Briefly talk about the Reload operation in PHP

Preface

Many seniors have warned us that reload can ensure the smoothness of the entire process. The so-called smoothness means that during the reload process, the old process will There will be no early termination before the current request. For many years, I never questioned this statement until one day, when I reloaded, a 502 error occurred and I had to think again.

How to reproduce the problem? Let us write a simple script to simulate:

<?php

sleep(11);
echo "foo";

?>

At this time, use a browser to browse this URL, and then immediately perform the reload operation, and you will see 502 error.

Is PHP so weak? Can’t even guarantee the basic smoothness of reload? The answer is of course no. In fact, our goal can be achieved through the

process_control_timeout

parameters. Unfortunately, this parameter defaults to 0, which means it does not take effect. This article sets it to 10s. Re-execute the previous experimental steps, and this time the results are output normally. However, if you do a few more experiments, you may find that sleep ends immediately when we reload. This is because sleep returns directly after receiving the signal from reload. Let us rewrite the script:

<?php

sleep(11);
echo "foo";
sleep(11);
echo "bar";

?>

Re-execute the previous experimental steps, you will find that the 502 error appears again. This is because although reload causes the first sleep to end immediately, the second sleep is still valid and exceeds the time limit of

process_control_timeout

. If we set

process_control_timeout

to 12s, then it's good again.

In this way, we only need to set a reasonable value for

process_control_timeout

to ensure the smoothness of the reload operation, but what is a reasonable value? If it is too small, it may not be effective. If it is too large, will there be any side effects? Let us repeat the last experiment with questions, but this time we add another monitor:

shell> watch -n1 'ps aux | grep php[-]fpm'

The purpose of this monitor is to observe the PHP- Regarding the changes in the number of FPM processes, in order to make the effect more obvious, it is recommended to change the startup mode of PHP-FPM to static mode, and at the same time, the number of processes should not be too many.

When we repeated the last experiment, we found that except for the process that was executing the request, other processes were killed directly, and the new process did not start immediately, and it was stuck until the last old process was executed. After that, the new process completes the startup process. During this period, if other requests come in, it will undoubtedly not be responded to immediately.

Based on our experiments, we can conclude that by default, PHP-FPM cannot guarantee smooth execution of the reload operation, and a reasonable

process_control_timeout

must be set. At the same time, we need to pay attention to However, the value cannot be set too large, otherwise the system may have a more serious request congestion problem.

Summary

The above is all about the Reload operation in PHP. For more related content, please pay attention to the PHP Chinese website (www.php.cn)!