On Thu, Mar 05, 2015 at 01:34:59PM +0800, Huang Ying wrote: > Hi, Mel, > > On Sat, 2015-02-28 at 15:30 +0800, Huang Ying wrote: > > On Sat, 2015-02-28 at 01:46 +0000, Mel Gorman wrote: > > > On Fri, Feb 27, 2015 at 03:21:36PM +0800, Huang Ying wrote: > > > > FYI, we noticed the below changes on > > > > > > > > git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master > > > > commit 3484b2de9499df23c4604a513b36f96326ae81ad ("mm: rearrange zone > > > > fields into read-only, page alloc, statistics and page reclaim lines") > > > > > > > > The perf cpu-cycles for spinlock (zone->lock) increased a lot. I > > > > suspect there are some cache ping-pong or false sharing. > > > > > > > > > > Are you sure about this result? I ran similar tests here and found that > > > there was a major regression introduced near there but it was commit > > > 05b843012335 ("mm: memcontrol: use root_mem_cgroup res_counter") that > > > cause the problem and it was later reverted. On local tests on a 4-node > > > machine, commit 3484b2de9499df23c4604a513b36f96326ae81ad was within 1% > > > of the previous commit and well within the noise. > > > > After applying the below debug patch, the performance regression > > restored. So I think we can root cause this regression to be cache line > > alignment related issue? > > > > If my understanding were correct, after the 3484b2de94, lock and low > > address area free_area are in the same cache line, so that the cache > > line of the lock and the low address area of free_area will be switched > > between MESI "E" and "S" state because it is written in one CPU (page > > allocating with free_area) and frequently read (spinning on lock) in > > another CPU. > > What do you think about this? >
My attention is occupied by the automatic NUMA regression at the moment but I haven't forgotten this. Even with the high client count, I was not able to reproduce this so it appears to depend on the number of CPUs available to stress the allocator enough to bypass the per-cpu allocator enough to contend heavily on the zone lock. I'm hoping to think of a better alternative than adding more padding and increasing the cache footprint of the allocator but so far I haven't thought of a good alternative. Moving the lock to the end of the freelists would probably address the problem but still increases the footprint for order-0 allocations by a cache line. -- Mel Gorman SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/