linux-kernel - Re: [memcg] 0f12156dff: will-it-scale.per_process

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Message-ID: <22a0156e-f74f-51c8-b7fd-9b5a375d7c81@kernel.dk>
Date:   Tue, 7 Sep 2021 11:18:21 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Tejun Heo <tj@...nel.org>, Roman Gushchin <guro@...com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        kernel test robot <oliver.sang@...el.com>,
        Vasily Averin <vvs@...tuozzo.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrei Vagin <avagin@...il.com>,
        Borislav Petkov <bp@...en8.de>, Borislav Petkov <bp@...e.de>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Jeff Layton <jlayton@...nel.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Michal Hocko <mhocko@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Serge Hallyn <serge@...lyn.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Yutian Yang <nglaive@...il.com>,
        Zefan Li <lizefan.x@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        kernel test robot <lkp@...el.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        Feng Tang <feng.tang@...el.com>,
        Zhengjun Xing <zhengjun.xing@...ux.intel.com>
Subject: Re: [memcg] 0f12156dff: will-it-scale.per_process_ops -33.6%
 regression

On 9/7/21 11:14 AM, Tejun Heo wrote:
> Hello,
> 
> On Tue, Sep 07, 2021 at 10:11:21AM -0700, Roman Gushchin wrote:
>> There are two polar cases:
>> 1) a big number of relatively short-living allocations, which lifetime is well
>>    bounded (e.g. by a lifetime of a task),
>> 2) a relatively small number of long-living allocations, which lifetime
>>    is potentially indefinite (e.g. struct mount).
>>
>> We can't use the same approach for both cases, otherwise we'll run into either
>> performance or garbage collection problems (which also lead to performance
>> problems, but delayed).
> 
> Wouldn't a front cache which expires after some seconds catch both cases?

A purely time based approach might be problematic, as you can allocate a
LOT of data in a short amount of time. Heuristics probably need to be a
hybrid of "time much time has passed" OR "we're over the front cache
threshold in terms of deferred accounting". But yes, I don't see why
we'd necessarily need different approaches for short vs long life times.

-- 
Jens Axboe