On 4/16/24 04:55, ylhsieh@xxxxxxxxxxx wrote:
Hi, there
In fio output, sometimes I see tail percentile latency to be 17112760320,
and it remains this value to 99.99%, why? Also, the mean latency is larger
than this value? (intuitively, tail latency should be larger than mean
latency...?) What's the reason and how to make it reasonable?
"clat_ns" : {
...
"mean" : 41081227631.728256,
...
"percentile" : {
"1.000000" : 1669332992,
"5.000000" : 10536091648,
"10.000000" : 17112760320,
"20.000000" : 17112760320,
...
"99.950000" : 17112760320,
"99.990000" : 17112760320
}
},
"lat_ns" : {
...
"mean" : 41087031313.177292,
...
}
Thank you
YL
CONFIDENTIALITY NOTE:
This e-mail and any attachments may contain confidential information
and/or personal data, which is protected by applicable laws. Please be
reminded that duplication, disclosure, distribution, or use of this e-mail
(and/or its attachments) or any part thereof is prohibited. If you receive
this e-mail in error, please notify us immediately and delete this mail as
well as its attachment(s) from your system. In addition, please be
informed that collection, processing, and/or use of personal data is
prohibited unless expressly permitted by personal data protection laws.
Thank you for your attention and cooperation.
Macronix International Co., Ltd.
=====================================================================
Yes, there is an upper bound. See the discussion in stat.h:
https://github.com/axboe/fio/blob/master/stat.h
The intention is to calculate the min, max, and mean without loss of
precision. But for the latency percentiles, each latency measurement is
assigned to a bucket that includes a range of values. The midpoint of
the bucket for the highest set of values is 17112760320 ns. Values
exceeding this are not tracked accurately.
You could increase FIO_IO_U_PLAT_GROUP_NR if you wanted to track long
latencies more accurately.
Vincent
