In order to reduce completion latency it makes sense to harvest completed
requests ASAP. Very fast backend device can complete requests just after
submission, so it is worth trying to check ring buffer in order to peek
completed requests directly after io_submit() has been called.
Indeed, this patch reduces the completions latencies and increases the
overall throughput, e.g. the following is the percentiles of number of
completed requests at once:
1th 10th 20th 30th 40th 50th 60th 70th 80th 90th 99.99th
Before 2 4 42 112 128 128 128 128 128 128 128
After 1 1 4 14 33 45 47 48 50 51 108
That means, that before the current patch is applied the ring buffer is
observed as full (128 requests were consumed at once) in 60% of calls.
After patch is applied the distribution of number of completed requests
is "smoother" and the queue (requests in-flight) is almost never full.
The fio read results are the following (write results are almost the
same and are not showed here):
Before
------
job: (groupid=0, jobs=8): err= 0: pid=2227: Tue Jul 19 11:29:50 2016
Description : [Emulation of Storage Server Access Pattern]
read : io=54681MB, bw=1822.7MB/s, iops=179779, runt= 30001msec
slat (usec): min=172, max=16883, avg=338.35, stdev=109.66
clat (usec): min=1, max=21977, avg=1051.45, stdev=299.29
lat (usec): min=317, max=22521, avg=1389.83, stdev=300.73
clat percentiles (usec):
| 1.00th=[ 346], 5.00th=[ 596], 10.00th=[ 708], 20.00th=[ 852],
| 30.00th=[ 932], 40.00th=[ 996], 50.00th=[ 1048], 60.00th=[ 1112],
| 70.00th=[ 1176], 80.00th=[ 1256], 90.00th=[ 1384], 95.00th=[ 1496],
| 99.00th=[ 1800], 99.50th=[ 1928], 99.90th=[ 2320], 99.95th=[ 2672],
| 99.99th=[ 4704]
bw (KB /s): min=205229, max=553181, per=12.50%, avg=233278.26, stdev=18383.51
After
------
job: (groupid=0, jobs=8): err= 0: pid=2220: Tue Jul 19 11:31:51 2016
Description : [Emulation of Storage Server Access Pattern]
read : io=57637MB, bw=1921.2MB/s, iops=189529, runt= 30002msec
slat (usec): min=169, max=20636, avg=329.61, stdev=124.18
clat (usec): min=2, max=19592, avg=988.78, stdev=251.04
lat (usec): min=381, max=21067, avg=1318.42, stdev=243.58
clat percentiles (usec):
| 1.00th=[ 310], 5.00th=[ 580], 10.00th=[ 748], 20.00th=[ 876],
| 30.00th=[ 908], 40.00th=[ 948], 50.00th=[ 1012], 60.00th=[ 1064],
| 70.00th=[ 1080], 80.00th=[ 1128], 90.00th=[ 1224], 95.00th=[ 1288],
| 99.00th=[ 1496], 99.50th=[ 1608], 99.90th=[ 1960], 99.95th=[ 2256],
| 99.99th=[ 5408]
bw (KB /s): min=212149, max=390160, per=12.49%, avg=245746.04, stdev=11606.75
Throughput increased from 1822MB/s to 1921MB/s, average completion latencies
decreased from 1051us to 988us.
Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
Message-id: 1468931263-32667-4-git-send-email-roman.penyaev@profitbricks.com
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
0ed93d84ed