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dual XP

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Frans

Registered
Joined
Jan 28, 2003
Hi all,

I've read the two articles describing how to run two XP CPU's in a dual configuration and an extensive post here at overclockers.com in the same topic. But I still really don't understand the actual difference between XP and MP processors, except the bridge on some batches. So.. What exactly differ an AMD XP cpu from an MP?
Two XP cpu's in an dual configuration is said to be stable.. is it really 100% stable and if not - what is causing it?

A doc published by Dave Jones concerning upcoming features in the new 2.6 kernel mentions this:

Running certain AMD processors in SMP boxes is out of spec, and will taint
the kernel with the 'S' flag. Running 2 Athlon XPs for example may seem to
work fine, but may also introduce difficult to pin down bugs.
In time it's likely this tainting will be extended to cover other out of
spec cases.

So, some of the best engineers says it's not that stable..

Question1:
Difference between XP and MP

Question2:
Is really dual XP stable?


regards,
Frans Englich
 
I currently run two XP's modified to appear as MP's by connecting that L5 bridge.

From what I understand and have read there is physically no difference between the two chips other than the L5 bridge change.... On the other hand there are some differences in quality control...

The MP's go through extensive testing and qualifications to ensure that they will run reliably and error free in a dual processor environment. They have also been reportedly the cream of the crop in terms of AMD, selects the best chips from its batches and designates them MP's. That is just hersay, that I've read from others and have no way to verify that second claim.

On the other hand XP's do not go through all of that testing to ensure complete compatibility/stability in an MP environment.

So with XP's its luck of the draw that the chip will perform correctly in an MP environment. I would suspect that there maybe quite a few that exhibit intermitten bugs, and seemingly random errors that don't effect most people most of the time.

Realistically if I wanted absolute gaurntee's that the system would perform reliably, I'd make sure I bought a couple of MP's. I'd also buy on high quality registered ECC ram too.

On the other hand more than likely the XP you buy will be just fine, no way to know other than experience. I personally run XP's in dual. and have had what I would think as an extremely stable system even while overclocked, runs prime95 forever, and runs all my software without crashing. but this is just my experience running my apps. I've really not heard too many people who run XP's that have run into problems. But then again everyone I talk to are hobbiest's and don't run applications for the most part that require abo****e stablity, and accuracy. Most of us are willing to live with a few glitches, not that I've seen any. Which is different than how a NASA engineer might see stablity for a system that plots rendevouz burns for the 2.5billion shuttle.
 
The difference between XP's and MP's are minimal. The MP has unlocked L1 bridges and joined L5 bridges to allow SMP use. Unlike Intel, AMD's SMP implementation is not down to the CPU's ability, rather the chipset. What I found interesting is that AMD's own tech docs had both the XP's and MP's listed as:

Multiprocessing support: point-to-point topology, with number of processors in SMP systems determined by chipset implementation

Read into that what you will, but I don't believe the myth that a few XP's will have bugs or won't work when correctly L5 modded.


Q2: Is Dual XP's Stable?

A: Sure is:

Code:
> uname -a; uptime
Linux TIGER3 2.4.19-gentoo-r10 #2 SMP Mon Jan 27 19:26:07 GMT 2003 i686 AMD Athlon(tm) MP 1900+ AuthenticAMD GNU/Linux
 20:03:35  up 79 days, 23:05,  4 users,  load average: 2.12, 2.00, 1.92
 
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I have five AMD duallies, all running modified Athlon XP's. All systems are stable and have worked perfectly.

Furthermore, I haven't personally heard from ANYONE who has had problems with their duallie, which can definitively be attributed to running dual XP's. I have heard of people blaming their problems on their XP's, but I haven't heard of ANYONE who has swapped their XP's for MP's and seen the problems go away.

There are different reasons you could have stability problems with a duallie. Common sources of problems are: insufficient or faulty power supply, faulty RAM, insufficient cooling, bad memory settings (too aggressive,) and power regulation problems (caused by the motherboard.)
 
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