On Fri, Sep 23, 2005 at 10:43:40AM -0500, Ron Johnson wrote:
> On Fri, 2005-09-23 at 14:45 +0200, Malte Cornils wrote:
> > Hello,
> >
> > we've been trying to make a program (ITK/VTK image processing for a
> > university
> > project) work. Unfortunately, the process needs slightly above 2 GiB of
> > virtual memory.
> >
> > Judging from the documentation I've seen, on 32bit systems I should be able
> > to
> > allocate up to 3 GiB of virtual memory (1 GiB of 32bit address space is
> > reserved for the kernel).
> >
> > However, our test case for this terminates when trying to allocate more
> > than 2
> > GiB of memory, even though we have a really big swap file.
> >
> > Does anyone have a clue why 2 GiB is the limit?
>
> http://www.puschitz.com/TuningLinuxForOracle.shtml#AddressMappingsOnLinux
> 0GB-1GB User space - Used for executable and brk/sbrk allocations
> (malloc uses brk for small chunks).
> 1GB-2GB User space - Used for mmaps (shared memory), shared libraries
> and malloc uses mmap (malloc uses mmap for large
> chunks).
> 2GB-3GB User space - Used for stack.
> 3GB-4GB Kernel Space - Used for the kernel itself.
>
>
> > Would using a swap
> > partition
> > instead of a swap file help? The 64G HIGHMEM kernel config option does not
> > seem to make a difference. On a real AMD64 system, the program works fine
> > with >2 GiB RAM, as was expected. Is there any way to make this work,
> > besides
> > changing the algorithm to use less than 2 GiB of memory?
>
> No.
Actually, some kludgery might help here.
It seems malloc restricts itself to the user space.
But if you were te replace malloc (or provide your own allocation
and freeing methods, which *miht* be possible in C++) you could do
the following.
Near the start of your program, allocate a *huge* array on the stack, like
char * hugepointer;
int main(...)
{
char huge[1000000000];
hugepointer = huge;
restofprogram(...);
}
Then have your private malloc (or allocation methods)
allocate space within huge, and when it's full, use
calls to the regular malloc for the rest.
-- hendrik
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