nginx introduces extra delay when talking to slow backend (probably FreeBSD kevent specific)

[Dmitry Marakasov]

Hi!

I've noticed strange behavior of nginx with slow uwsgi backend (turned
out to not be uwsgi specific, and acually reproduces with any backend)
on FreeBSD 11.2

If backend replies in less than a second, nginx doesn't introduce any
additional latency and replies in the same time. However if backend
replies no more than around 1.2 seconds, nginx introduces an extra delay
and replies in around 2.2 seconds.

I've gathered some details here, including the graph of nginx reply time
vs. backend reply time:

https://github.com/AMDmi3/nginx-bug-demo

It reproduces on FreeBSD 11.2 and around year old -CURRENT, but not the
recent -CURRENT, so I suspect this may be FreeBSD specific (probably
kevent-related) and already fixed.

Still, I'm posting to both related nginx and FreeBSD lists for this
problem to be known.

-- 
Dmitry Marakasov   .   55B5 0596 FF1E 8D84 5F56  9510 D35A 80DD F9D2 F77D
amdmi3 at amdmi3.ru  ..:              https://github.com/AMDmi3