linux-kernel - Re: IRQ balancing on a router

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Message-ID: <48E638B5.9040204@linux.intel.com>
Date:	Fri, 03 Oct 2008 08:22:29 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Jan Kasprzak <kas@...muni.cz>
CC:	linux-kernel@...r.kernel.org
Subject: Re: IRQ balancing on a router

Jan Kasprzak wrote:
> Arjan van de Ven wrote:
> : Jan Kasprzak <kas@...muni.cz> wrote:
> : > The result is
> : > that the CPU which receives IRQs for the uplink interface
> : > is 100 % busy (softirq mostly), while the other one is 90% idle.
> : 
> : one of the hard cases for irqbalance is that irqbalance doesn't have a
> : way to find out the actual cpu time spend in the handlers. For
> : networking it makes an estimate just based on the number of packets
> : (which is better than nothing)... but that breaks down if you have an
> : non-symmetry in CPU costs per packet like you have.
> : 
> : The good news is that irqthreads at least have the potential to solve
> : this "lack of information"; if not, we could consider doing a form of
> : microaccounting for irq handlers....
> 
> 	I am not sure whether this would help. In my case, the most of the
> in-kernel CPU time is not spend in the irq handler per se, but in softirq
> (i.e. checking the packet against iptables rules).

there is some consideration of making softirqs that are raised run as part of the irq thread.
or at least thoughts in that direction.
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