Re: [PATCH 7/8] mm: memcontrol: account "kmem" consumers in cgroup2 memory controller

Johannes Weiner
2015-12-10T15:16:39Z

On Thu, Dec 10, 2015 at 02:28:33PM +0100, Michal Hocko wrote:
> On Wed 09-12-15 14:30:38, Vladimir Davydov wrote:
> > From: Vladimir Davydov <vdavydov@virtuozzo.com>
> > Subject: [PATCH] mm: memcontrol: allow to disable kmem accounting for cgroup2
> >
> > Kmem accounting might incur overhead that some users can't put up with.
> > Besides, the implementation is still considered unstable. So let's
> > provide a way to disable it for those users who aren't happy with it.
>
> Yes there will be users who do not want to pay an additional overhead
> and still accoplish what they need.
> I haven't measured the overhead lately - especially after the opt-out ->
> opt-in change so it might be much lower than my previous ~5% for kbuild
> load.

I think switching from accounting *all* slab allocations to accounting
a list of, what, less than 20 select slabs, counts as a change
significant enough to entirely invalidate those measurements and never
bring up that number again in the context of kmem cost, don't you think?

There isn't that much that the kmem is doing, but for posterity I ran
kbuild test inside a cgroup2, with and without cgroup.memory=nokmem,
and these are the results:

default:
Performance counter stats for 'make -j16 -s clean bzImage' (3 runs):

715823.047005 task-clock (msec) # 3.794 CPUs utilized
252,538 context-switches # 0.353 K/sec
32,018 cpu-migrations # 0.045 K/sec
16,678,202 page-faults # 0.023 M/sec
1,783,804,914,980 cycles # 2.492 GHz
<not supported> stalled-cycles-frontend
<not supported> stalled-cycles-backend
1,346,424,021,728 instructions # 0.75 insns per cycle
298,744,956,474 branches # 417.363 M/sec
10,207,872,737 branch-misses # 3.42% of all branches

188.667608149 seconds time elapsed ( +- 0.66% )

cgroup.memory=nokmem
Performance counter stats for 'make -j16 -s clean bzImage' (3 runs):

729028.322760 task-clock (msec) # 3.805 CPUs utilized
258,775 context-switches # 0.356 K/sec
32,241 cpu-migrations # 0.044 K/sec
16,647,817 page-faults # 0.023 M/sec
1,816,827,061,194 cycles # 2.497 GHz
<not supported> stalled-cycles-frontend
<not supported> stalled-cycles-backend
1,345,446,962,095 instructions # 0.74 insns per cycle
298,461,034,326 branches # 410.277 M/sec
10,215,145,963 branch-misses # 3.42% of all branches

191.583957742 seconds time elapsed ( +- 0.57% )

I would say the difference is solidly in the noise.

I also profiled a silly find | xargs stat pipe to excercise the dentry
and inode accounting, and this was the highest kmem-specific entry:

0.27% 0.27% find [kernel.kallsyms] [k] __memcg_kmem_get_cache
|
---__memcg_kmem_get_cache
__kmalloc
ext4_htree_store_dirent
htree_dirblock_to_tree
ext4_htree_fill_tree
ext4_readdir
iterate_dir
sys_getdents
entry_SYSCALL_64_fastpath
__getdents64

So can we *please* lay this whole "unreasonable burden to legacy and
power users" line of argument to rest and get on with our work? And
then tackle scalability problems as they show up in real workloads?

Thanks.