netdev - Re: [PATCH 3/3] memcg: increase MEMCG_CHARGE

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Message-ID: <YwQ54pvNwy0/5u3C@P9FQF9L96D>
Date:   Mon, 22 Aug 2022 19:22:26 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Muchun Song <songmuchun@...edance.com>,
        Michal Koutný <mkoutny@...e.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Feng Tang <feng.tang@...el.com>,
        Oliver Sang <oliver.sang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>, lkp@...ts.01.org,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] memcg: increase MEMCG_CHARGE_BATCH to 64

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!