linux-kernel - Re: [PATCH RFC 00/14] The new slab memory controller

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Message-ID: <20190919214004.GA24631@castle.dhcp.thefacebook.com>
Date:   Thu, 19 Sep 2019 21:40:09 +0000
From:   Roman Gushchin <guro@...com>
To:     Suleiman Souhlal <suleiman@...gle.com>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "Shakeel Butt" <shakeelb@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        "Waiman Long" <longman@...hat.com>
Subject: Re: [PATCH RFC 00/14] The new slab memory controller

On Fri, Sep 20, 2019 at 06:10:11AM +0900, Suleiman Souhlal wrote:
> On Fri, Sep 20, 2019 at 1:22 AM Roman Gushchin <guro@...com> wrote:
> >
> > On Thu, Sep 19, 2019 at 10:39:18PM +0900, Suleiman Souhlal wrote:
> > > On Fri, Sep 6, 2019 at 6:57 AM Roman Gushchin <guro@...com> wrote:
> > > > The patchset has been tested on a number of different workloads in our
> > > > production. In all cases, it saved hefty amounts of memory:
> > > > 1) web frontend, 650-700 Mb, ~42% of slab memory
> > > > 2) database cache, 750-800 Mb, ~35% of slab memory
> > > > 3) dns server, 700 Mb, ~36% of slab memory
> > >
> > > Do these workloads cycle through a lot of different memcgs?
> >
> > Not really, those are just plain services managed by systemd.
> > They aren't restarted too often, maybe several times per day at most.
> >
> > Also, there is nothing fb-specific. You can take any new modern
> > distributive (I've tried Fedora 30), boot it up and look at the
> > amount of slab memory. Numbers are roughly the same.
> 
> Ah, ok.
> These numbers are kind of surprising to me.
> Do you know if the savings are similar if you use CONFIG_SLAB instead
> of CONFIG_SLUB?

I did only a brief testing of the SLAB version: savings were there, numbers were
slightly less impressive, but still in a double digit number of percents.

> 
> > > For workloads that don't, wouldn't this approach potentially use more
> > > memory? For example, a workload where everything is in one or two
> > > memcgs, and those memcgs last forever.
> > >
> >
> > Yes, it's true, if you have a very small and fixed number of memory cgroups,
> > in theory the new approach can take ~10% more memory.
> >
> > I don't think it's such a big problem though: it seems that the majority
> > of cgroup users have a lot of them, and they are dynamically created and
> > destroyed by systemd/kubernetes/whatever else.
> >
> > And if somebody has a very special setup with only 1-2 cgroups, arguably
> > kernel memory accounting isn't such a big thing for them, so it can be simple
> > disabled. Am I wrong and do you have a real-life example?
> 
> No, I don't have any specific examples.
> 
> -- Suleiman