Hello!

On Tue, Sep 28, 2021 at 10:27:55AM +0800, Zexuan Luo wrote:

> Hi, Nginx developers:
>
> Currently, the request_time uses ngx_timeofday to get the time, which
> finally will call gettimeofday. Meanwhile, the upstream_x_time series
> uses ngx_current_msec to calculate the time, which finally will call
> ngx_monotonic_time.
>
> On Linux, the gettimeofday will call clock_gettime(CLOCK_REALTIME,
> &ts) while the ngx_monotonic_time will call
> clock_gettime(CLOCK_MONOTONIC_COARSE, &ts).
>
> So the request_time uses CLOCK_REALTIME and the upstream_x_time series
> uses CLOCK_MONOTONIC_COARSE. As they are different sources, sometimes
> we observe that the upstream_response_time is larger than
> request_time. This behavior is unexpected and has caused bug in our
> software.
>
> A similar report can be also found in
> https://stackoverflow.com/questions/53978695/how-can-request-time-be-less-than-upstream-response-time-in-nginx.
>
> Is this behavior intended? Why not use the same time measurement for
> all metrics? Thanks for your reply.

The $upstream_response_time variable was changed to use
ngx_current_msec in 59fc60585f1e
(http://hg.nginx.org/nginx/rev/59fc60585f1e) as part of
$upstream_connect_time introduction, because it is easier to code
and more appropriate for time interval measurements, especially
following introduction of CLOCK_MONOTONIC usage.

The $response_time still uses ngx_timeofday() for mostly historic
reasons. Likely it will be changed to use ngx_current_msec,
though this needs to be done with care, notably because
ngx_current_msec can overflow on 32-bit platforms. At some point
there was a patch by Sergey Kandaurov to do the change, though it
was postponed due to questions on what happens on overflows in
limit_rate handling.

--
Maxim Dounin
http://mdounin.ru/
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