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From:   Shakeel Butt <shakeelb@google.com>
Date:   Tue, 7 Sep 2021 10:48:06 -0700
Message-ID: <CALvZod4=uY=5Zp_GCzjjMzCbs16856dR85SBWv-dpNc4kBrO+Q@mail.gmail.com>
Subject: Re: [memcg] 0f12156dff: will-it-scale.per_process_ops -33.6% regression
To:     Roman Gushchin <guro@fb.com>
Cc:     Tejun Heo <tj@kernel.org>, Jens Axboe <axboe@kernel.dk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        kernel test robot <oliver.sang@intel.com>,
        Vasily Averin <vvs@virtuozzo.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Andrei Vagin <avagin@gmail.com>,
        Borislav Petkov <bp@alien8.de>, Borislav Petkov <bp@suse.de>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Jeff Layton <jlayton@kernel.org>,
        Jiri Slaby <jirislaby@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        Michal Hocko <mhocko@kernel.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Serge Hallyn <serge@hallyn.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Yutian Yang <nglaive@gmail.com>,
        Zefan Li <lizefan.x@bytedance.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>, lkp@lists.01.org,
        kernel test robot <lkp@intel.com>,
        "Huang, Ying" <ying.huang@intel.com>,
        Feng Tang <feng.tang@intel.com>,
        Zhengjun Xing <zhengjun.xing@linux.intel.com>
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On Tue, Sep 7, 2021 at 10:31 AM Roman Gushchin <guro@fb.com> wrote:
>
> 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.

I think we need to prototype this to sensibly evaluate. Let me know if
you want to take a stab at this otherwise I can try.