Upstream prematurely closed connection while reading response header from upstream for no apparent reason?

Question

PHP

Sometimes such an error comes across in the nginx logs, while the request ends with a 502 error. The script is simple, without errors / warnings, spits out the answer in a split second. There is almost no load on the server. There are enough processes in the fpm pool. What other reasons could there be?

Answer 1

Take a traffic dump, at least - you will understand whether PHP really sends a whack. Well, it would be useful to compare the logs of the web server and the backend.