roadrunner: Too many opened tcp connections on low load

Recently I tried to do some stress testing stuff with my application (works with RR). I used artillery (https://artillery.io) to do this. When I firstly run the tests for 60 seconds with 10 requests/sec everything was fine, but many tcp connections were opened by RR process. On a longer distance (3 minutes) errors start to happen.

2020/10/15 11:43:45 http: Accept error: accept tcp [::]:80: accept4: too many open files; retrying in 10ms
2020/10/15 11:43:45 http: Accept error: accept tcp [::]:80: accept4: too many open files; retrying in 20ms
2020/10/15 11:43:45 http: Accept error: accept tcp [::]:80: accept4: too many open files; retrying in 40ms
2020/10/15 11:43:45 http: Accept error: accept tcp [::]:80: accept4: too many open files; retrying in 80ms
2020/10/15 11:43:45 http: Accept error: accept tcp [::]:80: accept4: too many open files; retrying in 160ms
2020/10/15 11:43:46 http: Accept error: accept tcp [::]:80: accept4: too many open files; retrying in 320ms
2020/10/15 11:43:46 http: Accept error: accept tcp [::]:80: accept4: too many open files; retrying in 640ms
2020/10/15 11:43:47 http: Accept error: accept tcp [::]:80: accept4: too many open files; retrying in 1s
2020/10/15 11:43:48 http: Accept error: accept tcp [::]:80: accept4: too many open files; retrying in 1s
2020/10/15 11:43:49 http: Accept error: accept tcp [::]:80: accept4: too many open files; retrying in 1s
2020/10/15 11:43:50 http: Accept error: accept tcp [::]:80: accept4: too many open files; retrying in 1s
2020/10/15 11:43:51 http: Accept error: accept tcp [::]:80: accept4: too many open files; retrying in 1s
2020/10/15 11:43:52 http: Accept error: accept tcp [::]:80: accept4: too many open files; retrying in 1s
2020/10/15 11:43:53 http: Accept error: accept tcp [::]:80: accept4: too many open files; retrying in 1s
2020/10/15 11:43:54 http: Accept error: accept tcp [::]:80: accept4: too many open files; retrying in 1s
2020/10/15 11:43:55 http: Accept error: accept tcp [::]:80: accept4: too many open files; retrying in 1s
2020/10/15 11:43:56 http: Accept error: accept tcp [::]:80: accept4: too many open files; retrying in 1s
2020/10/15 11:43:57 http: Accept error: accept tcp [::]:80: accept4: too many open files; retrying in 1s
2020/10/15 11:43:58 http: Accept error: accept tcp [::]:80: accept4: too many open files; retrying in 1s

lsof -a -p $(pidof rr) shows 30 worker pipes, some other connections and loads of tcp connections like ubuntu2004.localdomain:http->_gateway:58032. I’ve found only a temporary solution - increase opened files limit, but I think this is not the root of the problem. Maybe these connections are not closed immediately after the response and I don’t know how to fix that.