Apple's Last PowerPC Temptation

Yesterday, Apple introduced three new Power Mac G5s still based on the current PowerPC architecture. The new G5s are offered in one 2.0GHz configuration, one 2.3GHz and one 2.5GHz configuration, all of which are based on dual-core 90nm PowerPC 970MP CPUs. You can get Apple's pricing and more here but I just wanted to chime in with some of my thoughts on what's being offered:

Dual Core

All of Apple's new Power Mac G5s feature dual-core processors, but what it allows Apple to do is outfit the lower end Power Mac G5s with only a single processor and still offer the same number of concurrently executable threads as the older dual processor G5s. Granted you do lose some performance because the two cores now must share a single FSB, whereas the older dual processor machines had an independent FSB per processor. But any performance loss you'd see there is more than made up by the fact that each core now gets a full 1MB L2 cache.

The previous G5 cores were stuck with a relatively small (by today's standards) 512KB cache. It made the 90nm G5 die very small, but it also meant that performance wasn't as good as it could have been. One thing Johan found in his investigations with the G5 was that memory latency was pretty bad, and a small L2 cache does nothing to hide that.

For the two lower end G5s, the fact that the single CPUs are now dual core doesn't mean much, but the move to a 1MB L2 cache per core should result in a tangible performance increase in a lot of scenarios.

Apple does give up a bit of clock speed at the high end by moving to dual core, with the fastest G5 now topping out at 2.5GHz vs. 2.7GHz. The larger L2 cache will make up for some of that difference, but not all. Obviously the high end G5 now offers more than just a faster clock speed, it now features two dual-core CPUs. But, just like we've seen in the PC world, those applications that exhibit a high level of TLP will appreciate the dual dual-core CPU configuration, while others may actually run faster on the older dual 2.7GHz setup. For the most part, most OS X applications seem to be highly threaded in nature, and my money is on a dual dual-core configuration being the more desirable one.

DDR2-533

With the new G5s Apple has moved to DDR2-533, offering a total of 8.5GB/s of memory bandwidth. Unlike Intel's DDR2 platforms however, the G5s can actually use the added memory bandwidth. The G5 interfaces to the North Bridge via a bi-directional 64-bit FSB running at 1/2 the CPU clock speed. That means for a single 2.5GHz dual-core G5, there is about 10GB/s of bandwidth from the CPU to the North Bridge. For a dual dual-core 2.5GHz G5, that's 20GB/s of bandwidth as each CPU gets its own dedicated FSB. So in this case, there may actually be a tangible performance improvement from going to DDR2-533.

It is irritating that Apple didn't move to DDR2-667 yet, especially on their highest end configuration (and especially because it can use the bandwidth), but given Apple's relatively conservative nature whenever it comes to memory speeds it isn't a huge surprise.

PCI Express, at last

The move to dual-core is interesting, but given that the previous line of G5s were all dual processor to begin with, it's not a huge improvement. In my opinion, the biggest improvement to the new G5s is the move to PCI Express. And here's one thing I really do like about Apple, when they move to a new technology, they really move to it.

There isn't a single parallel PCI slot in the new G5s, instead you've got one x16 slot, two x4 slots and one x8 slot. The other interesting thing is that all of the PCI Express slots use a x16 connector, so although there is only one x16 slot (electrically), all four slots can fit a x16 card. Apple uses this support to their marketing advantage, by mentioning that the new G5s can support up to 8 displays through 4 dual-display PCIe graphics cards.

One thing that truly surprised me was the lack of two x16 (electrical) slots, meaning that these G5s aren't exactly configured for SLI. Given that NVIDIA is the GPU vendor of choice for Apple this time around, with the only PCIe GPU offerings coming from NVIDIA, you would think that Apple would put together a SLI-capable product with this line of G5s. There are a handful of reasons why this didn't happen and none of them really require too much thought. Apple seems to be very sensitive about preserving the usability of PCI (and now PCIe) slots, so occupying the area of virtually all four slots thanks to two large graphics cards probably wasn't at the top of their to-do list either.

Apple's new G5s get their choice of four different PCIe cards: a GeForce 6600 LE, 6600, 7800 GT and Quadro FX 4500. All of the cards support at least one dual-link DVI port, with the Quadro FX 4500 supporting two.

In the End

Apple has honestly done their best to make an attractive non-Intel Power Mac offering and obviously they had to. The dependency of OS X on high speed CPU/graphics communication means that the move to PCI Express graphics was a must, and I am pleased with the way in which Apple made that move. Offering four physical x16 slots, even if they aren't electrically x16, makes a lot of sense, and there shouldn't be any reason for PC motherboard makers to offer something similar. It may be a bit confusing, but the added flexibility is a definite benefit.

The move to dual core and DDR2 are both nice, but they will mean a lot more when we see the move to x86 next year. It is also interesting that none of Apple's performance comparisons are against x86 processors anymore :)