In our previous article we were already forced to conclude that MySQL 4.x does not scale tremendously well with multiple cores or threads, but since it is (still) the version on which our site runs, as well as many others, we wanted to have a look how the software would manage on the UltraSparc T1. As it turns out, the first two steps to two and four cores are smooth, but at the next, from four to six, things begin to disappoint. The final step to eight cores even proves to be barely noticeable in the results. Under heavy site load, doubling the number of cores to two yields 112% of performance gain, going to four cores still makes for an improvement of 78%, but the increase to eight only delivers an extra 18%.
In MySQL 5.0, a great deal of problems relating to scaling behaviour have been addressed, which is noticeable in the form of clearly higher performance peaks. In spite of that the move to eight cores still yields little performance increase, and moreover, some dramatic new behaviour appears to have been introduced. Performance collapses under heavier loads (more than 40 simultaneous users). To make things worse: the higher the peak, the sharper the drop. While for 25 simultaneous users we are still seeing logical results, bizarre situations start to crop up from 50 users and up, where one core turns out to be quicker than eight. The reasons for this are unclear: neither our system administrator nor the experts at Sun and MySQL were able to provide a solution. Experiments with various (beta) versions did not reveal any improvements, which means that for the moment, we shall just have to accept things the way they are.
