netdev - Re: [PATCH 7/7] slub: initial bulk free implementation

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Date:	Tue, 16 Jun 2015 11:04:45 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
cc:	Joonsoo Kim <js1304@...il.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-Netdev <netdev@...r.kernel.org>,
	Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [PATCH 7/7] slub: initial bulk free implementation

On Tue, 16 Jun 2015, Jesper Dangaard Brouer wrote:

> It is very important that everybody realizes that the save+restore
> variant is very expensive, this is key:
>
> CPU: i7-4790K CPU @ 4.00GHz
>  * local_irq_{disable,enable}:  7 cycles(tsc) - 1.821 ns
>  * local_irq_{save,restore}  : 37 cycles(tsc) - 9.443 ns
>
> Even if EVERY object need to call slowpath/__slab_free() it will be
> faster than calling the fallback.  Because I've demonstrated the call
> this_cpu_cmpxchg_double() costs 9 cycles.

But the cmpxchg also stores a value. You need to add the cost of the store
to the cycles.

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