linux-kernel - Re: [PATCH v6 00/19] The new cgroup slab memory controller

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Date:   Wed, 17 Jun 2020 13:24:21 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Roman Gushchin <guro@...com>, Shakeel Butt <shakeelb@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Linux MM <linux-mm@...ck.org>,
        Kernel Team <kernel-team@...com>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [PATCH v6 00/19] The new cgroup slab memory controller

On 6/17/20 5:32 AM, Roman Gushchin wrote:
> On Tue, Jun 16, 2020 at 08:05:39PM -0700, Shakeel Butt wrote:
>> On Tue, Jun 16, 2020 at 7:41 PM Roman Gushchin <guro@...com> wrote:
>> >
>> > On Tue, Jun 16, 2020 at 06:46:56PM -0700, Shakeel Butt wrote:
>> > > On Mon, Jun 8, 2020 at 4:07 PM Roman Gushchin <guro@...com> wrote:
>> > > >
>> [...]
>> > >
>> > > Have you performed any [perf] testing on SLAB with this patchset?
>> >
>> > The accounting part is the same for SLAB and SLUB, so there should be no
>> > significant difference. I've checked that it compiles, boots and passes
>> > kselftests. And that memory savings are there.
>> >
>> 
>> What about performance? Also you mentioned that sharing kmem-cache
>> between accounted and non-accounted can have additional overhead. Any
>> difference between SLAB and SLUB for such a case?
> 
> Not really.
> 
> Sharing a single set of caches adds some overhead to root- and non-accounted
> allocations, which is something I've tried hard to avoid in my original version.
> But I have to admit, it allows to simplify and remove a lot of code, and here
> it's hard to argue with Johanness, who pushed on this design.
> 
> With performance testing it's not that easy, because it's not obvious what
> we wanna test. Obviously, per-object accounting is more expensive, and
> measuring something like 1000000 allocations and deallocations in a line from
> a single kmem_cache will show a regression. But in the real world the relative
> cost of allocations is usually low, and we can get some benefits from a smaller
> working set and from having shared kmem_cache objects cache hot.
> Not speaking about some extra memory and the fragmentation reduction.
> 
> We've done an extensive testing of the original version in Facebook production,
> and we haven't noticed any regressions so far. But I have to admit, we were
> using an original version with two sets of kmem_caches.
> 
> If you have any specific tests in mind, I can definitely run them. Or if you
> can help with the performance evaluation, I'll appreciate it a lot.

Jesper provided some pointers here [1], it would be really great if you could
run at least those microbenchmarks. With mmtests it's the major question of
which subset/profiles to run, maybe the referenced commits provide some hints,
or maybe Mel could suggest what he used to evaluate SLAB vs SLUB not so long ago.

[1] https://lore.kernel.org/linux-mm/20200527103545.4348ac10@carbon/

> Thanks!
>