linux-kernel - Re: [patch] sched: align rq to cacheline boundary

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Date:	Tue, 10 Apr 2007 09:40:54 -0700
From:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	akpm@...ux-foundation.org, nickpiggin@...oo.com.au,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] sched: align rq to cacheline boundary

On Tue, Apr 10, 2007 at 08:24:22AM +0200, Ingo Molnar wrote:
> 
> * Siddha, Suresh B <suresh.b.siddha@...el.com> wrote:
> 
> > Align the per cpu runqueue to the cacheline boundary. This will 
> > minimize the number of cachelines touched during remote wakeup.
> 
> > -static DEFINE_PER_CPU(struct rq, runqueues);
> > +static DEFINE_PER_CPU(struct rq, runqueues) ____cacheline_aligned_in_smp;
> 
> ouch!! Now how did _that_ slip through. The runqueues had been 
> cacheline-aligned for ages. Or at least, they were supposed to be.

perhaps the per_cpu definition gave the impression of cacheline aligned too..

> 
> could you see any improvement in profiles or workloads with this patch 
> applied? (just curious - it's an obviously right fix)

We have seen 0.5% perf improvement on database workload on a 2 node setup.
0.5% is a very good improvement for this workload.

thanks,
suresh
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