On Fri, Aug 07 2009, Jens Axboe wrote: > > I'm not NAK'ing... just inserting some relevant NAPI field experience, > > and hoping for some numbers that better measure the costs/benefits. > > Appreciate you looking over this, and I'll certainly be posting some > more numbers on this. It'll largely depend on both storage, controller, > and worload. Here's a quick set of numbers, beating with random reads on a drive. Average of three runs for each, stddev is very low so confidence in the numbers should be high. With iopoll=0 (disabled), stock: blocksize IOPS ints/sec usr sys ------------------------------------------------------ 4k 48401 ~30500 3.36% 27.26% clat (usec): min=1052, max=21615, avg=10541.48, stdev=243.48 clat (usec): min=1066, max=22040, avg=10543.69, stdev=242.05 clat (usec): min=1057, max=23237, avg=10529.04, stdev=239.30 With iopoll=1 blocksize IOPS ints/sec usr sys ------------------------------------------------------ 4k 48452 ~29000 3.37% 26.47% clat (usec): min=1178, max=21662, avg=10542.72, stdev=247.87 clat (usec): min=1074, max=21783, avg=10534.14, stdev=240.54 clat (usec): min=1102, max=22123, avg=10509.42, stdev=225.73 The system utilization numbers are significant, I can say that for these three runs, the iopoll=0 numbers were 27.25%, 27.28%, and 27.26%. For iopoll=1, they were 26.44%, 26.26%, and 26.36%. The usr numbers were equally stable. The latencies numbers are too close to call here. On a slower box, I get: iopoll=0 blocksize IOPS ints/sec usr sys ------------------------------------------------------ 4k 13100 ~12000 3.37% 19.70% clat (msec): min=7, max=99, avg=78.32, stdev= 1.89 clat (msec): min=6, max=96, avg=77.00, stdev= 1.89 clat (msec): min=8, max=111, avg=78.27, stdev= 1.84 iopoll=1 blocksize IOPS ints/sec usr sys ------------------------------------------------------ 4k 13745 ~400 3.30% 19.74% clat (msec): min=8, max=91, avg=73.33, stdev= 1.66 clat (msec): min=7, max=90, avg=72.94, stdev= 1.64 clat (msec): min=6, max=103, avg=73.11, stdev= 1.77 Now, 13K iops isn't very much, so there isn't a huge performance difference here and system utilization is practically identical. If we were to hit 100k+ iops, I'm sure things would look different. If you look at the IO completion latencies, they are actually better. This box is a bit special, in that the 13k iops is purely limited by the softirq that runs the completion. The controller only generates irqs on a single CPU, so the softirqs all happen there (unless you use IO affinity by setting rq_affinity=1, in which case you can reach 30k IOPS with the same drive). Anyway, just a first stack of numbers. Both of these are with using the mpt sas controller. -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
