Re: [PATCH 2/2] mm, memcg: Optionally disable memcg by default using Kconfig
From: Johannes Weiner
Date: Wed May 20 2015 - 12:24:39 EST
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.
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