Messages in this thread | | | Date | Fri, 3 Oct 2008 10:29:41 -0400 | From | Jason Baron <> | Subject | Re: IRQ balancing on a router |
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On Fri, Oct 03, 2008 at 06:38:57AM -0700, Arjan van de Ven wrote: > > Hello, > > > > I have a dual-CPU router/firewall with five gigabit NICs. Recently I > > have found that irqbalance (0.55 from Fedora 9/x86_64) gives a > > suboptimal IRQ to CPU mapping on this box: > > > > During traffic spikes, it assings two NICs to one CPU, and the > > other three to the second CPU. However, this does not account for > > the fact that packets coming from the uplink interface are way more > > expensive to handle than the rest of the traffic: most iptables rules > > apply to the packets received from the uplink interface. 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. > > > > Setting the IRQ mapping by hand (uplink to one CPU, all the > > other NICs to the other CPU) makes a well balanced system (both CPUs > > 30-60 % busy). I am not sure whether my configuration is too special, > > but it might be worth trying to make irqbalance daemon cope also with > > this usage pattern. > > > > 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.... > >
perhaps, this could be addressed using tracepoints. The currently proposed ones are at the beginning and end of 'handle_IRQ_event()'. See: http://marc.info/?l=linux-kernel&m=121616099830280&w=2
thanks,
-Jason
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