| Re: strings and memory usage [message #55452] |
Wed, 22 August 2007 13:08  |
Jim Pendleton, ITT Vi
Messages: 13
Registered: August 2006
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Junior Member |
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Take a look at the IDL External Development Guide's discussion of
the IDL_Variable structure and the IDL_String structure. In
summary, an IDL_Variable (a descriptor) points to an IDL_String, which
is itself a descriptor and not just a null-terminated byte vector.
If anyone (anyone?!!) recognizes the call "OTS$SCopy_DX_DX()", you'll
understand the historical reason for this.
Jim P.
"Conor" <cmancone@gmail.com> wrote in message
news:1187808869.444817.321600@r23g2000prd.googlegroups.com.. .
> Does anyone know how IDL stores strings? I'm creating some very large
> string arrays and running out of memory when I shouldn't. So, for the
> following example I'm using the linux command 'top' to keep track of
> memory usage on a per-process basis. In the beginning, IDL is using
> 59 megabytes. Then, I create a string array with 5 million elements
> like this:
>
> test = strarr(5000000) + 'asdf'
>
> Now I have a string array with 5,000,000 elements, each with 4
> characters in it. According to top idl is now consuming 177
> megabytes! That means that each string takes up an average of 23
> bytes! To make matters worse, when I delete test (delvar,test) IDL
> drops back down to 120 megabytes!
>
> What in the world is going on? Naievly, I would expect a string array
> with strings 4 characters long to take up an absolute maximum of 8
> bytes per element (4 bytes for the characters, 2 bytes for the length,
> and maybe two bytes for pointers). Why is it taking up 23 bytes???
> Am I just confused about something? Also, why doesn't the memory
> usage drop back down to it's original value? I did notice one thing.
> When I then created more large variables, the memory usage didn't
> increase right away, so maybe IDL is clearing the memory but not
> releasing it to the operating system. Still, I find these problems
> very troubling. Is there something very wrong with the string arrays
> in IDL, or am I just being silly?
>
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