Zabbix have problem with threads number
It's needed in manualy adjust thread number on server/proxyes for handle all requests, and zabbix agent, probably, use only one threads for active checks (i don't understand it, why we can't fork new thread for every check?)

What i mean:
Zabbix server/proxy can adjust threads number, by threads % busy number.
Example:
threshol = 75.0;
if (zabbix_poller.pbusy > threshold)

As i think, this can solve problem with not enough threads for handling requests, and this logic can be applicable for all pollers.

On zabbix-agent, also, as i think and as i understand:
zabbix agent can just fork every active check in another threads.
I've a many, many active checks and in some cases, zabbix can't get in time data from it, and queue infinitely grow, its not a cool. As a workaroud i use zabbix passive checks for slow checks, because as i understand, zabbix agent prefork several threads, for handling passive checks.

Fix me, if i'm wrong in something.