netdev - Re: [RFC PATCH] slub: RFC: Improving SLUB performance with 38% on NO-PREEMPT

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Date:	Mon, 8 Jun 2015 11:58:42 +0200
From:	Jesper Dangaard Brouer <brouer@...hat.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Alexander Duyck <alexander.duyck@...il.com>,
	linux-mm@...ck.org, netdev@...r.kernel.org, brouer@...hat.com
Subject: Re: [RFC PATCH] slub: RFC: Improving SLUB performance with 38% on
 NO-PREEMPT

On Mon, 8 Jun 2015 04:39:38 -0500 (CDT)
Christoph Lameter <cl@...ux.com> wrote:

> On Mon, 8 Jun 2015, Jesper Dangaard Brouer wrote:
> 
> > My real question is if disabling local interrupts is enough to avoid this?
> 
> Yes the initial release of slub used interrupt disable in the fast paths.

Thanks for the confirmation.

For this code path we would need the save/restore variant, which is
more expensive than the local cmpxchg16b.   In case of bulking, we
should be able to use the less expensive local_irq_{disable,enable}.

Cost of local IRQ toggling (CPU E5-2695):
 *  local_irq_{disable,enable}:  7 cycles(tsc) -  2.861 ns
 *  local_irq_{save,restore}  : 37 cycles(tsc) - 14.846 ns

p.s. I'm back working on bulking API...

> > And, does local irq disabling also stop preemption?
> 
> Of course.

Thanks for confirming this.

-- 
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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