Notes/MiriadScalingLimitations?
Discussion with MarkCalabretta? 2007-06-07
As people have tried to image larger regions of sky with MIRIAD, there have been various reports of limitations in the size of image or cube that it can handle.
MIRIAD does its memory allocation by first allocating a chunk to itself and then doling pieces of that out on request.
The size of the initial chunk is set by a parameter called MAXBUF in inc/maxdim.h.
There is also a MAXBUF parameter for the C parts of MIRIAD, defined in inc/maxdimc.h, but used only by tasks that use routines from xyzio.c.
The parameter MAXDIM parameter in inc/maxdim.h, is often used to indicate the maximum size of maps (the longest side will be less than MAXDIM).
However the actual maximum that can successfully be allocated may vary from that suggested by MAXDIM, depending on how big MAXBUF is, whether the task processes the entire image or in a raster fashion, and whether any subregions have been allocated out of the space defined by MAXBUF. This last item adds some unpredictability to the whole issue; for example different ordering of clean windows may change the way in which the memory reserved by MAXBUF is fragmented, with the result that the packing is less efficient on some runs, and the maximum image size that can be accommodated may be different.
There are (now) comments in the include files noted above, that explain the upper limits in some detail.
For CABB, the biggest change is along the spectral (channel) axis; since MIRIAD usually handles each channel separately we do not expect that scaling will present much of a problem for single fields. Other parameters of interest for the spectral dimension are:
- MAXCHAN - maximum number of channels in spectral data (visibility or image cube). Default: 4097
- MAXWIN - maximum number of windows in visibility data. Default: 16
- MAXWIDE - maximum number of wideband channels (in visibility data?). Default: 18
Where we might strike problems is when people want to mosaic many pointings together; this is more common at high frequency where the primary beam is smaller.
As a footnote, the name MAXBUF is used all over the place in MIRIAD, in different contexts. It means different things in different contexts.
