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#### A.8.1.3 Tuning

This subsection describes the use of some toolbar buttons available in the match windows:

Tuning Parameters
  Displays tuning parameters alongside match parameters in the Match Criteria panel.
Full Profiling
  Increases the amount of timing and memory use information displayed in the Logging Panel.
Parallel Execution
  Runs the match in multi-threaded mode if the tables are large and if multiple processors are available. By default this is set; unsetting it should have no effect on the result apart from (usually) slowing the matching down. A limit (currently 6) on the parallelism of the matching is imposed since adding more processors tends to lead to diminishing returns.

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:

HEALPix k
  Used for sky-like matches. k is an integer value which determines the size of pixels into which the celestial sphere is decomposed for binning rows. The legal range is between 0 (corresponding to a pixel size around 60 degrees) and 20 (around 0.2 arcsec). In HEALPix language, k is log2(Nside).
Scale Factor
  Used for Cartesian-like matches. The scale factor is a floating point value which determines the size of the notional N-dimensional pixels into which the space is decomposed for binning rows, as a multiple of the match error. The smallest legal value is 1.

In either case, the number of rows per bin should be not too large, and not too small (though exactly what that means in quantitative terms is another matter). Larger bins/pixels generally mean less memory use and more CPU time, though that's not always the case.

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!

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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk