linux-kernel - Re: [PATCH 0/7] slub: Fastpath optimization (especially for RT) V1

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Message-ID: <20141211143518.02c781ee@redhat.com>
Date:	Thu, 11 Dec 2014 14:35:18 +0100
From:	Jesper Dangaard Brouer <brouer@...hat.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	akpm@...uxfoundation.org, rostedt@...dmis.org,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	linux-mm@...ck.org, penberg@...nel.org, iamjoonsoo.kim@....com,
	brouer@...hat.com
Subject: Re: [PATCH 0/7] slub: Fastpath optimization (especially for RT) V1

On Wed, 10 Dec 2014 10:30:17 -0600
Christoph Lameter <cl@...ux.com> wrote:

[...]
> 
> Slab Benchmarks on a kernel with CONFIG_PREEMPT show an improvement of
> 20%-50% of fastpath latency:
> 
> Before:
> 
> Single thread testing
[...]
> 2. Kmalloc: alloc/free test
[...]
> 10000 times kmalloc(256)/kfree -> 116 cycles
[...]
> 
> 
> After:
> 
> Single thread testing
[...]
> 2. Kmalloc: alloc/free test
[...]
> 10000 times kmalloc(256)/kfree -> 60 cycles
[...]

It looks like an impressive saving 116 -> 60 cycles.  I just don't see
the same kind of improvements with my similar tests[1][2].

My test[1] is just a fast-path loop over kmem_cache_alloc+free on
256bytes objects. (Results after explicitly inlining new func
is_pointer_to_page())

 baseline: 47 cycles(tsc) 19.032 ns
 patchset: 45 cycles(tsc) 18.135 ns

I do see the improvement, but it is not as high as I would have expected.

(CPU E5-2695)

[1] https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/time_bench_kmem_cache1.c
[2] https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/mm/qmempool_bench.c

-- 
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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