On 04/07/2017 02:38 AM, Joonsoo Kim wrote: > 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. Yes, but the downside is that compaction will create just the single high-order page that is requested, while previously it would also migrate away some more lower-order pages. When compacting for MIGRATE_UNMOVABLE allocation, we then can't steal extra pages, so next allocation might pollute a different pageblock. It's not a good tradeoff. >> >> 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. The low latency should not be at the expense of making long-term fragmentation worse. > 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>
