physical memory support under RHEL for Opteron

[Rick Stevens]

Guy Rouillier wrote:
> On Wed, 31 Mar 2004 11:52:32 -0800
> Rick Stevens <rstevens at vitalstream.com> wrote:
> 
> 
>>Chiu, PCM (Peter) wrote:
>>
>>>Hi,
>>>
>>>I am just about to order an dual Opteron 248 server to run under RHEL AS.
>>>
>>>I would like to double check the physical memory supported - we are getting
>>>8 x 2GB PC3200 or PC2700 memory on this server.
>>>
>>>I have found something a bit obscure on the RH web page:
>>>         ___http://www.redhat.com/software/rhel/as/_
>>>
>>>in that it said:
>>>
>>>**** 64GB is the maximum memory size for X86 systems. Maximum memory 
>>>sizes vary with other architectures.
>>>
>>>Can anyone confirm
>>>a. the maximum memory size for Opterons supported, and
>>>b. the maximum usable physical memory per process on user applications
>>
>>As I understand it, RHEL AS3 is available only as an x86 package (in
>>other words, 32-bit).  As such, it is limited to the same 64GB limit as
>>any 32-bit OS.  The per-process limit is 2^32 bytes (4GB).
> 
> 
> We just got this at work in order to run an AMD64 version of Oracle.  See the following: RH makes an AMD64-specific version of AS3, which is 64-bit.  Something is strange about the chart on that page, as they claim 64 GB on x86 but on 16 GB on AMD64.  How can you get > 4 GB physical memory on   32-bit hardware?  Are they talking about Intel's PAE?

It'd sure be nice if you'd wrap your message lines at 72 characters or
so.

The AMD 64 chip has a 40-bit physical address space with a 48-bit
virtual address space.  The 40-bit address space gives you 1TB of RAM,
the 48-bit virtual space gives you 280TB of address space.  I rather
doubt you'll find any motherboard in the near future that provides that
kind of RAM support for less than thousands of dollars.

I still don't know what AS3 on an X86_64 provides.  If they use the full
48-bit address space for apps, then a process has access to 280TB of
memory.
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- Rick Stevens, Senior Systems Engineer     rstevens at vitalstream.com -
- VitalStream, Inc.                       http://www.vitalstream.com -
-                                                                    -
-       "I'd explain it to you, but your brain might explode."       -
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