linux-kernel - Re: [PATCH v2 1/2] mm/slub: optimize alloc/free fastpath by removing preemption on/off

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Date:	Thu, 15 Jan 2015 21:28:26 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	rostedt@...dmis.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 1/2] mm/slub: optimize alloc/free fastpath by removing
 preemption on/off

On Thu, 15 Jan 2015, Andrew Morton wrote:

> > I saw roughly 5% win in a fast-path loop over kmem_cache_alloc/free
> > in CONFIG_PREEMPT. (14.821 ns -> 14.049 ns)
>
> I'm surprised.  preempt_disable/enable are pretty fast.  I wonder why
> this makes a measurable difference.  Perhaps preempt_enable()'s call to
> preempt_schedule() added pain?

The rest of the fastpath is already highly optimized. That is why
something like preempt enable/disable has such a disproportionate effect.

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