linux-kernel - Re: [RFC] time: xtime

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Date:	Fri, 6 May 2011 18:53:59 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <andi@...stfloor.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	john stultz <johnstul@...ibm.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Anton Blanchard <anton@...ba.org>
Subject: Re: [RFC] time: xtime_lock is held too long

> Then precise measurements have to be done on the source of cache misses, the 
> total cost of the timer interrupt, etc.

What we did was profiling the remote node cache misses
using raw offcore events, and the xtime code was high up.
I cannot share exact numbers for various reasons, but it was
an improvement on a hard to improve workload.

> I.e. this should be analyzed and improved properly, not just sloppily slapping 
> a few prefetches here and there, which wont really *solve* anything ...

Well it made things faster. That's definitely something.

I agree the cache lines need to be consolidated in a proper way. 

But well placed prefetches by themselves can be a useful tool,
and placing them based on stable profile results is not "sloppy".

-Andi
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