linux-kernel - Re: irq balancing question

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Message-ID: <45DF012D.3080802@astral.ro>
Date:	Fri, 23 Feb 2007 16:58:53 +0200
From:	Gergely Imre <imre.gergely@...ral.ro>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	linux-kernel@...r.kernel.org, hancockr@...w.ca
Subject: Re: irq balancing question

Arjan van de Ven wrote:
> On Fri, 2007-02-23 at 13:32 +0200, Imre Gergely wrote:
>> hi
>>
>> i have a SMP with 2x duo-core Opteron processors, and i have an ethernet card
>> (eth0). i want to balance the interrupts generated by the card to all 4
>> processors. 
> 
> you have only one card? then balancing like you say will only really
> hurt you; neither linux nor the chipset will do this.

in fact i have two cards, and 4 CPUs, but i was interested in then answer
Robert gave, that only _some_ machines distribute interrupts in hardware.
software distribution is obviously not good. consider this scenario:

you have one machine with 4 cpus, and two ethernet cards with a lot of
traffic on them. if you bind every card to one cpu, two of them are not used,
so you really use only half the power. not let's say you have so much traffic
(with limiting enabled, htb or something), that the two CPUs are on 100% all
the time, but the other two are doing nothing.

now if you could balance that to all 4 cpus, you could use all the power AND
no cpu would be used 100%.

but i guess the machine has to do it in hardware, and that was the real
question. irqbalance has no use here, i think, only if the hardware supports it.

is there a way to find out (some easy way) for sure, if one particular
hardware supports this interrupt distribution?

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