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

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Date:   Tue, 7 Sep 2021 10:31:01 -0700
From:   Roman Gushchin <guro@...com>
To:     Tejun Heo <tj@...nel.org>
CC:     Jens Axboe <axboe@...nel.dk>,
        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 Tue, Sep 07, 2021 at 07:14:45AM -1000, 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?

I'm not sure. For the second case we need to pack allocations from different
tasks/cgroups into a small number of shared pages. It means the front cache
should be really small/non-existing. For the first case we likely need a
substantial cache. Maybe we can do something really smart with scattering
the cache over multiple pages, but I really doubt.