Speed the Fruit

Turning technology from mumbo-jumbo into rich tasty gumbo

� New Apples Versus Comparable Dell | Main | Pro-ReplayTV Goes Forward �

August 15, 2002

Speed the Fruit

Unfortunately, according to Rob Art Morgan who has tested this, the new PowerMacs from Apple that use DDR (double data rate) memory like its Xserve rank-mount unit cannot access the memory any faster than the cheaper and slower SDRAM found in the previous system arch. A controller limits the data rate to 1 Gbps, while DDR could work more than twice as fast. Unfortunately, this makes mincemeat of the architecture, as it bus/memory-bounds 2D and 3D graphics and rendering.

In practical terms, my chart showing the Apple vs. Dell server comparison is now pointless: the new Macs can't compete with the same-price or cheaper Dells because the Macs are just as slow as the old Macs. You can boost the processor speed somewhat (jump to dual 1.25 GHz), but you can't extract nearly enough speed from the memory.

Posted by Glennf at August 15, 2002 7:41 PM

Trackback Pings

TrackBack URL for this entry:

Comments

Apple is very up front with this information. Everything on the system excep the CPU can access the DDR memory at full speed, but due to the way the G4 is currently implemented, the CPU can only talk to the System Controller at bus speed (see an excellent explanation of this at http://www.railheaddesign.com/). Still, with a 167 MHz bus, access to motherboard memory will be faster than ever.

And don't forget that the new Power Macs have 1 MB of level 3 cache per CPU -- and the top-end 1.25 GHz model will have twice that. When the CPU is working with the L1 and L2 cache, memory access is at full CPU speed. When it needs to access the L3 cache, there's a wide bus (128 bits) running at half CPU speed, or 433/500 MHz. It's only when the CPU needs to go to motherboard memory that the 1.33 GBps limitation on memory throughput comes into play.

The rest of the mobo support 2.67 GBps throughput, so things like DMA and virtual memory should be faster than on the old 133 MHz non-DDR machines.

Another issue not mentioned in your Dell comparison -- the Dell's throughput is 1.6 GBps per channel (which I assume means per CPU). That's only 20% better than the Mac (which I believe shares bandwidth with both CPUs, although this is outside my area of expertise).

The Intel Xeon processors have a smaller 512 KB cache per processor, which also means they'll have to access motherboard memory more frequently than a G4 with a 1 MB or 2 MB cache.

Anyhow, it's nonsense to state that your earlier comparison is nonsense just because the G4 CPU can't take full advantage of DDR memory. The G4 still has AltiVec to blow away the Xeon for Photoshop work. And it now uses less costly, readily available DDR memory.

But your comparison would be more fair if you put the 1 GHz model up against the Dell, since that would make prices much closer.

Posted by: Dan Knight at August 17, 2002 4:02 AM

Eh...his tests are fairly specious...I'd like to see some ones that stress the whole system, and look at disk I/O...also, the high end is now the middle, so that does make an ecconomic difference.

I definitely agree that the processor is starved, however. i just don't agree that the BreFeats folks were thorough, nor do I think it tells the whole story.

Posted by: mrmister at August 16, 2002 1:41 PM

Sun	Mon	Tue	Wed	Thu	Fri	Sat
				1	2	3
4	5	6	7	8	9	10
11	12	13	14	15	16	17
18	19	20	21	22	23	24
25	26	27	28	29	30	31