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DateMon, 16 Apr 2007 23:08:53 -0600
FromRobert Hancock <>
SubjectRe: Memory Allocation
Brian D. McGrew wrote:
> Good evening gents!
>
> I need some help in allocating memory and understanding how the system
> allocates memory with physical versus virtual page tables. Please
> consider the following snippet of code. Please, no wisecracks about bad
> code; it was written in 30 seconds in haste :-)

(snip)

> My test machine is a Dell Precision 490 with dual 5140 processors and
> 3GB of RAM. If I reduced kMaxSize to (2048 * 2048 * 236) is works.
> However, I need to allocate an array of char that is (2048 * 2048 * 256)
> and maybe even as large at (2048 * 2048 * 512).
>
> Obviously I have enough physical memory in the box to do this. However,
> I suspect that I'm running out of page table entries. Please, correct
> me if I'm wrong; but if I allocate (2048 * 2048 * 236) it work. When I

Pretty sure you're wrong.

> increment to 256 or 512 it fails and it is my suspicion that I just
> don't have enough more in kernel memory to allocate this much memory in
> user space.

Are you using a 32-bit kernel? If so, most likely you're hitting a limit
of the address space layout - there's just not enough room in the
address space for an allocation of this size.

>
> Because of a piece of 3rd party hardware, I'm forced to run the kernel
> in the 4GB memory model. What I need to be able to do is allocate an
> array of char (2048 * 2048 * (up to 512)) in user space *** AND *** I
> need the addresses that I get back to be contiguous, that's just the way
> my 3rd party hardware works.
>
> I'm inclined to believe that this in not specifically a Linux problem
> but maybe an architecture problem??? But maybe there is some kind of
> work around in the kernel for it??? I'd find it hard to believe that
> I'm the first one that ever needed to use this much memory.
>
> I ran this same code on two difference Macs. One of them a Powerbook G4
> with 4GB of RAM and it was successful. The other was a Macbook Pro with
> 4GB of RAM and it failed. Both running OS 10.4.9. And of course it
> runs just lovely on my Sun workstation with Solaris. Thus, I'm thinking
> it's an Intel/X86 issue!
>
> How the heck to I get past this problem in Linux on the X86 plateform???

--
Robert Hancock Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@nospamshaw.ca
Home Page: http://www.roberthancock.com/

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