linux-kernel - Re: [RFC PATCH 1/1] kernel/rcu/tree.c: simplify force_quiescent

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Date:	Tue, 17 Jun 2014 16:06:43 -0400
From:	Waiman Long <waiman.long@...com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Romanov Arya <romanov.arya@...il.com>,
	Pranith Kumar <bobby.prani@...il.com>,
	Josh Triplett <josh@...htriplett.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	torvalds@...ux-foundation.org
Subject: Re: [RFC PATCH 1/1] kernel/rcu/tree.c: simplify force_quiescent_state()

On 06/17/2014 01:37 PM, Paul E. McKenney wrote:
>
> Oh, and to answer the implicit question...  A properly configured 4096-CPU
> system will have two funnel levels, with 64 nodes at the leaf level
> and a single node at the root level.  If the system is not properly
> configured, it will have three funnel levels.  The maximum number of
> funnel levels is four, which would handle more than four million CPUs
> (sixteen million if properly configured), so we should be good.  ;-)
>
> The larger numbers of levels are intended strictly for testing.  I set
> CONFIG_RCU_FANOUT_LEAF=2 and CONFIG_RCU_FANOUT=2 on a 16-CPU system just
> to make sure that I am testing something uglier than what will be running
> in production.  A large system should have both of these set to 64,
> though this requires also booting with skew_tick=1 as well.
>
> 							Thanx, Paul

Thank for the clarification as I haven't looked deep into the code to 
see how many levels there are. I totally understand the impact cacheline 
contention can have on system performance. After all, this is what many 
of my patches are trying to address.

-Longman
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