I am trying to run a web proxy in Debian Buster. But while testing performance I saw only 7 or 8 parallel connections got established at a time, to narrow down the issue is not with my web proxy; i ran IPERF on the debian and from another linux client i ran iperf -c <debian buster IP> -P 10; the test do not run successfully.

If i change -P from 10 to 7, things are fine.

On the Debian Buster Side, I see connections stuck in SYN_SENT state with UNREPLIED status.

From netstat, I see,
<x> times the listen queue of a socket overflowed
<y> SYNs to LISTEN sockets dropped
where x and y are Equal.

Tried changing the TCP Backlog, SYN Backlog and disabled timestamp etc, nothing helped.

Any suggestions ?

Either the web server is configured to only allow 7 connections, or your proxy server is set that way.
Check with the web site (or test it while bypassing the proxy) to see how many simultaneous clients it allows. Then check the proxy config for the same. If the web server is not in your control you may need to work with their admin to figure this out.

If reply #2 isn't it then maybe some restriction or limit on some program either by design or because of load maybe?? Wonder if top will show any issue on it or other system monitor?