Weeks have passed since Apple's announcement of the Mac Pro, and while we wanted to conclude our look at the Mac Pro much earlier, like many Mac Pro users we ran into some serious performance issues under Windows XP.

With the Mac Pro performance issues resolved and some more time with the system under our belts, we're able to bring you the final part in our Mac Pro coverage. This time we're focusing on upgrading the memory and CPUs in the Mac Pro, as well as looking at its performance as a PC running Windows XP.

As a high end Xeon based machine that can run both Mac OS X and Windows XP, the Mac Pro has the potential to be the power user's dream. Today our task is to find out just how upgradable this machine is and how well it runs XP, if it can truly be your only system if you're a Mac and PC user.

FBD Revisited

Thus far the only real downside we've seen to the Mac Pro is its use of Fully Buffered DIMM (FBD). As we mentioned in our initial article discussing the Mac Pro's specifications, the FBD spec calls for a serial interface between memory controller and memory modules, while allowing the chips on the memory modules themselves to be regular mainstream DDR2 devices. A FBD memory controller talks to an AMB (Advanced Memory Buffer) on each memory module, which acts as a translation hub and buffer for all communication between the DDR2 devices on the module and the requests from the memory controller.

The major benefit of FBD is the ability to feature more memory modules per channel (up to 8 per channel), offering greater capacity for high end servers and workstations than even registered DDR2. The downside to FBD is that there is significant overhead and latency introduced by using a packetized interface and using the AMBs to translate from one interface technology to another (FBD to DDR2).

As we mentioned and proved in our previous articles, the number and configuration of FB-DIMMs in your Mac Pro can affect performance. The Intel 5000X chipset in the Mac Pro features two 144-bit FBD branches, each being the width of two FB-DIMMs (effectively giving the chipset four "channels"). Therefore you need at least two FB-DIMMs in the system (the width of a single FBD branch), but ideally you'd need at least four to have a hope of attaining peak bandwidth.

As some of our readers (and Intel) pointed out, the story doesn't just end at needing four FB-DIMMs. The rank of the FB-DIMMs can impact performance as well, and ideally each of your FB-DIMM modules would be dual rank modules. The rank of a DIMM is determined by dividing the width of all of the devices on the module by the width of the module itself. For example, a single rank FB-DIMM would have 9 DDR2 devices each being 8-bits wide. A dual rank FB-DIMM would be composed of 18 DDR2 devices, each still being 8-bits wide. All of our 512MB FB-DIMMs are single rank modules, while our 1GB and 2GB modules are dual rank.

The story doesn't end with rank though. Because of the dedicated read and write lanes between the memory controller and the AMBs on FB-DIMMs, you can be reading from one FB-DIMM while writing to another. So in theory, if you're running an application (or combination of applications) that have a lot of concurrent reads and writes going on you could stand to benefit from having more than one FB-DIMM per channel.

Based on all of the above information, it would seem like your best bet is to stick as many dual rank FB-DIMMs as you can afford in your system, and if that were the case then we'd be able to move on from here. Unfortunately it's not, because as we mentioned in previous articles, the more FB-DIMMs you have in your system, the higher access latencies will be to those additional FB-DIMMs.

What we then end up with is a tradeoff between more bandwidth and higher latency, so which route do you take? We've done a lot of testing and most of our tests seem to favor the four dual-rank FB-DIMM module configuration, but the number/configuration of modules really depends on your particular needs. We're still testing to figure out what the tangible real world performance differences are between the multitude of memory configurations, but for now just know that if you need maximum bandwidth you'll want 8 dual rank FB-DIMMs, but if you want lower latency you'll want fewer modules. Whether or not you'll see a performance difference will depend mostly on the application(s) you're running.

Third Party Memory Modules
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  • mostlyprudent - Tuesday, September 12, 2006 - link

    I have to agree with the first part of your post. I read the article and thought.."Why?". I have always understood the appeal of a Mac to be that you could upack it and get to work. If we start talking about running other OSs on a Mac or hacking software/drivers for better hardware supprt, then why pay the premium for a Mac? Reply
  • michael2k - Tuesday, September 12, 2006 - link

    Because a Mac is cheaper than an equivalently configured dual dual-core Xeon workstation from Dell orr HP? Reply
  • greylica - Tuesday, September 12, 2006 - link

    I see the rendering results and I Think its really will be a dream machine for 3D Rendering and not a gaming dream machine like allienware ( Now dell ) is, and therefore are a bunch of problems into running XP in this hardware that is not related here, XP is caped to the switch of 3GB and limited to 2GB per app, it´s nearly impossible to compare XP with Mac OS-X, the other downside is that I Didn´t see any review tolding you that you can read XP partition under Mac OS-X or even write to it.
    I Have so many doubts that windows Vista will not be caped in some way after seeing that the switch can really prejudice us to upgrade to another OS that MS is trying to sell.
    I will only switch my opinion about Microsoft when they finnaly release a patch to solve this caped windows in relation to this memory issue under adressing really what the hardware can do. If Mac OS-X haves 16GB of memory, XP only will see 3GB and 2GB per app, PAE ( Page adress extension ) is a nightmare for users and a dream for Microsoft.

    They are saying for all of employees now:
    - Memory Issues ? Push them to Vista !!!

    And it is not all, I discovered that windows 2000 is stronger than XP under heavy loads, and abandoned XP to rendering services, Linux, Mac OS-X and 2K are the best choices if you are a 3D professional and you hve a good workstation.

    Mac-OS-X have its own problems too before 10.3/10.4, it´s limited too to 2GB per app. But right now this is completely solved, giving to you all of your machine.

    I Have certainty to tell that professionals will really benefit from 2- quad core clovertowns under 3D content creation.

    XP was not designed to meet professionals demands, its a S.O. to play games.
    Then, do not polute your Mac, and do not buy a mac to use XP, use a budget computer that deserves XP if you wish to use this.

    If you are a pro, use windows 2000 instead of XP, it´s faster, reliable, secure.
    Windows 2000 helps your productivity. Course... Without the cosmetics...
    Reply
  • Pirks - Tuesday, September 12, 2006 - link

    Are there any decent 64-bit rendering apps like Maya/Lightwave/etc that run natively on XP x64? Reply
  • splines - Tuesday, September 12, 2006 - link

    After seeing what you've said about wishing for a C2D/DDR2 Mac, I'd be interested in how the new iMac C2D with perhaps the 7600GT option stands up in gaming to a comparable PC. Sure, it's nowhere near as configurable (or as boast-worthy) as a Mac Pro, but it does seemingly offer a solution somewhere in the middle.

    That said, the 24" iMac is the only one currently supporting a 7600GT upgrade, and it does boost the price a bit. On a cost - performace ratio, in GPU-limited applications the Mac still seems to be overpriced compared to a PC.

    For interest's sake, however, it's something I'd read carefully.
    Reply
  • JackPack - Tuesday, September 12, 2006 - link

    Just wondering, what's the stepping on the Clovertowns used? Reply
  • Imaginer - Tuesday, September 12, 2006 - link

    I am not entirely convinced of the apple craze. I still prefer my own customability of my system and XP is really not a bad OS to work with. That and I am cheap and really dont want to shell out money for the apple package deal they have going. Reply
  • Calin - Tuesday, September 12, 2006 - link

    I just want to tell you that the MacPros are workstations - and their price reflect that. You buy one for lots and lots of reasons, the last of them are games.
    Those are used to make money - and in some cases, having twice the horsepower lets the employee that use them work twice as fast.
    I do prefer my own customability of my system (even if I barely used it), and XP is (now) not a bad OS to work with. But MacPros are for me just as much as an SGI Octane would be (very useful for tasks I wouldn't touch with a barge pole)
    Reply
  • tuteja1986 - Tuesday, September 12, 2006 - link

    <b>I just want to tell you that the MacPros are workstations - and their price reflect that. You buy one for lots and lots of reasons,</b> the last of them are games.
    Those are used to make money - and in some cases, having twice the horsepower lets the employee that use them work twice as fast.
    I do prefer my own customability of my system (even if I barely used it), and XP is (now) not a bad OS to work with. But MacPros are for me just as much as an SGI Octane would be (very useful for tasks I wouldn't touch with a barge pole)

    What the hell are you talking about :*(

    Windows Workstation with the same spec that cost cheaper could do everything that Mac workstation could do :*( only real real reason i would buy a new Mac pro if i was Video editing.
    Reply
  • Calin - Tuesday, September 12, 2006 - link

    Assuming you want a workstation capable of accessing 16GB of RAM (and using two processors), your options are a bit more reduced. There was an article on Anandtech, and the Mac Pro (the most expensive) was just a couple of hundred dollars more expensive than the sum of its components (and operating system I think). Reply

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