Most modern CPUs are either 32- or 64-bit – this is the number of bits of data used in each instruction. So, 64-bit should be twice as fast as 32-bit? Well, no. It depends on how much you need - if you're performing an operation on a 20-bit number, it will run at the same speed on 64- and 32-bit machines.
This word length can also affect how the CPU addresses the RAM. See the 32- vs 64-bit processors below for how different lengths affect performance.
One of the biggest aspects of CPU performance is the number of cores. In effect, each core is a processor in its own right that can run software with minimal interference with the other cores. As with the word length, the number of cores can't simply be multiplied by the clock speed to determine the power of the CPU. A task can take advantage of multiple CPU cores only if it has been multi-threaded. This means that the developer split the program up into different sub-programs that can each run on a different core.
Not all tasks can be split up in this way. Running a single-threaded program on a multi-core CPU will not be any faster than running it on a single core - however, you will be able to run two single-threaded programs on a multi-core CPU faster than the two would run on a single core.
We tend to think of memory as something a computer has a single lump of, and divides up among the running programs. But it's more nuanced than this. Rather than being a single thing, it's a hierarchy of different levels.
Typically, the faster the memory the more expensive it is, so most computers have a small amount of very fast memory, called cache, a much larger amount of RAM, and some swap that is on the hard drive and functions as a sort of memory overflow.
When it comes to CPUs, it's the cache that's most important, since this is on the chip. While you can add more RAM and adjust the amount of swap, the cache is fixed.
Cache is itself split into levels, with the lower ones being smaller and faster than higher ones. So, in light of all this, it can be difficult to know how different configurations will perform in different situations. Rather than try to work out how computers should perform with different CPU configurations, we've run a series of tests on them to find out how they perform.
The processors we're looking at are:
AMD Phenom II X4 3400Mhz Quad Core (Cache: 4x64KB level 1, 4x512KB level 2 and 6MB level 3) £79.00
AMD Phenom II X6 Six Core 3300Mhz (Cache: 6x512KB level 2, 6MB Level 3) £100.27
Intel i5-2500K 3.6Ghz (Cache: 2x32KB level 1, 256KB level 2, 6MB level 3) £162.43
We're running all of them at their recommended clock speeds. Over-clocking is an art in itself, and could squeeze additional performance out of each of these processors, but it's beyond the scope of this article.
In an ideal world, we'd test each of these with exactly the same motherboard so we could eliminate any differences here. However, different CPUs have different pin setups, and so they don't fit physically into the same motherboards (and they wouldn't work if they did).
We can see that the Intel processor outperformed the AMD ones in almost every area. This isn't surprising, as it costs twice as much as the cheapest one. In a few areas - the Apache static page test for example - it performed twice as well.
What is perhaps more surprising is that it almost always outperformed the Phenom II X6 despite having two fewer cores and only slightly faster clock speed. The only significant exceptions to this were the John the Ripper password cracking test and some of the GraphicsMagic tests. These are a highly parallel test, which could take full advantage of the extra processing in the extra processing power of the X6.
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