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Date:   Wed, 17 Jun 2020 15:31:10 +0100
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Roman Gushchin <guro@fb.com>, Shakeel Butt <shakeelb@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christoph Lameter <cl@linux.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Kernel Team <kernel-team@fb.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Jesper Dangaard Brouer <brouer@redhat.com>
Subject: Re: [PATCH v6 00/19] The new cgroup slab memory controller
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On Wed, Jun 17, 2020 at 01:24:21PM +0200, Vlastimil Babka wrote:
> > 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.
> 

Last time the list of mmtests configurations I used for a basic
comparison were

db-pgbench-timed-ro-small-ext4
db-pgbench-timed-ro-small-xfs
io-dbench4-async-ext4
io-dbench4-async-xfs
io-bonnie-dir-async-ext4
io-bonnie-dir-async-xfs
io-bonnie-file-async-ext4
io-bonnie-file-async-xfs
io-fsmark-xfsrepair-xfs
io-metadata-xfs
network-netperf-unbound
network-netperf-cross-node
network-netperf-cross-socket
network-sockperf-unbound
network-netperf-unix-unbound
network-netpipe
network-tbench
pagereclaim-shrinker-ext4
scheduler-unbound
scheduler-forkintensive
workload-kerndevel-xfs
workload-thpscale-madvhugepage-xfs
workload-thpscale-xfs

Some were more valid than others in terms of doing an evaluation. I
followed up later with a more comprehensive comparison but that was
overkill.

Each time I did a slab/slub comparison in the past, I had to reverify
the rate that kmem_cache_* functions were actually being called as the
pattern can change over time even for the same workload.  A comparison
gets more complicated when comparing cgroups as ideally there would be
workloads running in multiple group but that gets complex and I think
it's reasonable to just test the "basic" case without cgroups.

-- 
Mel Gorman
SUSE Labs