nginx proxy_cache: limit parallel requests to a backend

I am using nginx as a reverse proxy to my backend(s). Configuration is pretty basic, e.g. the core is just: upstream myservice { server 127.0.0.1:80; server 123.123.123.123:80; } location / { proxy_pass http://myservice; proxy_set_header Host myservice; } Now my service is computationally very heavy, and I would like nginx to limit the number of active parallel (simultaneous) requests to a single