[j-nsp] 32-Bit JunOS on the 64-Bit Routing Engines

Joel jaeggli joelja at bogus.com
Thu Aug 25 13:47:07 EDT 2011

On 8/24/11 06:25 , Keegan Holley wrote:
> Sent from my iPhone
> On Aug 24, 2011, at 9:13 AM, Chris Adams <cmadams at hiwaay.net> wrote:
>> Once upon a time, Keegan Holley <keegan.holley at sungard.com> said:
>>> Interestingly enough my SE told us this is possible at lease on our Mx480 and MX960 boxes. Our lab boxes are otherwise engaged at the moment so we havent tested. One note regarding general computing though.  The processor can only address 4G (3.8 or so actually) of ram with a 32 bit word size.  So even if you get the re's running the 32 bit code they will only register 4G of the precious 16G.
>> Well, that isn't entirely true.  Intel added the Physical Address
>> Extension to the Pentium Pro many years ago (and virtually everything
>> claiming to be i686 compatible has PAE).  PAE allows the OS kernel to
>> address up to 36 bits of RAM (64G), just not all at once.
> I've never heard of this actually being used though.  Maybe I'm wrong though.  Most people just modified their code to support 64 bit and stopped there. I also haven't seen any boards RE's or regular Mobo's with 32 bit procs and support for more that the 4G of RAM.

There are plenty of machines that do. virtually every intel system since
the pentium pro  (except the atom) has the hardware if not the bios
support for doing so, that's not germain to the question of whether it's
feasible/useful in an embedded system. In particular, in a system (like
for example a firewall) where kernel datastructures may represent the
overwhelming source of memory utilization, the  PAE performance hit may
trivially overwhelm the value of any memory that can otherwise be freed
up for userspace.

64bitness has been the prefered approach for intel based servers since
about 2003, but the embedded lifecycle runs on it's own timeline.
>>  In general, a
>> given program can still only see 32 bits, unless it does special bank
>> switching.
>> I don't know about PAE support on FreeBSD or JUNOS, but it does exist in
>> all x86 Juniper hardware.
> Interesting.
>> -- 
>> Chris Adams <cmadams at hiwaay.net>
>> Systems and Network Administrator - HiWAAY Internet Services
>> I don't speak for anybody but myself - that's enough trouble.
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