Here are a few example configuration settings for servers of different memory sizes running both MySQL and a web server on the same machine. These are not perfect, but they are good starting points.
For servers with 512MB RAM:
thread_cache_size=50 key_buffer=40M table_cache=384 sort_buffer_size=768K read_buffer_size=512K read_rnd_buffer_size=512K query_cache_limit=2M query_cache_size=16M query_cache_type=1 thread_concurrency=2*CPU skip-innodb
For servers with 1GB RAM:
thread_cache_size=80 key_buffer=150M table_cache=512 sort_buffer_size=1M read_buffer_size=1M read_rnd_buffer_size=768K query_cache_limit=4M query_cache_size=32M query_cache_type=1 thread_concurrency=2*CPU skip-innodb
For servers with 2GB RAM:
thread_cache_size=80 key_buffer=350M table_cache=1024 sort_buffer_size=2M read_buffer_size=2M read_rnd_buffer_size=768K query_cache_limit=4M query_cache_size=64M query_cache_type=1 thread_concurrency=2*CPU skip-innodb
Once WordPress has been up and running for some time you can tweak your settings by running MySQLTuner, a Perl script that analyzes your MySQL performance and, based on the statistics it gathers, gives recommendations about which variables you should adjust to increase performance. With MySQLTuner, you can tune your my.cnf file to tease out the last bit of performance from your MySQL server and make it work more efficiently.
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