listen() backlog upper limits

listen() backlog upper limits

even though a lot was said on the topic, I am still stumped.
I experiment with a monster linux server capable of handling proper load
ramps, presumably many thousand connections a second. Now, if i check
default listen() queue:
#cat /proc/sys/net/core/somaxconn
128
which couldn't be actual queue size at all. I suspect it might be a
legacy, and actual size is given by this:
#cat /proc/sys/net/ipv4/tcp_max_syn_backlog
2048
However, man tcp says the latter is connections awaiting ACK from clients,
which is different from total number of connections having not yet been
accepted, which is what listen() backlog is.
So my question is how can I increase listen() backlog, and how to get/set
upper limit of it (right before kernel recompilation)?