Analysis: Dual Core technology

By David Fearon

Posted on 27 May 2005 at 11:02

After the massive acceleration of Intel's dual-core processor plans, dual core has arrived. And it's set to make a huge impact.

The processor previously codenamed Smithfield has become two parts: the Pentium D and the higher-end Pentium Extreme Edition (EE). One of the first dual-core PCs in the country was hand-delivered to us by Intel.

Inside the machine was a Pentium EE processor, with two independent execution cores each running at 3.2GHz. Note that the name is 'Pentium Extreme' and not 'Pentium 4 Extreme'. Intel has dropped the Pentium 4 name for its dual-core CPUs, although it seems likely it will be retained for its future single-core products (see The roadmap, below).

What's the difference?
Whereas the original EE processors were essentially repackaged Xeons, the new EE is a direct variant of the Pentium D. Both have 1MB of Level 2 cache per core, an 800MHz front side bus, plus EM64T and Execute Disable Bit support. The only significant difference is that the Extreme has Hyper-Threading on both its cores, which the D lacks. The EE is also clock multiplier unlocked to allow for overclocking.

Compatibility
The new parts come in the same LGA775 package as current Pentium 4s, but dreams of being able to pop one into an existing motherboard are misplaced. Dual-core designs won't work in current 915 and 925X Express
chipset motherboards; the system will immediately shut itself down. You'll need a new motherboard with one of the two new chipsets: 945 and 955X. Our test system was supplied with a 955X-equipped motherboard in the form of an Intel D955XBK. Technical details were still sketchy at the time of going to press, but it seems clear that they're relatively minor tweaks of the existing 915 and 925 chipsets (see issue 118, p50).

Only 3.2GHz?
For the time being, yes. Intel itself makes little secret of the fact that the basis of the first dual-core processors is the standard Prescott core, with some extra arbitration logic to allow for such things as power management. With two to cool via one heatsink, they can't run too fast.

Beyond Hyper-Threading
Despite Intel's enthusiasm, Hyper-Threading - where a single-core processor allows two threads to share execution resources simultaneously - has been only a qualified success. The problem for developers has been the relatively opaque low-level mechanisms by which the CPU handles threaded code. It's difficult to determine, ahead of time, if the performance boost will be worth the effort or if resource contention will hamstring the process. Dual core changes the landscape. Developers can look at their code at the algorithmic level, determine which operations can usefully be executed in parallel and get a very good idea of the benefits before they start coding. Since dual-core processors are basically two completely
separate CPUs in one physical package, you can be practically guaranteed that two independent operations taking ten seconds each will take ten seconds on a dual-core design but 20 seconds on a single core. True dual core is a performance no-brainer, so long as the application in question is algorithmically suited to multithreading (see The pitfalls of multithreading).

Don't Forget your Thermals
Contrary to various rumours circulating before their release, the new dual-core parts aren't especially power efficient: they consume less power per core than current top-end Pentium 4s, but only by dint of their relatively low clock speeds. The TDP (thermal design power) of both the Pentium D and Pentium EE is 130W - higher than any previous desktop part. That said, the new parts do feature EIST (Enhanced Intel SpeedStep Technology), reducing frequency and voltage when possible under operating