linux-kernel - Re: NO_HZ migration of TCP ack timers

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Message-ID: <20100218095529.GA31681@kryten>
Date:	Thu, 18 Feb 2010 20:55:30 +1100
From:	Anton Blanchard <anton@...ba.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	arun@...ux.vnet.ibm.com, tglx@...utronix.de, davem@...emloft.net,
	linux-kernel@...r.kernel.org, arjan@...radead.org,
	venkatesh.pallipadi@...el.com
Subject: Re: NO_HZ migration of TCP ack timers


Hi Andi,

> If the nohz balancer CPU is otherwise idle, shouldn't it have enough
> cycles to handle acks for everyone? Is the problem the cache line
> transfer time?

Yeah, I think the timer spinlock on the nohz balancer cpu ends up being a
global lock for every other cpu trying to migrate their ack timers to it.

> Sounds like something that should be controlled by the cpufreq governour's
> idle predictor? Only migrate if predicted idle time is long enough.
> It's essentially the same problem as deciding how deeply idle to put
> a CPU. Heavy measures only pay off if the expected time is long enough.

Interesting idea, it seems like we do need a better understanding of
how idle a cpu is, not just that it is idle when mod_timer is called.

Anton
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