On 07/08/14 09:55, Or Gerlitz wrote: > On Fri, Jul 4, 2014 at 12:59 PM, Bart Van Assche <bvanassche@xxxxxxx> wrote: >> >> [...] the blk-iopoll framework defers work to softirq >> context. This means that a context switch from interrupt to softirq >> context has to occur before SRP completion processing can start. Recent >> measurements on current hardware have shown that such a context switch >> takes about 0.5 microseconds. Since I prefer to keep the latency of SRP >> I/O as low as possible I haven't looked further into using blk-iopoll for the SRP initiator driver. > > In the same manner that SoftirqD is the right place to process > received packets in the networking space, it should be > the case for IO completions in the storage space too. Note that > typical packet latency in latest/fastest 10g NICs go below 5us > when using NAPI so if we (== NIC drivers) are happy with the CS > overhead there, we (== SCSI LLD storage drivers whose > latency is in the area of 15-20us) should be happy here too. > Specifically, the CS cost is amortized across multiple IOs. Hello Or, We might each be referring to different concepts here. What you are referring to is average response time for QD > 1. What I was referring to is average response time for QD = 1. The measurements I ran with fio have shown that the response time for QD = 1 is lower when processing completions in interrupt context compared to processing interrupts in softirq context. Bart. -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
