I think this is an fio bug.
Description: When specifying ramp_time, cpu may show over 100% (when using a
polling mode driver).
If ramp_time is not specified, cpu values appear to be correct.
You normally don't see this problem because most drivers don't utilize high CPU
usage.
You will have to use some custom driver that does polling, for example
(something like http://www.dpdk.org/).
OS: Any (but tested on Linux CentOS 6.6)
---------
Job file:
[global]
thread=1
group_reporting=1
direct=1
ioengine=libaio
time_based=1
ramp_time=10
runtime=10
bs=4096
filename=/dev/polling
[test]
rw=read
numjobs=1
iodepth=1
-------------
Fio output:
test: (g=0): rw=randread, bs=4K-4K/4K-4K/4K-4K, ioengine=libaio, iodepth=1
fio-2.2.12
Starting 1 thread
test: (groupid=0, jobs=1): err= 0: pid=27094: Wed Nov 25 11:23:07 2015
read : io=486652KB, bw=48660KB/s, iops=12165, runt= 10001msec
slat (usec): min=0, max=16, avg= 2.15, stdev= 0.60
clat (usec): min=34, max=2323, avg=79.77, stdev=20.54
lat (usec): min=36, max=2325, avg=81.79, stdev=20.54
clat percentiles (usec):
| 1.00th=[ 58], 5.00th=[ 66], 10.00th=[ 69], 20.00th=[ 73],
| 30.00th=[ 74], 40.00th=[ 74], 50.00th=[ 76], 60.00th=[ 85],
| 70.00th=[ 88], 80.00th=[ 89], 90.00th=[ 90], 95.00th=[ 91],
| 99.00th=[ 92], 99.50th=[ 93], 99.90th=[ 94], 99.95th=[ 121],
| 99.99th=[ 1048]
bw (KB /s): min= 0, max=48904, per=95.03%, avg=46242.40, stdev=10885.36
lat (usec) : 50=0.92%, 100=99.01%, 250=0.05%, 500=0.01%, 750=0.01%
lat (usec) : 1000=0.01%
lat (msec) : 2=0.01%, 4=0.01%
cpu : usr=164.11%, sys=4.55%, ctx=27, majf=0, minf=3013
IO depths : 1=163.5%, 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 : total=r=121663/w=0/d=0, short=r=0/w=0/d=0, drop=r=0/w=0/d=0
latency : target=0, window=0, percentile=100.00%, depth=1
Run status group 0 (all jobs):
READ: io=486652KB, aggrb=48660KB/s, minb=48660KB/s, maxb=48660KB/s, mint=1000
1msec, maxt=10001msec
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