linux-kernel - Re: [PATCH 2/2] smp_call_function: use rwlocks on queues rather than rcu

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Date:	Sat, 23 Aug 2008 09:34:57 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Ingo Molnar <mingo@...e.hu>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] smp_call_function: use rwlocks on queues rather than rcu

On Fri, Aug 22, 2008 at 11:35:46AM -0700, Jeremy Fitzhardinge wrote:
> Andi Kleen wrote:
> > Right now my impression is that it is not well understood why
> > the kmalloc makes the IPI that much slower. In theory a kmalloc
> > shouldn't be all that slow, it's essentially just a 
> > "disable interrupts; unlink object from cpu cache; enable interrupts"
> > with some window dressing. kfree() is similar.
> >
> > Does it bounce a cache line on freeing perhaps?
> 
> I think it's just an assumption that it would be slower.  Has anyone
> measured it?

It's likely slower than no kmalloc because
there will be more instructions executed, the question is just how much.

> 
> (Note: The measurements I posted do not cover this path, because it was
> on a two cpu system, and it was always using the call-single path.)

Ah so it was already 25% slower even without kmalloc? I thought
that was with already. That doesn't sound good. Any idea where that slowdown 
comes from?

-Andi
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