Re: strange stack limit behavior when allocating more than 2GB mem on 32bit machine

["Michał Nazarewicz" <m.nazarewicz@samsung.com>]

> On Fri, 21 Aug 2009, Joe wrote:
>> Thanks for your explanation. However as you can see, I got 2GB mem and
>> ~10GB swap, totally 12GB.
>>
>> With ulimit -s 10240(KB), I can allocate 2.5GB, I guess these are in
>> swap, right?
>> With ulimit -s unlimited, as you said, kernel reserved 1GB, stack
>> reserved 2GB, there are still 12-3=9GB left??
>>
>> Why did malloc failed, instead of allocating this abundant swap space?

On Fri, 21 Aug 2009 11:37:50 +0200, Nicholas Mc Guire <mcguire@lzu.edu.cn> wrote:
> Initializing 12GB swap will not change the address space limit unless you
> enabled high-memory support in the kernel (CONFIG_HIGHMEM64G=y in your .config)
> the limit is not the physical memory (RAM+swap) but the 32bit address space.

This won't help either since CONFIG_HIGHMEM64G (ie. Physical Address Extension)
affects only physical (not virtual) address space.  Because Joe has 1 GiB
HIGHMEM64G it will only slow his computer down (virtual->physical address
translation requires one more read when HIGHMEM64G is enabled).  Even with
gazillion GiBs of RAM CONFIG_HIGHMEM64G won't help here because the problem
is *virtual* address space limits which is 32-bit on 32-bit x86 systems
(regardless high memory (be it 4G or 64G) being enabled).

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