National Radio
Astronomy Observatory

3.3.1.2 A Brief Note Regarding plotms Memory Usage

In order to provide a wide range of flexible interactive plotting options while minimizing the I/O burden, plotms caches the data values for the plot (along with a subset of relevant meta-info) in as efficient a manner as possible. For plots of large numbers of points, the total memory requirement can be quite large. plotms attempts to predict the memory it will require (typically 5 or 6 bytes per plotted point when only one axis is a data axis, depending upon the data shapes involved), and will complain if it believes there is insufficient memory to support the requested plot. For most practical interactive purposes (plots that load and draw in less than a few or a few 10s of minutes), there is usually not a problem on typical modern workstations (attempts to plot large datasets on small laptops might be more likely to encounter problems here).

The absolute upper limit on the number of simultaneously plotted points is currently set by the ability to index the points in the cache. For modern 64 bit machines, this is about 4.29 billion points (requiring around 25GB of memory). (Such plots are not especially useful interactively, since the I/O and draw become prohibitive.)

In general, it is usually most efficient to plot data in modest chunks of not more than a few hundred million points or less, either using selection or averaging. Note that all iterations are (currently) cached simultaneously for iterated plots, so iteration is not a way to manage memory use. A few hundred million points tends to be the practical limit of interactive plotms use w.r.t. information content and utility in the resulting plots, especially when you consider the number of available pixels on your screen.

Since datasets are growing very large, options for plotting arbitrarily large numbers of points—probably in a non-interactive mode—are under consideration for a future release.

More information about CASA may be found at the CASA web page

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