Re: Although CONFIG_IRQBALANCE is enabled IRQ's don't seem to be balanced very well

[Stijn Eeckhaut <stijn.eeckhaut@intec.ugent.be>]

Nauman Tahir wrote:
> On 1/10/06, Jesper Juhl <jesper.juhl@gmail.com> wrote:
> > On 1/10/06, Martin Bligh <mbligh@mbligh.org> wrote:
> > > Jesper Juhl wrote:
> > > > On 1/10/06, Martin Bligh <mbligh@mbligh.org> wrote:
> > > > > Josef Sipek wrote:
> > > > > > On Tue, Jan 10, 2006 at 12:14:42PM +0100, Jesper Juhl wrote:
> > > > > >
> > > > > > > Do I need any userspace tools in addition to CONFIG_IRQBALANCE?
> > > > > >
> > > > > > Last I checked, yes you do need "irqbalance" (at least that's what
> > > > > > the package is called in debian.
> > > > >
> > > > > Nope - you need the kernel option turned on OR the userspace daemon,
> > > > > not both.
> > > >
> > > > Ok, good to know.
> > > >
> > > > > If you're not generating interrupts at a high enough rate, it won't
> > > > > rotate. That's deliberate.
>
> What I have read is that first CPU is used more for interrupts to use
> the concept of maximizing cache locality. Probably kernel is
> optimizing this even with CONFIG option enabled.
>
> > > > Hmm, and what would count as "a high enough rate"?

This is what I tested a few months ago:

Test system: 2 dual Pentium3 systems
- with 2.6.11 kernel and kernel IRQ balancing;
- each with an Intel dual port E1000 NIC (e1000 driver 6.0.54);
- both systems connected back-to-back to each other with 2 links.

Test 1:
- I started 1 UDP flow (< 23 Mbps) on the first link with the Iperf 
network performance measurement tool. For a UDP bandwidth lower than 23 
Mbps the interrupt rate at the receiver interface was lower than 2000 
interrupts per second. In this case all interrupts were distributed to 
CPU 0. 2000 interrupts per second seemed to be the threshold for the 
interrupts to be distributed to 1 CPU.

Test 2:
- Then I started 1 UDP flow of 600 Mbps on the first link. 8000 
interrupts per second were generated by the receiver interface. 
Approximately half of the interrupts were distributed to CPU 0, the 
other half to CPU 1.

Test 3:
- Then I did a test with 2 UDP flows of 600 Mbps, each over their own 
link. 8000 interrupts per second were generated by both receiver 
interfaces. All interrupts generated by the 1st interface were 
distributed to CPU 0, all interrupts generated by the 2nd interface were 
 distributed to CPU 1.


> > > > I just did a small test with thousands of  ping -f's through my NIC
> > > > while at the same time giving the disk a good workout with tons of
> > > > find's, sync's & updatedb's - that sure did drive up the number of
> > > > interrupts and my load average went sky high (amazingly the box was
> > > > still fairly responsive):
> > > >
> > > > root@dragon:/home/juhl# uptime
> > > > 22:59:58 up 12:43,  1 user,  load average: 1015.48, 715.93, 429.07
> > > > but, not a single interrupt was handled by CPU1, they all went to CPU0.
> > > >
> > > > Do you have a good way to drive up the nr of interrupts above the
> > > > treshhold for balancing?
> > >
> > > Is it HT? ISTR it was intelligent enough to ignore that. But you'd
> > > have to look at the code to be sure.
> >
> > Dual Core Athlon 64 X2 4400+
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