[daip] memory

[Eric Greisen]

Marina Alexandrovna Kounkel wrote:
> Hello! 
> 
> I was curious, is there any way to feed aips more memory than 6114 MB at which setmaxap maxes out? Considering we have several dozens of GB available on a dedicated machine, this seems to be a rather small number. And/or, is there any other way at all to increase the speed of the operations? 
> 
> On a somewhat unrelated subject, when running imagr, what is the best imsize/cellsize to use? Our notes do specify 8192/0.1, but we are not completely sure whether such large numbers are entirely appropriate.
>

Memory: The 6114 Mbytes is the limit of a 32-bit integer to address a 
single array.  The pseudo-AP is a single array (of double precision 
words) and we use 32-bit integers to address it.  To change the code to 
use 64-bit integers would be prohibitively expensive.  And it would 
probably not produce better performance - the algorithms are optimized 
fairly well for single-thread code.  The best performance win would be 
to be able to go multi-thread - again essentially impossible with such a 
large base of code.

One thing that is not obvious is that the operating system uses the 
memory not used by tasks for the disk IO - it is possible that your data 
will fit in RAM (if you have a lot) or that the work file in IMAGR will 
fit in RAM.  If that is the case, the task will run very fast, if that 
is not the case it will be very much slower.  So the large memory is not 
wasted.

You can make some operations be "parallel" by running more than one task 
at the same time - imaging  each spectral window in separated copies of 
aips (AIPS1, APPIS2, AIPS3, etc).  Some powerful machines can manage 
this - but be careful not to use the same disk files as input or head 
contention will occur.

Your IMAGR parameters are simply WRONG.  The cell size and image size 
must be set depending on your input data and the size of the region in 
the sky that you wish to image.  In many cases, with longer wavelength 
data anyway, multiple facets are required (at least in AIPS).  The task 
SETFC will recommend cell size and image size and facet locations.  I 
use it by first setting CELLSIZE and IMSIZE to 0.  After running the 
task I delete the output text file and select cell sizes and image sizes 
  to be "nicer"  (0.1 asec is nicer than 0.104598 for example).  Then I 
re-run it to set the locations of the facets, making a BOXFILE for input 
to IMAGR.

Eric Greisen