The Dual Core Dual

Both Intel and AMD, but particularly Intel, are making a big thing about their dual Core CPUs – effectively two CPUs on a single “package” and are head to head to get sales of them.

For buyers these CPUs raise questions, such as: -

Is it worth buying a dual core system rather than one with a single core CPU?

If a dual core system is better, which supplier’s CPU is better for the job?

How do the Dual Core systems from Intel and AMD differ (if at all)?

To provide the answers it’s worth starting with a bit of background.

When they introduced the Pentium 4 Intel’s designers bottled in a problem that has now come back to bite them – the Pentium 4 has to run at high clock speeds to deliver its performance. The difficulty Intel now face is that as a result of the original design decisions, squeezing more out of the Pentium 4 architecture means raising the already high clock speeds and this is getting harder and harder. Circuit delays mean that the chip must be ever more compact to run at higher clock speeds and this drive for compactness is now seriously restricted by circuit delays and the problems of laying down circuits that are only a few molecules wide.

Heat dissipation is also a massive obstacle - as it goes up by an exponent of the clock speed. Hence, assuming some changes in the silicon, a 25% increase in clock speed can easily result in 50% increase in heat dissipation. It is a formidable engineering problem to find a way to get the increasing amounts of heat out of a shrinking CPU core. Further to counter the architectural issues of the Pentium 4, Intel have been increasing the amount of second level on chip-cache and this extra on-chip memory adds even more to thermal output.

The AMD design team went a different route with the Athlon 64, producing an architecture that yields more per clock cycle. The result is that the AMD Athlon 64 3000 only has to run at 1.8GHz to deliver roughly the same performance as a Pentium 4 530 (which is clocked at 3Ghz). At 3.7Ghz (the fastest Pentium 4) Intel’s engineers seem to be signalling that they can not go too much further while at 2.8Ghz the Athlon FX5700 can produce better performance and yet still AMD have leeway to further jack up performance.

Comparing power consumption for similarly configured systems shows one practical result of the Athlon’s lower clock cycle, with the Athlon 3000 using about 150watts and the Pentium 530 200 watts. In practice this means that the Pentium CPU needs to dissipate around 50 watts more heat.

So how to squeeze more performance from an architecture that has been stretched to the limit and has major thermal problems must have been really worrying Intel’s management, particularly when their arch rivals, AMD, have some way to go before their architecture hits the speed-up buffer. The result of all this head scratching at Intel is Dual Core. If you can’t get a CPU to run faster and have pretty much lost the single CPU power battle to your competitor then sell the customer two slower ones - a particularly attractive commercial solution as the chip-makers can now get both CPUs onto a single package costing little more to make than the single CPU version.

Over at AMD, Dual Core is seen in a different light and is as a way to give users who can benefit from a multiple CPU system a solution without having to resort to the extra cost of a full blown dual CPU set-up. This difference in approach is also reflected in the construction of the solutions from the two vendors. Both put two CPUs on a single large lump of silicon however Intel’s architecture is cruder. With the Pentium D the two CPUs share a single system bus and communication between the CPUs and to system RAM all goes through the single bus – which is likely to be a bottleneck if both CPUs are utilised. AMD have significantly re-engineered the Athlon 64 in the Dual Core version to include a high-speed inter-processor communications channel. There is still a common memory controller and “HyperTransport” (memory access channel).

In summary Dual Core is a key strategic issue for Intel while it is a currently a high-end concept for AMD. With their current CPU architecture Intel must sell the Dual Core concept to users who need more CPU power (and those who don’t need it but think they do) or these high end users will end up migrating to AMD. This has been reflected in Intel’s initial pricing with the entry level product, a Dual Core 2.8Ghz CPU (Pentium D 820) being very aggressively priced at around 50% more than the single core 2.8GHz CPU i.e. only adding another £70 or so to the selling price of a system. This is not much over the price of the 3.2Ghz single core CPU – so that it looks an attractive option for many corporate purchasers of higher performance computers. One can ream masses of information from the way Intel’s Machiavellian marketing department set prices and in practice this overlap of prices means that Intel’s marketing department want dual core purchases to replace single core for higher spec PCs. As customers migrate upwards in performance Intel expect to see multiple core CPUs taking a significant chunk of Pentium 4 business.

Over at AMD their marketing department seems, not surprisingly, to have seen Dual Core as initially a “Premium” product i.e. one than generates more profit and does not displace single core sales. Hence AMD’s starting point was a far more powerful dual core 3500+ CPU (dual 2.2GHz) that was priced at a premium level i.e. at around three times the price of the single 3500+ CPU. Somewhat in response to Intel’s pricing this price has since been adjusted downwards and a lower entry level CPU introduced (a dual core 3200+ CPU) but one that is still about twice the price of the single core version. This suggests that AMD still see dual core as a premium product and indeed what else should they think as their single CPU architecture has plenty of leeway to cope with speed ups.

Overall, Intel would like users to buy the multi-core concept as soon as possible i.e. before they decide that an AMD solution is the only way to get more CPU power. Intel’s nightmare scenario is that if the bulk of customers have still not bought the multi core concept within a couple of years then many power users are likely to migrate to AMD.

So, having given all this background, is Dual Core worth going for at present? Partly the answer lies in your buying preference. Do you buy only Intel, only AMD or the CPU that is most cost effective?

Intel Purchasers

If you prefer Intel CPU’s and are buying higher performance systems then Dual Core does look attractive price wise, presumably 2 x 2.8GHz of CPU power equals 5.6Ghz - much better than one 3.2GHz CPU? There are though some down sides of Dual Core.

First is the heat and power issue. Current single core Pentium 4s are already dissipating uncomfortably large amounts of heat and yet a typical Dual Core Pentium system is likely to dissipate yet another 50 – 70 watts or so more heat than an equivalently priced single core one. So, with the current method of cooling PCs, which involves blowing a hurricane through them, the roar of fans is likely to mark out the room full of Dual Core systems. An office with ten dual core systems could also easily have 3KW being dissipated into the air i.e. air conditioning is a must in summer. The cost of the extra power should also not be overlooked as it could well add £50 or so to the power bill for the system over its working life.

Next there is the motherboard chip-set issue. The Dual Core Pentium 4s will not run on any of the common motherboard chip-sets - only a small number of very recently introduced ones. New motherboard chip-sets tend to mean buggy hardware and BIOS which take a few months to fix. This is a good reason to hold off a bit on other than trial purchases until the products stabilise – unless there is an overwhelming application case for Dual Core.

Finally of course comes the question of the value of having two CPUs rather than one and this all depends on the workload. Multiple CPUs may be highly desirable on a server or mainframe that is handling many simultaneous jobs that can be nicely distributed around the CPUs but the arrangement is less likely to be of major benefit on an office PC or a workstation. True, any PC has to run lots of itsy bitsy background tasks such as operating system, I/O and anti-virus and these can be run in the background on the second CPU. But at the end of the day it is likely to be one or two major tasks that are the issue if a single core Pentium 4 system is CPU bottlenecked. Hence for many users a second core is unlikely to be of much benefit and indeed if a user decides to buy a dual core system rather than a faster single core then the effect may well be counterproductive. Remember also that many systems that appear to be CPU bound are in practice throttled back by memory bandwidth and a Dual Core Pentium system still has to rely on a single 800MHz front Side Bus and the same memory (and hard disk) sub-system as a single core one.

Of course this situation will be inverted if the user is running or can run more major tasks simultaneously e.g. trying to compile two program modules at once or is printing a major graphics file and can get on with something else that uses the second core while the first one processes the print job. Similar reasoning applies if the application and/or the compiler can make use of multiple CPUs. For example rendering and ray tracing are quite likely to gain significant benefit from having a second CPU core available. For such users a Pentium D solution makes sense.

AMD Purchasers

If you tend to buy AMD then the situation is a bit simpler as the Athlon 64 still has a way to go with their single core architecture and are not providing the same financial incentive to induce users to go multi-core. So it is still significantly cheaper to get more power by just getting a faster single CPU system. The current top end “commodity” CPU is the Athlon 64 4000+ and if this is not fast enough then there is still the FX5500 and 5700. However these are expensive and then the nature of the workload needs to dictate if a Dual Core system is preferred – much as it does with Intel as above.

If Dual Core seems attractive with AMD then there are not the same drawbacks as with the Intel Dual Core solutions as:-

Dual Core Athlons will work on the current socket 939 motherboards – reducing the risk of chip-set and BIOS issues.

Athlon 64 CPUs dissipate significantly less heat than their Intel equivalents and they have a better “Cool and Quiet” system (which regulates the CPU as workload drops and hence reduces power consumption under low CPU load). The overall result is that heat is not such a big issue with Athlon 64 single and Dual Core CPUs as it is with the Pentium 4. (That AMD CPUs run cooler than their Intel equivalents is a major turn round as historically keeping AMD CPUs cool has been a bigger problem than with Intel’s – but no longer).

The Floating Voter

If you don’t have a CPU supplier preference and need a high performance workstation then first of all you need decide which of the basic single CPUs (Pentium 4 or Athlon 64) is better. Generally AMD are likely to be the winners as their high end CPUs run cooler are generally faster and are cheaper. “Generally” is an important word here as the major architectural differences mean that there are some situations where the equivalent Pentium 4s can run quicker. The later models include larger caches, which while not being particularly efficient can still score if a kernel can nestle nicely in the cache. For example, the AMD CPUs win out on most things like rendering and ray tracing and most of the Science Mark 2 benchmark suite and when running Linpack with smaller matrix sizes. However larger (but not too large) matrix sizes give the 2Mb cache Pentium 4s (600 series) the edge – as the 2Mb cache allows a matrix to sit in the Level 2 cache.

If you have a 64-bit operating system and application then this is likely to mean that AMD will move further ahead.

Next you need to think if your workload will benefit from a multi-core approach. If it does not then AMD will generally stay as the winner as they have the benefit of their better single CPU architecture, better dual core architecture and lower power consumption. Again “generally” is important as at the lower end a rich multitasking 32-bit work mix that does not involve too much memory hammering is likely to run pretty well on a cheaper Pentium D 820 or 830 system. In that situation the Intel product makes sense.

The Future

CPU vendors want us to continue to buy faster CPUs at higher prices and the fabricators are making bigger and bigger chips. In “mainframe” computing single CPUs systems are a thing of the past and multiple processors within a single system commonplace and PCs have typically followed a few years behind mainframes. In reality, whatever the arguments pro and con are today for Dual Core, five years down the line a multi-core CPU package is likely to be a feature of most, if not all, new upper end PCs.