netdev - Re: [RFC PATCH 0/3] Network stack, first user of SLAB/kmem

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Date:	Wed, 9 Sep 2015 14:59:19 +0200
From:	Jesper Dangaard Brouer <brouer@...hat.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Alexander Duyck <alexander.duyck@...il.com>,
	netdev@...r.kernel.org, akpm@...ux-foundation.org,
	linux-mm@...ck.org, aravinda@...ux.vnet.ibm.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	iamjoonsoo.kim@....com, brouer@...hat.com
Subject: Re: [RFC PATCH 0/3] Network stack, first user of SLAB/kmem_cache
 bulk free API.

On Tue, 8 Sep 2015 12:32:40 -0500 (CDT)
Christoph Lameter <cl@...ux.com> wrote:

> On Sat, 5 Sep 2015, Jesper Dangaard Brouer wrote:
> 
> > The double_cmpxchg without lock prefix still cost 9 cycles, which is
> > very fast but still a cost (add approx 19 cycles for a lock prefix).
> >
> > It is slower than local_irq_disable + local_irq_enable that only cost
> > 7 cycles, which the bulking call uses.  (That is the reason bulk calls
> > with 1 object can almost compete with fastpath).
> 
> Hmmm... Guess we need to come up with distinct version of kmalloc() for
> irq and non irq contexts to take advantage of that . Most at non irq
> context anyways.

I agree, it would be an easy win.  Do notice this will have the most
impact for the slAb allocator.

I estimate alloc + free cost would save:
 * slAb would save approx 60 cycles
 * slUb would save approx  4 cycles

We might consider keeping the slUb approach as it would be more
friendly for RT with less IRQ disabling.

-- 
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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