On Thu, Jan 28, 2010 at 02:56:31PM +0100, Tim Edwards wrote:
> We're trying to work out memory limits for 32-bit versions of SL4 and 5.


AFAIC, the effective hard limits for 32-bit Linux are 3 GBytes per user
process (they may have squeezed it up to 3.5 GBytes) and 4 GBytes
of physical RAM for the machine.

If your machine has more than 4 Gbytes of physical RAM, you should
be running a 64-bit OS. In this case, 32-bit processes are still
limited to 3-3.5 Gbytes of memory, there is no way around that.

The "by the specs" "paper" limits may say that 32-bit Linux kernels
can use more memory, and it might even work in practice, but be aware
that it requires that the Linux kernel use some page table magic
and relies on funny Intel CPU extenstions (PAE & co). As I understand,
this leads to noticable drop in performance.

I am now curious why are you interested in running 32-bit Linux
when 64-bit Linux was "invented" specifically to "fix" the
memory limits.


K.O.







> Redhat's page (http://www.redhat.com/rhel/compare/) says that the
> maximums are 64GB or RHEL4 and 16GB for RHEL5 (I guess because they
> dropped the HUGEMEM kernel RPM in RHEL5).
> 
> This page (http://www.scientificlinux.org/documentation/misc/limits)
> says that it's 64GB in SL4 but gives no information for SL5. So two
> questions:
> 
> Does SL4 i386 have a 'HUGEMEM' kernel build or do you just build those
> features into the normal -smp kernel build in order to support 64GB RAM?
> 
> What is the memory limit on SL5 i386?
> 
> Thanks
> 
> Tim Edwards

-- 
Konstantin Olchanski
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