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Re: memory allocation on Macs [message #60188 is a reply to message #60073]

Mon, 05 May 2008 13:23

Karl[1]
Messages: 79
Registered: October 2005

Member

On May 2, 1:13 pm, pgri...@gmail.com wrote:
> Yes, you're right that I can allocate all the 7 GB (and more ) in
> different IDL
> sessions. So there seems to be a limit indeed on how much memory one
> single
> IDL session (or in general , one process) can use up, but there isn't
> a limit for
> total usage (which, though I am sure there are a number of technical
> reason
> for it, seems a bit silly, after all if the system as a whole can
> access more
> than 4 GB, why shouldn't parts of the system be allowed to do the
> same?)

Because it is a 32-bit application. One key difference between 32-bit
and 64-bit applications is that the pointers maintained by a 32-bit
application are 32 bits in size, and the pointers maintained by a 64-
bit application are 64 bits in size. This happens at compile time.
So, your 32-bit application simply cannot address more than 4GB at a
time due to its fundamental pointer size. Note that a 64-bit
application will have a larger storage requirement due to the larger
pointers.

The memory management unit on the 32-bit CPU, something that you
cannot directly access, can address more than 4GB worth of RAM since
it can map more than 4GB among several processes. Here, it is
probably mapping larger chunks of memory, or pages, rather than
individual bytes, so it isn't as hard as it sounds. But it is the MMU
that locates the memory pages assigned to a 4GB process among the 7GB
of installed memory and translates their physical addresses to 32-bit
virtual addresses for the 4GB process.

While there are lots of ways to emulate bigger address spaces and ways
to fit bigger problems onto small machines, it may often be much
easier to move to a 64-bit address space.

Karl

>
> FYI, this is a Xeon machine in Mac OS X 10.4, so it is a 64 bit
> processor
> in a 32 bit OS running a 32 bit application.
>
> Anyway, thanks to all. I can cope with reading a few arrays off the
> disk
> from time to time.
>
> Ciao,
> Paolo

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[Message index]

		Re: memory allocation on Macs By: pgrigis on Fri, 02 May 2008 12:13
		Re: memory allocation on Macs By: Brian Larsen on Fri, 02 May 2008 10:56
		Re: memory allocation on Macs By: Karl[1] on Fri, 02 May 2008 09:36
		Re: memory allocation on Macs By: pgrigis on Thu, 01 May 2008 14:00
		Re: memory allocation on Macs By: pgrigis on Thu, 01 May 2008 13:56
		Re: memory allocation on Macs By: Brian Larsen on Thu, 01 May 2008 13:08
		Re: memory allocation on Macs By: pgrigis on Thu, 01 May 2008 12:06
		Re: memory allocation on Macs By: pgrigis on Thu, 01 May 2008 11:25
		Re: memory allocation on Macs By: Rick Towler on Thu, 01 May 2008 11:15
		Re: memory allocation on Macs By: Brian Larsen on Thu, 01 May 2008 10:54
		Re: memory allocation on Macs By: Kenneth P. Bowman on Fri, 02 May 2008 12:39
		Re: memory allocation on Macs By: Karl[1] on Mon, 05 May 2008 13:23
		Re: memory allocation on Macs By: ekvys101 on Fri, 02 May 2008 16:57

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