AIPS HELP file for SETMAXAP in 31DEC22
As of Fri Jul 1 11:18:46 2022
SETMAXAP: verb to set maximum dynamic pseudo-AP memory
No adverbs: desired size in Megabytes is given as an
immediate argument. If that argument is missing or
out of range the current size and limits are shown.
Example: SETMAXAP(0) displays current and allowed
limits; SETMAXAP(256) sets limit to 256 Mbytes or
32 Mega words of double precision memory.
Use: The pseudo-AP in AIPS now uses dynamic memory in a size suited
to the current problem. AIPS has algorithms that can handle
situations in which the problem does not fit into memory. But
it is very hard to determine a reasonable memory to use, since
most operating systems will allow the task to ask for more
memory than it can actually provide. When this happens, the
task will thrash (page fault a lot) and take much more time than
if it were using a smaller memory and one of the small memory
algorithms. This verb lets you adjust this important system
parameter to guide AIPS tasks to work as efficiently as
The initial limit will be set to 528 Mbytes, but you are allowed
to go up to 2000 Mbytes on 32-bit addressing machines and 12288
Mbytes on 64-bit addressing machines.
The pseudo-AP now uses double precision (8 byte) words. The
limits above are then 192 and 768 Mega words, resp.
Note that operating systems often use up to 512 Mbytes of RAM
these days and there are other programs running on your machine
such as editors, e-mail, and other AIPS tasks.
SETMAXAP takes an immediate argument giving the desired limit on
AP size in Megabytes. If the argument is missing or out of
range, the verb will display the current size limit and the
range of allowed size limits. If the argument is in the legal
range, the system parameter will be changed and known to all
AIPS tasks starting on that computer after the verb is run. If
the machine is a multi-user (public) machine, please exercise
caution and be responsible.
Memory is used for the following:
1. Basic operating system functions
2. AIPS.EXE, TV, TeK, message server processes
3. AIPS task - basic needs plus pseudo-AP
4. "swap-memory" used by the OS to hold some or all of your
disk file(s) in memory rather than use much slower disk
5. memory types 3 and 4 for every other task in the system
Note that a task such as IMAGR running on a multi-facet case
can use a lot of pseudo-AP at times when it makes the initial
beams and makes or remakes all the facet images at once. In
between these operations it uses less memory but still reads and
rereads the IMAGR work file repetitively plus at least one image
file. It will work a great deal better if the IMAGR work file
can stay in the swap-memory. Therefore you should estimate the
size of that work file and the costs of parts 1 and 2 before
deciding to max out the pseudo-AP memory. If there is no way
the work file will fit in swap-memory, then you are better off
maxing out the pseudo-AP. In any case, beware of running
several large AIPS tasks at once - they can end up clobering
each other for memory with the agregate taking much longer to
run than it would have taken running them sequentially.
The verb SIZEFILE will tell you the size of your UV data set in
magabytes, returning the value in adverb FSIZE. If you are
imaging all of the IFs and spectral channels in that data set
together, then this will be about the size of your work file.
If you are doing only one of the channels/IFs at a time, the
work file will be slightly larger than FSIZE/n where n is the
total number of channels times IFs in the input data set.