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/ _______________________________________________ nginx-devel mailing list nginx-devel@nginx.org http://mailman.nginx.org/mailman/listinfo/nginx-devel
