Suggest setting request_terminate_timeout_track_finished = yes in Performance Tuning page

[tom--]

PHP 7.3 introduced the FPM ini config directive request_terminate_timeout_track_finished. I think the Performance Tuning of the Yii 2 def guide should suggest turning it on.

I quote PHP Manual below and there's a discussion of why this config was needed here.

request_terminate_timeout mixed

The timeout for serving a single request after which the worker process will be killed. This option should be used when the 'max_execution_time' ini option does not stop script execution for some reason. A value of '0' means 'Off'. Available units: s(econds)(default), m(inutes), h(ours), or d(ays). Default value: 0.

request_terminate_timeout_track_finished bool

The timeout set by request_terminate_timeout is not engaged after a fastcgi_finish_request or when application has finished and internal shutdown functions are being called. This directive will enable timeout limit to be applied unconditionally even in such cases. Default value: no. Since PHP 7.3.0.

Yii's session, error handler and logging components (which are by convention defaults in Yii apps) all register shutdown functions.

So if an FPM request to a Yii app gets stuck after finishing then the request_terminate_timeout will not kill the FPM worker and it will hang indefinitely. Iiuc, PHP devs considered this a bug in FPM and introduced request_terminate_timeout_track_finished as the fix. But it is off by default.