On Wed, May 20, 2015 at 01:50:45PM +0100, Mel Gorman wrote: > memcg was reported years ago to have significant overhead when unused. It > has improved but it's still the case that users that have no knowledge of > memcg pay a small performance penalty. > > This patch adds a Kconfig that controls whether memcg is enabled by default > and a kernel parameter cgroup_enable= to enable it if desired. Anyone using > oldconfig will get the historical behaviour. It is not an option for most > distributions to simply disable MEMCG as there are users that require it > but they should also be knowledgable enough to use cgroup_enable=. > > This was evaluated using aim9, a page fault microbenchmark and ebizzy > but I'll focus on the page fault microbenchmark. It can be reproduced > using pft from mmtests (https://github.com/gormanm/mmtests). Edit > configs/config-global-dhp__pagealloc-performance and update MMTESTS to > only contain pft. This is the relevant part of the profile summary > > /usr/src/linux-4.0-chargefirst-v2r1/mm/memcontrol.c 3.7907 223277 > __mem_cgroup_count_vm_event 1.143% 67312 > mem_cgroup_page_lruvec 0.465% 27403 > mem_cgroup_commit_charge 0.381% 22452 > uncharge_list 0.332% 19543 > mem_cgroup_update_lru_size 0.284% 16704 > get_mem_cgroup_from_mm 0.271% 15952 > mem_cgroup_try_charge 0.237% 13982 > memcg_check_events 0.222% 13058 > mem_cgroup_charge_statistics.isra.22 0.185% 10920 > commit_charge 0.140% 8235 > try_charge 0.131% 7716 > > It's showing 3.79% overhead in memcontrol.c when no memcgs are in > use. Applying the patch and disabling memcg reduces this to 0.51% > > /usr/src/linux-4.0-disable-v2r1/mm/memcontrol.c 0.5100 29304 > mem_cgroup_page_lruvec 0.161% 9267 > mem_cgroup_update_lru_size 0.154% 8872 > mem_cgroup_try_charge 0.153% 8768 > mem_cgroup_commit_charge 0.042% 2397 > > pft faults > 4.0.0 4.0.0 > chargefirst disable > Hmean faults/cpu-1 1509075.7561 ( 0.00%) 1508934.4568 ( -0.01%) > Hmean faults/cpu-3 1339160.7113 ( 0.00%) 1379512.0698 ( 3.01%) > Hmean faults/cpu-5 874174.1255 ( 0.00%) 875741.7674 ( 0.18%) > Hmean faults/cpu-7 601370.9977 ( 0.00%) 599938.2026 ( -0.24%) > Hmean faults/cpu-8 510598.8214 ( 0.00%) 510663.5402 ( 0.01%) > Hmean faults/sec-1 1497935.5274 ( 0.00%) 1496585.7400 ( -0.09%) > Hmean faults/sec-3 3941920.1520 ( 0.00%) 4050811.9259 ( 2.76%) > Hmean faults/sec-5 3869385.7553 ( 0.00%) 3922299.6112 ( 1.37%) > Hmean faults/sec-7 3992181.4189 ( 0.00%) 3988511.0065 ( -0.09%) > Hmean faults/sec-8 3986452.2204 ( 0.00%) 3977706.7883 ( -0.22%) > > Low thread counts get a small boost but it's within noise as memcg overhead > does not dominate. It's not obvious at all at higher thread counts as other > factors cause more problems. The overall breakdown of CPU usage looks like > > 4.0.0 4.0.0 > chargefirst-v2r1disable-v2r1 > User 41.81 41.45 > System 407.64 405.50 > Elapsed 128.17 127.06 This is a worst case microbenchmark doing nothing but anonymous page faults (with THP disabled), and yet the performance difference is in the noise. I don't see why we should burden the user with making a decision that doesn't matter in theory, let alone in practice. We have CONFIG_MEMCG and cgroup_disable=memory, that should be plenty for users that obsess about fluctuation in the noise. There is no reason to complicate the world further for everybody else. -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
