There’s a lot of confusion about dual-processor systems. Is there much benefit over a single-processor one? Do you have to use special software, and how do you manage the process? Is there actually any point in having more than one processor?

The facts

In pre-OS X days the problem with dual processor Macs was that only applications which were written to take specific advantage both CPUs would stand a chance of waking up the extra hardware. Back when Apple had its MP Power Macs and Daystar dabbled with its Genesis Mac clones there were no more than a handful of these applications, and even then the multiple-processor support was limited to a few specific features. This led to a crippling chicken and egg situation: users didn’t buy multiple-processor Macs because software developers didn’t support them directly, while those developers didn’t bother making their applications use extra processors because there weren’t enough multiple-brained Macs out there to make it worth their while.

Fortunately, this all changed with Mac OS X. Apple took the essential step of assuming full responsibility for managing the use of multiple processors by ensuring the core of the operating system itself would handle the task itself, as a matter of course. With the heart of the OS in charge like never before and eager to put different tasks onto idle CPUs whenever possible, users could see the benefit of the new generation of twin processor Macs no matter what software they ran.

This doesn’t mean that every application will automatically split its tasks across every CPU in a well-endowed Mac. That’s something which requires explicit multi-threading within the software, putting different parts of an overall task into separate threads and sharing those out across the available processors. Producing multi-threaded software is now much easier for developers, and with OS X’s underlying architecture it makes a certain amount of sense even in single-processor Macs, so it is no longer such a rarity.

Put simply, regular, unthreaded applications send out processing requests sequentially, and the OS simply treats such applications as a complete single general thread. When you run two such applications each will be assigned to a different processor, so one application’s data processing work isn’t forced to queue up and share space with another’s. With modern multi-threaded applications things are broken down further. Each thread is assigned to the most appropriate processor, with the decision-making done by the Mac OS. Either way, it is entirely transparent to the end user. The result is a more responsive, reliable machine even under heavy loads. While this in no way means a dual 2GHz Mac is like a 4GHz single-processor one, it does provide double the muscle of a single CPU equivalent.

Using multiple CPUs makes sense for the processor developers directly, too. As the cycle speed of CPUs rises through the multiple-gigahertz range the problem of heat becomes more and more serious. By using two processors at a given speed you can do roughly as much as the same processor design running at twice the frequency. Following in the footsteps of high-end PC and workstation manufacturers, in 2004 Apple switched from just fans to a design which uses a water-cooled radiator system as well in order to control the heat of the dual processors in its fastest Power Mac G5s.

Variations on the idea of sharing the processor load to reduce heat are being tried, with the two key approaches being twin chip cores in one CPU and full-on multiple CPUs. The idea behind twin-core processors is that they use different parts of their internal CPU structure at different times. By this means, heat can be reduced and therefore processor performance can increase. This is a valuable direction in processor development, although a cynic might add that one of the reasons for the twin core line of development could be because, in the Windows world, only Windows 2000 and XP support actual multiple processors at the OS level, and even there it isn’t quite as well integrated as it is in Unix-based operating systems such as Mac OS X. Until very recently in the PC industry, multiple-processor systems were generally seen as being suited to server setups rather than regular desktop uses.

Unlike older Macs and some other platforms, dual-processor Power Mac G5s and the newer dual core Intel-based Macintoshes manage their twin brains efficiently and automatically, whether your applications are multi-threaded or not. Where they are, you can expect to see real-world performance boosts of around 1.8 times over a single processor of the same GHz rating. Regardless of the internal workings of your software, a dual-processor Mac will be more reliable and responsive across the board.

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