On Wed, Jun 29, 2016 at 01:55:55PM -0700, David Rientjes wrote: > On Wed, 29 Jun 2016, Vlastimil Babka wrote: > > > On 06/29/2016 03:39 AM, David Rientjes wrote: > > > It's possible that the freeing scanner can be consistently expensive if > > > memory is well compacted toward the end of the zone with few free pages > > > available in that area. > > > > > > If all zone memory is synchronously compacted, say with > > > /proc/sys/vm/compact_memory, and thp is faulted, it is possible to > > > iterate a massive amount of memory even with the per-zone cached free > > > position. > > > > > > For example, after compacting all memory and faulting thp for heap, it > > > was observed that compact_free_scanned increased as much as 892518911 4KB > > > pages while compact_stall only increased by 171. The freeing scanner > > > iterated ~20GB of memory for each compaction stall. > > > > > > To address this, if too much memory is spanned on the freeing scanner's > > > freelist when releasing back to the system, return the low pfn rather than > > > the high pfn. It's declared that the freeing scanner will become too > > > expensive if the high pfn is used, so use the low pfn instead. > > > > > > The amount of memory declared as too expensive to iterate is subjectively > > > chosen at COMPACT_CLUSTER_MAX << PAGE_SHIFT, which is 512MB with 4KB > > > pages. > > > > > > Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx> > > > > Hmm, I don't know. Seems it only works around one corner case of a larger > > issue. The cost for the scanning was already paid, the patch prevents it from > > being paid again, but only until the scanners are reset. > > > > The only point of the per-zone cached pfn positions is to avoid doing the > same work again unnecessarily. Having the last 16GB of memory at the end > of a zone being completely unfree is the same as a single page in the last > pageblock free. The number of PageBuddy pages in that amount of memory > can be irrelevant up to COMPACT_CLUSTER_MAX. We simply can't afford to > scan 16GB of memory looking for free pages. We need to find a root cause of this problem, first. I guess that this problem would happen when isolate_freepages_block() early stop due to watermark check (if your patch is applied to your kernel). If scanner meets, cached pfn will be reset and your patch doesn't have any effect. So, I guess that scanner doesn't meet. We enter the compaction with enough free memory so stop in isolate_freepages_block() should be unlikely event but your number shows that it happens frequently? Maybe, if we change all watermark check on compaction.c to use min_wmark, problem would be disappeared. Anyway, could you check how often isolate_freepages_block() is stopped and why? In addition, I worry that your previous patch that makes isolate_freepages_block() stop when watermark doesn't meet would cause compaction non-progress. Amount of free memory can be flutuated so watermark fail would be temporaral. We need to break compaction in this case? It would decrease compaction success rate if there is a memory hogger in parallel. Any idea? Thanks. > > > Note also that THP's no longer do direct compaction by default in recent > > kernels. > > > > To fully solve the freepage scanning issue, we should probably pick and finish > > one of the proposed reworks from Joonsoo or myself, or the approach that > > replaces free scanner with direct freelist allocations. > > > > Feel free to post the patches, but I believe this simple change makes > release_freepages() exceedingly better and can better target memory for > the freeing scanner. > > -- > 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> -- 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>