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Date: Fri, 22 Aug 2008 12:14:37 -0500
From: Christoph Lameter <cl@linux-foundation.org>
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To: paulmck@linux.vnet.ibm.com
CC: Pekka Enberg <penberg@cs.helsinki.fi>, Ingo Molnar <mingo@elte.hu>,
       Jeremy Fitzhardinge <jeremy@goop.org>,
       Nick Piggin <nickpiggin@yahoo.com.au>, Andi Kleen <andi@firstfloor.org>,
       "Pallipadi, Venkatesh" <venkatesh.pallipadi@intel.com>,
       Suresh Siddha <suresh.b.siddha@intel.com>,
       Jens Axboe <jens.axboe@oracle.com>,
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       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/2] smp_call_function: use rwlocks on queues rather	than
 rcu
References: <48AE0883.6050701@goop.org> <20080822062800.GQ14110@elte.hu> <84144f020808220006n25d684b1n9db306ddc4f58c4c@mail.gmail.com> <48AEC6B2.1080701@linux-foundation.org> <20080822151156.GA6744@linux.vnet.ibm.com>
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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.