From: Vincent Fu <vincentfu@gmail.com>
To: ylhsieh@mxic.com.tw, fio@vger.kernel.org
Subject: Re: fio output percentile latency seems upper bounded
Date: Tue, 16 Apr 2024 11:25:03 -0400 [thread overview]
Message-ID: <551f4e4a-8aba-43f6-ae20-439041b87310@gmail.com> (raw)
In-Reply-To: <OFA9096527.7152DEBE-ON48258B01.0030FF36-48258B01.003110F5@mxic.com.tw>
On 4/16/24 04:55, ylhsieh@mxic.com.tw 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
>
>
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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
prev parent reply other threads:[~2024-04-16 15:25 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-16 8:55 fio output percentile latency seems upper bounded ylhsieh
2024-04-16 15:25 ` Vincent Fu [this message]
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