linux-kernel - Re: v2.6.21.4-rt11

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Message-ID: <Pine.LNX.4.64.0706180948250.4751@schroedinger.engr.sgi.com>
Date:	Mon, 18 Jun 2007 09:54:18 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Dinakar Guniguntala <dino@...ibm.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	suresh.b.siddha@...el.com, pwil3058@...pond.net.au,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: v2.6.21.4-rt11

On Mon, 18 Jun 2007, Srivatsa Vaddagiri wrote:

> This particular machine, elm3b6, is a 4-cpu, (gasp, yes!) 4-node box i.e 
> each CPU is a node by itself. If you don't have CONFIG_NUMA enabled,
> then we won't have cross-node (i.e cross-cpu) load balancing.
> Fortunately in your case you had CONFIG_NUMA enabled, but still were
> hitting the (gross) load imbalance.
> 
> The problem seems to be with idle_balance(). This particular routine,
> invoked by schedule() on a idle cpu, walks up sched-domain hierarchy and
> tries to balance in each domain that has SD_BALANCE_NEWIDLE flag set.
> The nodes-level domain (SD_NODE_INIT) however doesn't set this flag,
> which means idle cpu looks for (im)balance within its own node at most and

The nodes-level domain looks for internode balances between up to 16 
nodes. It is not restricted to a single node. The balancing on the
phys_domain level does balance within a node.
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