Here are 4k sequential read results (cache=none) to check whether we see an ioeventfd performance regression with virtio-blk.
The idea is to use a small blocksize with an I/O pattern (sequential reads) that is cheap and executes quickly. Therefore we're doing many iops and the cost virtqueue kick/notify is especially important. We're not trying to stress the disk, we're trying to make the difference in ioeventfd=on/off apparent. I did 2 runs for both ioeventfd=off and ioeventfd=on. The results are similar: 1% and 2% degradation in MB/s or iops. We'd have to do more runs to see if the degradation is statistically significant, but the percentage value is so low that I'm satisfied. Are you happy to merge virtio-ioeventfd v6 + your fixups? Full results below: x86_64-softmmu/qemu-system-x86_64 -m 1024 -drive if=none,file=rhel6.img,cache=none,id=system -device virtio-blk-pci,drive=system -drive if=none,file=/dev/volumes/storage,cache=none,id=storage -device virtio-blk-pci,drive=storage -cpu kvm64,+x2apic -vnc :0 fio jobfile: [global] ioengine=libaio buffered=0 rw=read bs=4k iodepth=1 runtime=2m [job1] filename=/dev/vdb ioeventfd=off: job1: (groupid=0, jobs=1): err= 0: pid=2692 read : io=2,353MB, bw=20,080KB/s, iops=5,019, runt=120001msec slat (usec): min=20, max=1,424, avg=34.86, stdev= 7.62 clat (usec): min=1, max=11,547, avg=162.02, stdev=42.95 bw (KB/s) : min=16600, max=20328, per=100.03%, avg=20084.25, stdev=241.88 cpu : usr=1.14%, sys=13.40%, ctx=604918, majf=0, minf=29 IO depths : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0% submit : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0% complete : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0% issued r/w: total=602391/0, short=0/0 lat (usec): 2=0.01%, 4=0.01%, 10=0.01%, 20=0.01%, 50=0.01% lat (usec): 100=0.01%, 250=99.89%, 500=0.07%, 750=0.01%, 1000=0.02% lat (msec): 2=0.01%, 4=0.01%, 10=0.01%, 20=0.01% Run status group 0 (all jobs): READ: io=2,353MB, aggrb=20,079KB/s, minb=20,561KB/s, maxb=20,561KB/s, mint=120001msec, maxt=120001msec Disk stats (read/write): vdb: ios=601339/0, merge=0/0, ticks=112092/0, in_queue=111815, util=93.38% ioeventfd=on: job1: (groupid=0, jobs=1): err= 0: pid=2692 read : io=2,299MB, bw=19,619KB/s, iops=4,904, runt=120001msec slat (usec): min=9, max=2,257, avg=40.43, stdev=11.65 clat (usec): min=1, max=28,000, avg=161.12, stdev=61.46 bw (KB/s) : min=15720, max=19984, per=100.02%, avg=19623.26, stdev=290.76 cpu : usr=1.49%, sys=19.34%, ctx=591398, majf=0, minf=29 IO depths : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0% submit : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0% complete : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0% issued r/w: total=588578/0, short=0/0 lat (usec): 2=0.01%, 4=0.01%, 10=0.01%, 20=0.01%, 50=0.01% lat (usec): 100=0.01%, 250=99.86%, 500=0.09%, 750=0.01%, 1000=0.02% lat (msec): 2=0.01%, 4=0.01%, 10=0.01%, 50=0.01% Run status group 0 (all jobs): READ: io=2,299MB, aggrb=19,619KB/s, minb=20,089KB/s, maxb=20,089KB/s, mint=120001msec, maxt=120001msec Disk stats (read/write): vdb: ios=587592/0, merge=0/0, ticks=110373/0, in_queue=110125, util=91.97% Stefan