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. > Most of the work in my system is spent in enrypt/decrypting traffic. Right now all this is done in a tasklet within the softirqd and hence all landing up on the same CPU. On the receive side it'a packet handler that handles the traffic. On the tx side it's done within the transmit path of the packet. So would re-architecting this to move the rx packet handler to a different kernel thread(with smp affinity to one CPU) and tx to a different kernel thread(with SMP affinity to a different CPU) be advisable. What's the impact on cache miss and slowpath/fastpath in network stack.
Thx, -Venkat -----Original Message----- From: Chris Snook [mailto:[EMAIL PROTECTED] Sent: Wednesday, September 12, 2007 2:45 PM To: Venkat Subbiah Cc: linux-kernel@vger.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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
