Possible limitation of ngx_http_limit_req_module

[jwroblewski]

My use case is that upstreams are supposed to return within ~100ms,
therefore using burst is not an option. I wanted to use limit_req to filter
out traffic which is exceeds my backend's processing capacity, but
apparently it is not the right tool to use, if it only operates with
millisecond-precision... Could you please document this limitation?

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,258803,258824#msg-258824