Why can upstream timed out (110: Connection timed out) while connecting to upstream occur?

Question

Nginx

"upstream timed out (110: Connection timed out) while connecting to upstream" errors occasionally appear, especially when some bot is indexing the pages. It is clear that this means that the server did not return the result in time. But the pages themselves are rendered very quickly, there are no slow queries on them, and the server is not particularly loaded. What else can cause such errors? Where can you dig?

Answer 1

Options:
1) connections run out on the backend that nginx accesses
2) Descriptors run out on the server

Answer 2

As usual, there is little information, so you can’t figure out the problem.
As a rule, the error is treated by increasing the
proxy_read_timeout 300s;
also look at these directives:
proxy_connect_timeout 300s;
proxy_send_timeout 300s;
They are all by default 60 sec
+ if you have php-fpm, there may be
fastcgi_send_timeout
fastcgi_read_timeout
See everything, test, check