Don't Assume – Per Session Buffers

MySQL has a number of global buffers, i.e. your SGA. There are also a number of per session/thread buffers that combined with other memory usage constitutes an unbounded PGA. One of the most common errors in mis-configured MySQL environments is the setting of the 4 primary per session buffers thinking they are global buffers.

Global buffers include:

    The four important per session buffers are:

    I have seen people see these values > 5M. The defaults range from 128K to 256K. My advice for any values above 256K is simple. What proof do you have this works better? When nothing is forthcoming, the first move is to revert to defaults or a maximum of 256K for some benchmarkable results. The primary reason for this is MySQL internally as quoted by Monty Taylor – for values > 256K, it uses mmap() instead of malloc() for memory allocation.

    These are not all the per session buffers you need to be aware of. Others include thread_stack, max_allowed_packet,binlog_cache_size and most importantly max_connections.

    MySQL also uses memory in other areas most noticeably in internal temporary tables and MEMORY based tables.

    As I mentioned, there is no bound for the total process memory allocation for MySQL, so some incorrectly configured variables can easily blow your memory usage.


    About “Don’t Assume”

    “Don’t Assume” is a series of posts to help the Oracle DBA understand, use and appreciate the subtle differences and unique characteristics of the MySQL RDBMS in comparison to Oracle. These points as essential to operate MySQL effectively in a production environment and avoid any loss of data or availability.

    For more posts in this series be sure to follow the mysql4oracledba tag and also watch out for MySQL for Oracle DBA presentations.

    The MySQLCamp for the Oracle DBA is a series of educational talks all Oracle DBA resources should attend. Two presentations from this series IGNITION and LIFTOFF will be presented at the MySQL Users Conference 2010 in Santa Clara, April 2010 This series also includes JUMPSTART and VELOCITY. If you would like to here these presentations in your area, please contact me.