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

Jesper Dangaard Brouer
2020-06-19T08:33:48Z

On Thu, 18 Jun 2020 18:30:13 -0700
Roman Gushchin <guro@fb.com> wrote:

> On Thu, Jun 18, 2020 at 11:31:21AM +0200, Jesper Dangaard Brouer wrote:
> > On Thu, 18 Jun 2020 10:43:44 +0200
> > Jesper Dangaard Brouer <brouer@redhat.com> wrote:
> >
> > > On Wed, 17 Jun 2020 18:29:28 -0700
> > > Roman Gushchin <guro@fb.com> wrote:
> > >
> > > > On Wed, Jun 17, 2020 at 01:24:21PM +0200, Vlastimil Babka wrote:
> > > > > 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@fb.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@fb.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/
> > > >
> > > > Oh, Jesper, I'm really sorry, somehow I missed your mail.
> > > > Thank you, Vlastimil, for pointing at it.
> > > >
> > > > I've got some results (slab_bulk_test01), but honestly I fail to interpret them.
> > > >
> > > > I ran original vs patched with SLUB and SLAB, each test several times and picked
> > > > 3 which looked most consistently. But it still looks very noisy.
> > > >
> > > > I ran them on my desktop (8-core Ryzen 1700, 16 GB RAM, Fedora 32),
> > > > it's 5.8-rc1 + slab controller v6 vs 5.8-rc1 (default config from Fedora 32).
> > >
> > > What about running these tests on the server level hardware, that you
> > > intent to run this on?
> >
> > To give you an idea of the performance difference I ran the same test
> > on a Broadwell Intel(R) Xeon(R) CPU E5-1650 v4 @ 3.60GHz.
> >
> > The SLUB fastpath:
> > Type:kmem fastpath reuse Per elem: 60 cycles(tsc) 16.822 ns
> >
> >
> > > >
> > > > How should I interpret this data?
> > >
> > > First of all these SLUB+SLAB microbenchmarks use object size 256 bytes,
> > > because network stack alloc object of this size for SKBs/sk_buff (due
> > > to cache-align as used size is 224 bytes). Checked SLUB: Each slab use
> > > 2 pages (8192 bytes) and contain 32 object of size 256 (256*32=8192).
> > >
> > > The SLUB allocator have a per-CPU slab which speedup fast-reuse, in this
> > > case up-to 32 objects. For SLUB the "fastpath reuse" test this behaviour,
> > > and it serves as a baseline for optimal 1-object performance (where my bulk
> > > API tries to beat that, which is possible even for 1-object due to knowing
> > > bulk API cannot be used from IRQ context).
> > >
> > > SLUB fastpath: 3 measurements reporting cycles(tsc)
> > > - SLUB-patched : fastpath reuse: 184 - 177 - 176 cycles(tsc)
> > > - SLUB-original: fastpath reuse: 178 - 153 - 156 cycles(tsc)
> > >
> >
> > For your SLAB results:
> >
> > SLAB fastpath: 3 measurements reporting cycles(tsc)
> > - SLAB-patched : 161 - 160 - 163 cycles(tsc)
> > - SLAB-original: 174 - 170 - 159 cycles(tsc)
> >
> > I find it strange that SLAB is slightly better than SLUB (in many
> > measurements), because SLUB should have an advantage on this fast-path
> > quick reuse due to the per-CPU slabs. Maybe this is also related to
> > the CPU arch you are using?
> >
> >
> > > There are some stability concerns as you mention, but it seems pretty
> > > consistently that patched version is slower. If you compile with
> > > no-PREEMPT you can likely get more stable results (and remove a slight
> > > overhead for SLUB fastpath).
> > >
> > > The microbenchmark also measures the bulk-API, which is AFAIK only used
> > > by network stack (and io_uring). I guess you shouldn't focus too much
> > > on these bulk measurements. When bulk-API cross this objects per slab
> > > threshold, or is unlucky is it use two per-CPU slab, then the
> > > measurements can fluctuate a bit.
> > >
> > > Your numbers for SLUB bulk-API:
> > >
> > > SLUB-patched - bulk-API
> > > - SLUB-patched : bulk_quick_reuse objects=1 : 187 - 90 - 224 cycles(tsc)
> > > - SLUB-patched : bulk_quick_reuse objects=2 : 110 - 53 - 133 cycles(tsc)
> > > - SLUB-patched : bulk_quick_reuse objects=3 : 88 - 95 - 42 cycles(tsc)
> > > - SLUB-patched : bulk_quick_reuse objects=4 : 91 - 85 - 36 cycles(tsc)
> > > - SLUB-patched : bulk_quick_reuse objects=8 : 32 - 66 - 32 cycles(tsc)
> > >
> > > SLUB-original - bulk-API
> > > - SLUB-original: bulk_quick_reuse objects=1 : 87 - 87 - 142 cycles(tsc)
> > > - SLUB-original: bulk_quick_reuse objects=2 : 52 - 53 - 53 cycles(tsc)
> > > - SLUB-original: bulk_quick_reuse objects=3 : 42 - 42 - 91 cycles(tsc)
> > > - SLUB-original: bulk_quick_reuse objects=4 : 91 - 37 - 37 cycles(tsc)
> > > - SLUB-original: bulk_quick_reuse objects=8 : 31 - 79 - 76 cycles(tsc)
> >
> > Your numbers for SLAB bulk-API:
> >
> > SLAB-patched - bulk-API
> > - SLAB-patched : bulk_quick_reuse objects=1 : 67 - 67 - 140 cycles(tsc)
> > - SLAB-patched : bulk_quick_reuse objects=2 : 55 - 46 - 46 cycles(tsc)
> > - SLAB-patched : bulk_quick_reuse objects=3 : 93 - 94 - 39 cycles(tsc)
> > - SLAB-patched : bulk_quick_reuse objects=4 : 35 - 88 - 85 cycles(tsc)
> > - SLAB-patched : bulk_quick_reuse objects=8 : 30 - 30 - 30 cycles(tsc)
> >
> > SLAB-original- bulk-API
> > - SLAB-original: bulk_quick_reuse objects=1 : 143 - 136 - 67 cycles(tsc)
> > - SLAB-original: bulk_quick_reuse objects=2 : 45 - 46 - 46 cycles(tsc)
> > - SLAB-original: bulk_quick_reuse objects=3 : 38 - 39 - 39 cycles(tsc)
> > - SLAB-original: bulk_quick_reuse objects=4 : 35 - 87 - 87 cycles(tsc)
> > - SLAB-original: bulk_quick_reuse objects=8 : 29 - 66 - 30 cycles(tsc)
> >
> > In case of SLAB I expect the bulk-API to be slightly faster than SLUB,
> > as the SLUB bulk code is much more advanced.
>
> So again it looks like a patched version is only slightly worse if we're taking
> the smallest number in each series. Is it a correct assumption?

Yes, I guess that is a good way to look at these somewhat fluctuating numbers.

--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer