I am seeing some tg3 reports occasionally that show a fair number of interrupts-per-second, even though tg3 is 100% NAPI. It seems to me that as machines get faster, and amount of memory increase [xlat: less waiting for free RAM in all parts of the kernel, and less GFP_ATOMIC alloc failures], the likelihood that a NAPI driver can process 100% of the RX and TX work without having to reqquest subsequent iterations of dev->poll(). NAPI's benefits kick in when there is some amount of system load. However if the box is fast enough to eliminate cases where system load would otherwise exist (interrupt and packet processing overhead), the NAPI "worst case" kicks in, where a NAPI driver _always_ does ack some irqs mask irqs ack some more irqs process events unmask irqs whereas a non-NAPI driver _always_ does ack irqs process events When there is load, the obvious NAPI benefits kick in. However, on super-fast servers, SMP boxes, etc. it seems likely to me that one can receive well in excess of 1,000 interrupts per second, simply because the box is so fast it can run thousands of iterations of the NAPI "worst case", above. The purpose of this email is to solicit suggestions to develop a strategy to fix what I believe is a problem with NAPI. Here are some comments of mine: 1) Can this problem be alleviated entirely without driver changes? For example, would it be reasonable to do pkts-per-second sampling in the net core, and enable software mitigation based on that? 2) Implement hardware mitigation in addition to NAPI. Either the driver does adaptive sampling, or simply hard-locks mitigation settings at something that averages out to N pkts per second. 3) Implement an alternate driver path that follows the classical, non-NAPI interrupt handling path in addition to NAPI, by logic similar to this[warning: off the cuff and not analyzed... i.e. just an idea]: ack irqs call dev->poll() from irq handler [processes events until budget runs out, or available events are all processed] if budget ran out, mask irqs netif_rx_schedule() [this, #3, does not address the irq-per-sec problem directly, but does lessen the effect of "worst case"] Anyway, for tg3 specifically, I am leaning towards the latter part of #2, hard-locking mitigation settings at something tests prove is "reasonable", and in heavy load situations NAPI will kick in as expected, and perform its magic ;-) Comments/feedback requested. Jeff - : send the line "unsubscribe linux-net" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
