| Re: call_external for file I/O [message #13190 is a reply to message #13186] |
Mon, 26 October 1998 00:00   |
David Foster
Messages: 341
Registered: January 1996
|
Senior Member |
|
|
Charlie Solomon wrote:
> The data is in a disk file initially, the map_file routine puts the relevant
> part of the file into memory and sets those pages for sequential access, so
> that when IDL accesses the data in memory it is not based on an algorithm
> like most recently used. This is optimal for image data in which you may
> need to access different subsets of the array at different times.
>
> I agree that it is fast, but the method we use on our Solaris machine seems
> much faster for the same files. I don't have time data available right now,
> but I could get it if needed.
Charlie -
I'm wondering if what you are coming up against is a storage order
problem, in which the elements of the array are being accessed in
an order that is not optimal given the order of storage of the
elements in memory. Ideally the dimension that varies fastest in
memory should be incremented first.
We've run up against this problem before, and have written a C
routine to optimize access to a 3D array in memory. If you're
interested in checking it out let me know.
You also might want to look into Eric Korpela's VARRAY utility
that allows you to have memory mapped files:
http://sag-www.ssl.berkeley.edu/~korpela/mmap/
korpela@islay.ssl.berkeley.edu
Dave
--
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~
David S. Foster Univ. of California, San Diego
Programmer/Analyst Brain Image Analysis Laboratory
foster@bial1.ucsd.edu Department of Psychiatry
(619) 622-5892 8950 Via La Jolla Drive, Suite 2240
La Jolla, CA 92037
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~
|
|
|
|
comp.lang.idl-pvwave archive
Members
Search
Help
Login
Home
