Since upgrading to Moodle 1.8.1 (from 1.5.2) the server is behaving very differently, I have attached some charts that illustrate just how differently!
1) The number of running MySQL threads has increased from 1-2 to averaging maybe 5 with some significant peaks.
2) The number of connected threads has not really changed
3) The CPU usage is constantly running at 99.9% almost all of it taken by MySQL.
4) The number of users accessing the system seems to have little bearing on the performance, i.e. it is constantly "running hot", even during the small hours when there are few users accessing.
I have statsitics enabled, and after one week they are still not up to date, I can see that these have been populating tables during the day time, although I have set this process to stop at 06:00.
We have rebooted the server, and restarted MySQL, however the system just re-consumes its resources.
I have increased the thread cache size to 40, and set the timeout on connections to 5 minutes, but this has not had any impact on performance. I have also meddled with the query cache, gain to no avail.
I am not using a PHP accelerator which I will look into, however, as the server has not changed in terms of OS / DB version I do not believe that will solve the problem
If anyone has any thoughts on this I would love to hear them!!