On Wed, 22 Nov 2017 15:52:55 +0100 Vlastimil Babka <vbabka@xxxxxxx> wrote: > On 11/22/2017 03:33 PM, Johannes Weiner wrote: > > From: Vlastimil Babka <vbabka@xxxxxxx> > > > > The goal of direct compaction is to quickly make a high-order page available > > for the pending allocation. The free page scanner can add significant latency > > when searching for migration targets, although to succeed the compaction, the > > only important limit on the target free pages is that they must not come from > > the same order-aligned block as the migrated pages. > > > > This patch therefore makes direct async compaction allocate freepages directly > > from freelists. Pages that do come from the same block (which we cannot simply > > exclude from the freelist allocation) are put on separate list and released > > only after migration to allow them to merge. > > > > In addition to reduced stall, another advantage is that we split larger free > > pages for migration targets only when smaller pages are depleted, while the > > free scanner can split pages up to (order - 1) as it encouters them. However, > > this approach likely sacrifices some of the long-term anti-fragmentation > > features of a thorough compaction, so we limit the direct allocation approach > > to direct async compaction. > > > > For observational purposes, the patch introduces two new counters to > > /proc/vmstat. compact_free_direct_alloc counts how many pages were allocated > > directly without scanning, and compact_free_direct_miss counts the subset of > > these allocations that were from the wrong range and had to be held on the > > separate list. > > > > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> > > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> > > --- > > > > Hi. I'm resending this because we've been struggling with the cost of > > compaction in our fleet, and this patch helps substantially. > > > > On 128G+ machines, we have seen isolate_freepages_block() eat up 40% > > of the CPU cycles and scanning up to a billion PFNs per minute. Not in > > a spike, but continuously, to service higher-order allocations from > > the network stack, fork (non-vmap stacks), THP, etc. during regular > > operation. > > > > I've been running this patch on a handful of less-affected but still > > pretty bad machines for a week, and the results look pretty great: > > > > http://cmpxchg.org/compactdirectalloc/compactdirectalloc.png > > Thanks a lot, that's very encouraging! Yup. Should we proceed with this patch for now, or wait for something better to come along? -- 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>
