linux-kernel - Re: irq load balancing

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Date:	Wed, 12 Sep 2007 17:44:58 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Venkat Subbiah <venkats@...nite.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: irq load balancing

Venkat Subbiah wrote:
> Most of the load in my system is triggered by a single ethernet IRQ.
> Essentially the IRQ schedules a tasklet and most of the work is done in the
> taskelet which is scheduled in the IRQ. From what I read looks like the
> tasklet would be executed on the same CPU on which it was scheduled. So this
> means even in an SMP system it will be one processor which is overloaded.
> 
> So will using the user space IRQ loadbalancer really help?

A little bit.  It'll keep other IRQs on different CPUs, which will prevent other 
interrupts from causing cache and TLB evictions that could slow down the 
interrupt handler for the NIC.

> What I am doubtful
> about is that the user space load balance comes along and changes the
> affinity once in a while. But really what I need is every interrupt to go to
> a different CPU in a round robin fashion.

Doing it in a round-robin fashion will be disastrous for performance.  Your 
cache miss rate will go through the roof and you'll hit the slow paths in the 
network stack most of the time.

> Looks like the APIC  can distribute IRQ's dynamically? Is this supported in
> the kernel and any config or proc interface to turn this on/off.

/proc/irq/$FOO/smp_affinity is a bitmask.  You can mask an irq to multiple 
processors.  Of course, this will absolutely kill your performance.  That's why 
irqbalance never does this.

	-- Chris
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