's met by ASYNC compaction. > It resets the scanner position and SYNC compaction would start the > scan at the beginning of the zone every time. It would make cached > position useless and inefficient. > > 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>
