here's a sixcore with 32 GB RAM. I've installed MySQL 5.1.47 (backport). Config is nearly standard, except max_connections, which is set to 2000. On the other hand there is PHP5.3/FastCGI on nginx. There is a very simple php application which should be served. NGINX can handle thousands of request parallel on this machine. This application accesses MySQL via mysqli. When using non-persistent connections in mysqli there is a problem when reaching 100 concurrent connections.
[error] 14074#0: *296 FastCGI sent in stderr: "PHP Warning: mysqli::mysqli():  Resource temporarily unavailable (trying to connect via unix:///tmp/mysqld.sock) in /var/www/libs/db.php on line 7
I've no idea to solve this. Connecting via tcp to mysql is terrible slow. The interesting thing is, when using persistent connections (add 'p:' to hostname in mysqli) the first 5000-10000 thousand requests fail with the same error as above until max connections (from webserver, set to 1500) is reached. After the first requests MySQL keeps it 1500 open connections and all is fine, so that I can make my 1500 concurrent requests. Huh? Is it possible, that this is a problem with PHP FastCGI?