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

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Message-ID: <20080822182915.GG6744@linux.vnet.ibm.com>
Date:	Fri, 22 Aug 2008 11:29:15 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Pekka Enberg <penberg@...helsinki.fi>, Ingo Molnar <mingo@...e.hu>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Andi Kleen <andi@...stfloor.org>,
	"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 12:14:37PM -0500, Christoph Lameter wrote:
> Paul E. McKenney wrote:
> 
> >> RCU is problematic because it lets cachelines get cold. A hot cacheline that
> >> is used frequently read and written to by the same cpu is very good thing for
> >> performace.
> > 
> > So on your these large boxes, read-only cachelines are preferentially
> > ejected from the cache, so that one should write to per-CPU data
> > occasionally to keep it resident?  Or is the issue the long RCU grace
> > periods which allow the structure being freed to age out of all relevant
> > caches?  (My guess would be the second.)
> 
> The issue are the RCU grace period that are generally long enough to make the
> cacheline fall out of all caches.

Would it make sense to push the freed-by-RCU memory further up the
hierarchy, so that such memory is not mistaken for recently freed
hot-in-cache memory?

							Thanx, Paul
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