Mac Pro and RAM: The saga continues

Posted by Pierre Igot in: Macintosh
February 7th, 2007 • 11:42 am

On Feb. 1, I wrote about a problem I was having with Apple RAM modules for my Mac Pro. I had 2 x 512 MB and 2 x 1 GB of RAM. If I installed only the 2 x 512 MB or only the 2 x 1 GB, the Mac Pro worked with no problems. But as soon as I installed both the 2 x 512 MB and the 2 x 1 GB modules, following Apple’s instructions for the installation (i.e. putting 2 modules in the top riser and 2 modules in the bottom riser), I would get kernel panics all the time. I also sent a report on this problem to the ever-reliable MacInTouch web site.

I phoned Apple about the issue and they elected to send me replacement modules for the 2 x 1 GB of RAM, which arrived yesterday. I tried with the new modules, and I had the exact same problem with kernel panics when following Apple’s instructions for the positioning of the modules.

Meanwhile, on Feb. 2, MacInTouch reader Eric Taylor replied with very valuable information about “2-way interleaving” and “4-way interleaving” and suggested that I try installing all 4 modules in the top riser.

I did just that, and am happy to report that this appears to have eliminated the kernel panics!

Now, it should be stressed that putting all 4 modules in the top riser directly contradicts the instructions in Apple’s own manual for the Mac Pro (as well as the visual instructions printed on the inside of the Mac Pro’s side panel).

But, according to what Eric Taylor says, the instructions given by Apple are only the procedure that is required if you want the Mac Pro to use 4-way interleaving, which normally gives higher memory performance. If you put all 4 modules in the same riser, the Mac Pro still works, but only uses the RAM in 2-way interleaving, which gives lower memory performance.

Out of curiosity, I tried other combinations:

  • 2 x 1 GB in riser A and 2 x 1 GB in riser B gives kernel panics
  • 2 x 1 GB and 2 x 512 MB in riser A and 2 x 1 GB in riser B gives no kernel panics, but the Mac Pro does not see the memory in riser B and says I only have 3 GB of RAM installed, all in riser A.

So sometimes the Mac Pro kernel panics, sometimes it simply fails to detect some of the RAM altogether.

Today (Feb. 7), I got back on the phone with Apple and was transferred to an “engineer.” I explained the whole thing again. However, this particular engineer did not seem to be aware of the interleaving thing. In his view, the tests we had done showed that there was nothing wrong with the RAM modules themselves, but possibly with the riser cards, or with the logic board. He started talking about taking the Mac Pro to a service provider to get it properly tested.

I mentioned the fact that the closest provider is a 3-hour drive away, and that I had already gone through a lot of back-and-forth with the provider with my G5 Quad last year, which turned out to be beyond repair and was eventually replaced with… this very Mac Pro that I am now having problems with!

I reiterated, once again, what MacInTouch reader Eric Taylor said about interleaving, and he said that he was actually going to try and talk to other engineers and see whether they would suggest replacing the riser cards, or something else, before asking me to go on the 6-hour round trip.

He gave me his contact information and said he would get back to me later on today or tomorrow.

I must say that I find it a bit surprising that even Apple’s own engineers appear to be unaware of some of the basic features of their latest hardware. But I also understand his approach, which is to treat this is as a situation possibly indicating another kind of hardware defect. In any case, I am glad that he was understanding enough and will try to communicate with other engineers before forcing me to endure yet another trip to my service provider.

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