Re: [PATCH 3/3] memcg: increase MEMCG_CHARGE_BATCH to 64

[Roman Gushchin <roman.gushchin@linux.dev>]

On Mon, Aug 22, 2022 at 09:34:59PM +0200, Michal Hocko wrote:
> On Mon 22-08-22 11:37:30, Roman Gushchin wrote:
> [...]
> > I wonder only if we want to make it configurable (Idk a sysctl or maybe
> > a config option) and close the topic.
> 
> I do not think this is a good idea. We have other examples where we have
> outsourced internal tunning to the userspace and it has mostly proven
> impractical and long term more problematic than useful (e.g.
> lowmem_reserve_ratio, percpu_pagelist_high_fraction, swappiness just to
> name some that come to my mind). I have seen more often these to be used
> incorrectly than useful.

A agree, not a strong opinion here. But I wonder if somebody will
complain on Shakeel's change because of the reduced accuracy.
I know some users are using memory cgroups to track the size of various
workloads (including relatively small) and 32->64 pages per cpu change
can be noticeable for them. But we can wait for an actual bug report :)

> 
> In this case, I guess we should consider either moving to per memcg
> charge batching and see whether the pcp overhead x memcg_count is worth
> that or some automagic tuning of the batch size depending on how
> effectively the batch is used. Certainly a lot of room for
> experimenting.

I'm not a big believer into the automagic tuning here because it's a fundamental
trade-off of accuracy vs performance and various users might make a different
choice depending on their needs, not on the cpu count or something else.

Per-memcg batching sounds interesting though. For example, we can likely
batch updates on leaf cgroups and have a single atomic update instead of
multiple most of the times. Or do you mean something different?

Thanks!