Subject: Re: slab-nomerge (was Re: [git pull] device mapper changes for 4.3)

From: Jesper Dangaard Brouer <[email-masked]>
Date: Mon, 7 Sep 2015 23:17:15 +0200

On Mon, 7 Sep 2015 13:22:13 -0700
Linus Torvalds <[email-masked]> wrote:

> On Mon, Sep 7, 2015 at 2:30 AM, Jesper Dangaard Brouer
> <[email-masked]> wrote:
> >
> > The slub allocator have a faster "fastpath", if your workload is
> > fast-reusing within the same per-cpu page-slab, but once the workload
> > increases you hit the slowpath, and then slab catches up. Slub looks
> > great in micro-benchmarking.
> >
> > And with "slab_nomerge" I get even high performance:
> 
> I think those two are related.
> 
> Not merging means that effectively the percpu caches end up being
> bigger (simply because there are more of them), and so it captures
> more of the fastpath cases.

Yes, that was also my theory.  As manually tuning the percpu sizes gave
me almost the same boost.

> Obviously the percpu queue size is an easy tunable too, but there are
> real downsides to that too. 

The easy fix is to introduce a subsystem specific percpu cache that is
large enough for our use-case.  That seems to be a trend. I'm hoping to
come up with something smarter that every subsystem can benefit from. 
E.g some heuristic that can dynamic adjust SLUB according to the usage
pattern. I can imagine something as simple as a counter for every
slowpath call, that is only valid as long as the jiffies count matches
(reset to zero, and store new jiffies cnt).  (But I have not thought
this through...)

> I suspect your IP forwarding case isn't so
> different from some of the microbenchmarks, it just has more
> outstanding work..

Yes, I will admit that my testing is very close to micro benchmarking,
and it is specifically designed to pressure the system to its limits[1].
Especially the minimum frame size is evil and unrealistic, but the real
purpose is preparing the stack for increasing speeds like 100Gbit/s.

> And yes, the slow path (ie not hitting in the percpu cache) of SLUB
> could hopefully be optimizable too, although maybe the bulk patches
> are the way to go (and unrelated to this thread - at least part of
> your bulk patches actually got merged last Friday - they were part of
> Andrew's patch-bomb).

Cool. Yes, it is only part of the bulk patches. The real performance
boosters are not in yet (but I need to make them work correctly with
memory debugging enabled before they can get merged).  At least the
main API is in, which allows me to implement use-case easier in other
subsystems :-)

[1] http://netoptimizer.blogspot.dk/2014/09/packet-per-sec-measurements-for.html
-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Sr. Network Kernel Developer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer

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