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MySQL Error: Too many connections

Just a strange number from MySQL:

By default 151 is the maximum permitted number of simultaneous client connections in MySQL 5.5

And then the issue related to it:

MySQL uses one thread per client connection and many active threads are performance killer. Usually a high number of concurrent connections executing queries in parallel can cause significant slowdown and increase chances for deadlocks. Prior to MySQL 5.5, it doesn’t scale well although MySQL is getting better and better since then — but still if you have hundreds of active connections doing real work (this doesn’t count sleeping [idle] connections) then the memory usage will grow. Each connection requires per thread buffers. Also implicit in memory tables require more memory plus memory requirement for global buffers. On top of that, tmp_table_size/max_heap_table_size that each connection may use, although they are not allocated immediately per new connection.

Facebook has been doing a lot of work in this area. Just one example: Making MySQL accept connections faster; I strongly encourage you to read if you have a busy MySQL server.

Original title and link: MySQL Error: Too many connections (NoSQL database©myNoSQL)