On Fri, 2009-08-28 at 09:25 -0400, Rik van Riel wrote: > raz ben yehuda wrote: > > > yes. latency is a crucial property. > > In the case of network packets, wouldn't you get a lower > latency by transmitting the packet from the CPU that > knows the packet should be transmitted, instead of sending > an IPI to another CPU and waiting for that CPU to do the > work? Hello Rik If I understand what you are saying, you say that I pass 1.5K packets to a offline CPU ? If so, then this is not what I do, because you are very right, it does not make any sense. I do not pass packets to an offline cpu , i pass assignments. an assignment is a buffer with some context of what do with it (like aio) and a buffer is of ~1MB. Also, the offline processor holds the network interface as it own interface. No two offline processors transmit over a single interface.( I modified the bonding driver to work with offline processor for that ). I am aware of network queue per processors, but benchmarks proved this was better.( I do not have these benchmarks now). Also these engines do not release any sk_buffs to the operating system, these packets are being reused over and over to reduce latency of allocating memory and cache misses. Also, in some cases I disabled the transmit interrupts and I released packets ( --skb->users was still greater than 0, not really release ) in an offline context.I learned it from the chelsio driver. This way, I reduced more load from the operating system. It proved to be better in large 1Gbps arrays and was able to remove atomic_inc atomic_dec in some variants of the code, atomic operations cost a lot. in MSI cards I did not find it useful.in the example i showed, i use MSI and system is almost idle. Also, as I recall , IPI will not pass to an offladed processor. offsced it runs NMI. Also, I would to express my apologies if any of this correspondence seems to be as I am trying to PR offsched. I am not. > Inter-CPU communication has always been the bottleneck > when it comes to SMP performance. Why does adding more > inter-CPU communication make your system faster, instead > of slower like one would expect? > -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
