| freeing memory in programs [message #3933] |
Thu, 23 March 1995 07:03  |
pjclinch
Messages: 27
Registered: May 1993
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Junior Member |
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I'm writing a Wave Widgets program that uses some big arrays (256 * 256 *
128 MRI image stacks). Some operations require temporary arrays to hold
significant chunks of this, but I don't want these arrays around full
time to choke the memory. I can't use DLEVAR as I'm not at the main
program level, so how do I free up memory? If I had assigned an array
by, say
TmpArr = MAKE_ARRAY(256,256,128, /INT)
would a subsequent use of something like TmpArr = BYTE(0) or
TmpArr = MAKE_ARRAY(1) free up the memory, or would it still be taken up
but no longer accessible?
Curiously, Pete.
--
Peter Clinch Dundee Teaching Hospitals NHS Trust
voice: 44 1382 660111 x 3637 snail: Directorate of Medical Physics
fax: 44 1382 640177 Ninewells Hospital
email: p.j.clinch@dundee.ac.uk Dundee DD1 9SY Scotland UK
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| Re: Freeing memory [message #7026 is a reply to message #3933] |
Fri, 13 September 1996 00:00  |
Christian Soeller
Messages: 46
Registered: August 1996
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Member |
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Phil Williams <williams@irc.chmcc.org> writes:
> My question is why? And why is there very little descrpancy in the
> help,/mem for the heap used? The sysmon looks like alot more than a 1%
> difference?
It is because of the way that memory allocation in a process under unix works.
Following example, from an freshly started idl process:
IDL> i=fltarr(1000000)
IDL> help,/mem
heap memory in use: 4075014, calls to MALLOC: 138, FREE: 36
ps -el for the process:
30 S 1115 21705 21701 0 26 20 * 3070:1673 8838e3e0 pts/0 0:01 idl
i.e. Unix tells you that the process uses 3070 blocks of memory (1673 in
physical RAM)
But then say in IDL
IDL> i=0
IDL> help,/mem
heap memory in use: 74976, calls to MALLOC: 140, FREE: 37
Now ps -el tells you
30 S 1115 21705 21701 0 26 20 * 3070:1676 8838e3e0 pts/0 0:01 idl
i.e. still the same amount of memory is used by the process, even more is now
in physical RAM.
The paradox is resolved by knowing that memory that has been returned by IDL
(internally using some equivalent of the free (3C) function from the
allocation library) is not(!) being returned to the operating system but
merely put in the list of free blocks maintained by the allocation library
routines. Here is what the info of the GNU allocation library tells you:
> Occasionally, free can actually return memory to the operating system
> and make the process smaller. Usually, all it can do is allow a later
> later call to malloc to reuse the space. In the meantime, the space
> remains in your program as part of a free-list used internally by
> malloc.
So, if you have a temporary computation that uses a *huge* amount of memory
you will still see that in sysmon after the associated memory has been
internally freed and the heap memory in IDL looks much smaller. Another
related issue is memory fragmentation...
Obviously, if you restart the process from scratch and restore only the saved
variables you won't incurr these costs.
Regards,
Christian
------------------------------------------------------------ --------
Christian Soeller mailto: csoelle@sghms.ac.uk
St. Georges Hospital Medical School Dept. of Pharmacology
Cranmer Terrace London SW17 0RE
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| Re: Freeing memory [message #7028 is a reply to message #3933] |
Thu, 12 September 1996 00:00 |
rigby
Messages: 16
Registered: September 1995
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Junior Member |
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In article 31DF@irc.chmcc.org, Phil Williams <williams@irc.chmcc.org> () writes:
>
> I have a function that reads data and does some base processing. As the
> procedure is running I monitor my system memory using SGI's sysmon.
> After the function is complete I use some % of memory.
>
> IDL> help,/mem
> heap memory in use: 13710221, calls to MALLOC: 25781, FREE: 25676
>
> I then save all the data to a file and quit idl.
>
> I then start idl and restore the data and compile the function and there
> is less memory used.
>
> IDL> help,/mem
> heap memory in use: 13709644, calls to MALLOC: 119, FREE: 14
>
> My question is why?
I don't know if it explains your observations, but the IDL command
history buffer uses up heap memory. Notice what "help,/mem" returns
as the history list gets longer.
---
--
Wayne Rigby
GE Corporate Research and Development
rigby@crd.ge.com
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