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18.1.20 Use persistent connections

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If you connect to a database with each script, consider using a persistent connection rather than a normal connection. For MySQL users, that is the difference between using mysql_pconnect() rather than mysql_connect(). Persistent connections remain connected even after your script has ended, which means that the next time a script asks for a connection, it uses the one that is already open - this saves a lot time negotiating passwords and such that can otherwise be used to execute important code.

Switching to persistent connections does not require any other change than adding a "p" in the function name - the parameters are still the same. If your database server is not on the same machine as your web server, consider using CLIENT_COMPRESS as the fifth parameter to your mysql_connect() /mysql_pconnect() call - it allows MySQL to compress data to save space, can drastically lower network bandwidth and transfer speed, particularly when reading in lots of data.

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Andrey - 29 Aug 2008

Use persistent connections with care because they can be dangerous (a reason they don't exist in the ext/mysqli extension). MySQL can handle a lot of open connections but even then the burded on context-switching and keeping a stack for every thread (MySQL is multithreaded) is big. With some thread library implementations MySQL handles up to 1024 connections (threads) but a higher number can be reached with others. However, a lot of memory is allocated for 3000 threads for example.

Also an Apache instance can server a simple HTML but the MySQL connection is cached in it and not used. With Apache 1.3.x the connections are kept in the every apache process - apache uses processes not threads.

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