A plot that helps explain why the desktop computer industry is in trouble

Desktop computer sales are falling.

Desktop (and laptop) machines are just not improving in speed at the same rate as they have historically, and so sales are on the slide – there is no need to buy a faster machine when the one you have already is not much slower than an expensive replacement.

(I am writing this on a six year old laptop – it is, relatively speaking, quite slow, but still not so slow that I have been bothered to buy a faster machine.)

This lack of increased speed up is because, although “Moore’s Law” of doubling the number of transistors on a piece of silicon still applies, the related effect of “Dennard Scaling” – which kept the power needed by those transistors under control – has broken down.

The solution for this ought to be to use more but less energy-hungry, chips (CPUs). If we can get our software to efficiently use an array of not-so-fast chips rather than a single fast chip then we can still see faster computers. This is why computers (an increasingly mobile phones, because they too will soon be in trouble) are now sold as “multicore” devices.

But this has its limits. Unless we can find better ways for these chips to co-operate in software then current hardware models limit the ability to speed machines up.

And that, sort of, is where my chart comes in.

slowWhat the chart shows is that, using current software designs, adding more processors to tackling a problem can actually decrease the speed at which the problem is processed, and certainly shows no sign of speeding it up.

The chart – not to be taken as gospel as the data is very rough and ready and not properly normalized – shows how many instructions (in a simulator) an array of processors will execute per million or so processing cycles – essentially 10 means100,000), 20 means 200,000 and so on.

But the piece of information the chart does not show is that as we move from left to right the number of processors being applied to the problem is being increased and yet the number of instructions being executed is either more or less constant or even – as the first few processors are added – falling. After about 29,000,000,000 ticks the number of processors being applied begins to fall (from a peak of 16 to 11 at the far right) but again the number of instructions being processed is close to constant on average (the red line).

This isn’t because the code is hopelessly inefficient (for instance the processors are added or removed as new threads of execution come on-stream or finish) but because they compete with each other to access the available memory. This is an example of what computer scientists call “thrashing” – as the cores force each others’ preferred pieces of memory out of the system in an effort execute their own code (and this takes time), only to have the same thing done to them. Processors spend far more time waiting for their code and data to be fetched into memory than they do actually executing it.

Unless and until this fundamental problem is fixed or bypassed then the desktop computing industry will be in recession.

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