This subsection describes the use of two toolbar buttons available in the match windows:
The parameters such as Max Error visible by default in the Match Criteria panel define what counts as a match and must be filled in for the match to proceed.
Optionally, you can see and adjust another set of parameters known as Tuning parameters. These will not affect the result of the match, but may affect its performance, in terms of how much CPU time and memory it takes. Most of the time, you can forget about this option, since TOPCAT attempts to pick reasonable defaults, but if your match is very slow (especially if it's unexpectedly slow given the sizes of the tables involved), or if it's failing with messages about running out of memory, then adjusting the tuning parameters may help.
To view the tuning parameters, use the Tuning Parameters () toolbar button or menu item. This will add display of tuning parameters to the match parameters in the Match Criteria panel. Suggested values are filled in by default, and may be affected by the match parameters that you fill in; you can play around with different values and see the effect on match performance. If you do this, it is useful to use also the Full Profiling () toolbar button to turn on full profiling. This will cause additional information on the amount of CPU time and memory used by each step to be displayed in the logging panel at the bottom. There is a small penalty in CPU time required to gather this information, which is one reason it is not turned on by default.
What tuning parameters are available depends on the match type you have chosen. Some additional description is available as tooltips if you hover over the query field. In most cases, they are one or other of the following:
Even if you happen to have a good understanding of how the matching algorithm works (and you probably don't), this kind of tuning is a bit of a black art, and depends considerably on the details of the particular match. In some cases however it is quite possible to improve the time taken by a match, or the size of table which can be matched in a given amount of memory, by a factor of several. If you want to try to improve performance, try the default, adjust the tuning parameters slightly, look at how the performance changes by examining the logging output, and maybe repeat to taste.
Another thing which can affect speed and memory usage is whether your Java Virtual Machine is running in 32-bit or 64-bit mode. There are pros and cons of each - sometimes one will be better, sometimes the other. If you need to improve performance, experiment!