lsof shows ~800 before starting the server and goes to about 880 upon start. netstat shows ~120 lines (all included, even headers). Not an active server, I'm playing on the side with it, so no load except my control test. I see similar result for the open file limit set to 900, 1024 and 2048. I start the server and immediately do an HTTP request, getting response successfully. Leave the server running for about 10-15minutes without any HTTP request. lsof stays pretty stable at around 880. netstat shows ~120 lines (all included, even headers). Check lsof and netstat and you see the same numbers. Make an HTTP request and it doesn't get a response, the server log shows the asyncio message that @SynclabIO posted. The error message only happens upon making the new HTTP request. no change on lsof and netstat. At this point I couldn't find any remedy to make it work again. If I move the task to background, change the ulimit and move back to the foreground it just Sigfault.