Are Two Brains Better Than One?
Dan Knight - 2003.03.24
Q. When Apple couldn't get faster G4s from Motorola, it released dual processor models with the slogan "Two Brains Are Better Than One." Is that really true, or is it just hype?
A. Multiple processor computing on the Mac OS goes back to the quad-processor DayStar Genesis MP 528, which was released in October 1995. At the time, Apple's fastest model was the Power Mac 9500/132. With four 132 MHz 604 processors and the right software, the Genesis could smoke it.
Without the right software, three of the four CPUs spent their time idle. Although DayStar came up with a patch for the Mac OS to support multiple processors - and even licensed it to Apple for some dual processor models - neither the classic Mac OS nor most applications could take advantage of multiprocessing.
Photoshop could, and Photoshop mavens made the quad-processor Genesis line a big hit among the graphics crowd. With faster processors, the Genesis PM Plus eventually offered four 233 MHz 604e processors to this demanding market.
That all ended in August 1997, the same month Apple discontinued the Power Mac 9600/200MP. It wasn't that multiple processors was a bad idea, but the fact that a single 350 MHz 604e or 266 MHz G3 offered comparable performance to two 200 MHz 604e processors in Photoshop - and about 75% more everywhere else.
Introduced in October 2000, the Power Mac G4 promised 500 MHz performance - and finally delivered it in February 2001. Come Macworld Expo in July 2001, Motorola still hadn't delivered a faster G4, so Apple released the "Mystic" (a.k.a. Gigabit Ethernet) Power Mac G4, which has dual G4 processors running at 450 or 500 MHz.
This is when the "Two Brains Are Better Than One" campaign was launched. However, the Classic Mac OS had no more dual processor support than DayStar had offered seven years earlier. A few applications in the graphics and video fields supported both processors, but by and large most Mac applications and the Mac OS itself were unable to benefit from the second CPU.
I'll let history speak for itself: When Apple replaced the Mystic G4s with the 5-slot "Digital Audio" models in January 2002, only one of the five configurations offered two CPUs - and it was a middle of the line unit running at 533 MHz vs. a single 733 MHz G4 on the top-end Power Mac.
At least under the Classic Mac OS, unless you ran programs designed for multiple processors, two brains weren't any better than one.
Mac OS X
That all changed with Mac OS X, which was released in March 2001. From the BSD core through the Aqua display engine, the new operating system was designed to support multiple processors. And although the Quicksilver and Quicksilver 2002 lines had only included one dual-processor model, the Power Macintosh G4 MDD (mirrored drive doors) contained exclusively dual processor models.
Oddly enough, the current Power Mac G4 models have only a single CPU in the entry-level model - and offers it at the lowest cost yet for a Power Mac G4.
But it begs the question: Are two brains really better than one? Put another way, is today's entry-level single CPU 1 GHz Power Mac G4 a more powerful computer than last year's entry-level dual 867 MHz Power Mac G4?
As we're fond of saying at Low End Mac, there are no easy answers. Under the classic Mac OS a 1 GHz single processor machine would outperform a dual processor 867 MHz machine in almost every task - but the new Power Macs don't even boot the classic Mac OS, making that a moot point.
Finding meaningful Mac OS X benchmarks isn't easy. Here's what I've been able to locate:
In a monumental comparison dated March 13, 2003, the new 1 GHz Power Mac is 16% slower than the January 2002 dual 1 GHz model, which is their benchmark. The dual 867 MHz model is only 2% slower than it.
In a file search, the new 1 GHz machine is 13% slower than the dual 1 GHz model, and the dual 867 falls midway between the two at 93%.
In MacSpeedZone's Word and Excel macro tests using Office X, the dual 867 lags at 84-88%, while the single processor 1 GHz model has 90-94% the performance of the dual processor 1 GHz benchmark machine. Office X apparently doesn't take real advantage of the second CPU.
Neither does AppleWorks, which shows similar results.
For serious number crunching, the AltiVec Fractal benchmark show the dual 867 at the expected 87% level compared with a dual 1 GHz machine. Today's single CPU 1 GHz model comes in at 50%, and the old single CPU 867 offers half the performance of the dual G4/867.
MacSpeedZone also tests in a multitasking setup with iTunes ripping, AppleWorks searching and replacing, and the Finder copying - all at the same time. The single processor G4/1 GHz scored 75% on this test, while the dual G4/867 offers 95% the performance of their baseline dual G4/1 GHz machine.
MacSpeedZone also runs several other benchmarks, but some of them seem more directly tied to hard drive and CD-ROM speed than CPU performance. Based on the multitasking results, all else being equal it appears that a dual processor system will offer about one-third more performance when background tasks are taking place.
In an older report, Bare Feats finds that Final Cut Pro 3.0 performs a "render and blur" much faster on a dual G4/533 (132 sec.) than on a single processor G4/933 (180 sec.). Dual 800 MHz and 1 GHz machines are faster yet. Had a dual 933 been available, it would have completed the test in roughly 116 seconds - 35% faster than a single processor machine.
Another set of benchmarks pits the same G4/933 against the same dual processor machines, and even the dual G4/533 outperforms the 933 in the Photoshop, iMovie, and Cinema 4D tests. With the right software, dual processing rocks.
Of course, we should stress here that Bare Feats tends to look for the kind of applications that really demand G4 horsepower, most of which also benefit greatly from dual processors.
In the old days, Macbench 3, 4, and 5 were the standards. Not only did Macworld publish the results, but they also made the benchmarking software available to anyone.
No longer. For the past few years, Macworld has been using Speedmark, their own in-house benchmark suite that seems to get updated a few times a year, making direct comparisons between Macs of different ages almost impossible.
Last December they gave Speedmark 3.2 scores to the original Quicksilver 2002 dual 1 GHz Power Mac G4, the new DDR version, and the then top-end 1.25 GHz Power Mac G4. These scores were 163, 161, and 184 respectively, and those ratings are relative to a 700 MHz eMac, which is assigned a score of 100.
At this point, Macworld has not yet published benchmark results for the new Power Mac G4 models, nor can I find any Speedmark results for the G4/867 dual.
A year earlier, the G4/800 dual processor had earned a Speedmark 2.1 score of 203, the single CPU G4/867 came in at 215, the G4/733 rated 183, and the older dual G4/533 managed 176. This time all scores are relative to a 350 MHz 1999 iMac, which is assigned a score of 100. And Speedmark 2.x was based on Mac OS 9.x, not OS X.
A few months before that, Macworld compared the single processor G4/450 and G4/500 with their new dual-processor replacements - also using Speedmark 2.1. Here are the ratings: G4/450, 146; G4/500, 159; G4/450 dual, 158; G4/500 dual, 165.
Macworld has gone from having consistent, meaningful, widely verifiable benchmarks to creating a proprietary suite of tests that changes from time to time, making comparisons over time virtually impossible.
Where Macworld dropped the ball, Xbench has picked it up. Xbench required Mac OS X 10.2 or later and reports simple scores for memory, CPU, threading, Quartz, OpenGL, the user interface, and the hard drive. Phil's Xbench Benchmarks Site has a nice collection of user submitted results.
There are some oddities. The same computer with different amounts of memory will have different memory scores - the more RAM, the lower the score. Quartz and OpenGL will vary with the OS, drivers, and video card installed. And disk tests will also vary depending on the brand and model of hard drive in the tested computer.
Xbench's CPU score doesn't seem to benefit much from dual processors. Comparing single- and dual-processor G4/533 machines, the dual only scores 8% higher. But at least it gives us a standard benchmark we can use to compare our OS X machines.
Are two brains better than one? Yes, but how much better is the unanswered question. Or, more precisely, the question with no easy answer. The brief form of this article:
Q. How much better is a dual-processor Power Mac than a single processor one?
A. It depends.
For some tasks, dual processors can be nearly twice as fast, so today's dual 1.42 GHz G4 Power Mac could potentially offer comparable performance to a theoretical 2.5-2.8 GHz G4, and last year's dual 867 MHz model could reach the levels of a 1.5-1.7 GHz G4 - if such a thing existed today.
But for other tasks, you may see no difference at all.
One or two CPUs is only part of the equation. Other factors that improve performance are a bigger and/or larger L2 and/or L3 cache, a faster memory bus, whether your system has enough physical RAM or needs to page out to virtual memory (VM) all the time, how fast your hard drive is when VM is being used, what version of the Mac OS you're using, whether Quartz Extreme is offloading display from the CPU to your video card, your video card and screen resolution, how many programs you have active, which programs they are, and what haxies and other system modifications may be installed on your Mac.
Mac OS X is very different from the Classic Mac OS - much more different than you'd imagine by looking at it through its interface. It uses memory differently. It manages resources differently. It handles programs differently - even if one application doesn't support multiple processors, the OS itself can juggle other programs so they make more use of the other CPU to balance things out.
There are no easy answers when it comes to a complex, modern operating system like Mac OS X.
Like I said, it depends.
That said, things are likely to improve as more and more programs are written specifically for Mac OS X instead of simply ported over from the Classic Mac OS or other platforms - and also tuned for multiple processors. On average, we could see dual processor models offering 30% to 50% more performance overall than a single processor machine. Over time, that should get even better. It's mostly up to the developers to offer MP support in their software.
Real World Comparison
Both have a 133 MHz system bus, 1 MB of L3 cache per CPU, Nvidia GeForce 4 graphics, 256 MB of RAM, and a 60 GB hard drive, but the dual 867 also has the ability to boot into OS 9 if/when you need to. Both are currently selling for about US$1,500, but when running OS X, overall performance (thanks to the second CPU) should more than offset the 13% slower clock speed, perhaps offering 15-20% more power on average - and up to 70% more in some instances.
For the same price, I'd call it a no brainer and go for the older dual 867.
Too bad Apple doesn't offer dual processor PowerBooks....
Update: One further benefit of dual-processor Macs comes in an unexpected place - Classic Mac OS performance. With two CPUs, the Classic Environment has full access to one CPU while OS X uses the other CPU to handle keyboard and mouse input, graphics, network access, disk reads and writes, etc. This means that the CPU handling the Classic Mac OS has less to do than it does when booted natively. An unexpected bonus for those of us who still use Classic Mode!
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