posted by Nicholas Blachford on Mon 15th Nov 2004 20:42 UTC
IconLast year IBM introduced a cut-down POWER4 CPU called the PowerPC 970 and Apple promptly put it into their PowerMac line. IBM are not standing still; the POWER5 is out and rumours have long hinted at a successor to the 970 being in development. What should we expect?

I predicted [1] that PowerPC would get the lead over x86 CPUs in 2004. With the non appearance of the 3GHz 970 however this has not been the case, though that's not to say it's losing either [2]. However, anyone can select benchmarks which show any processor running ahead of others, the real story is a lot more complex.

According to SPEC benchmarks the Itanium 2 is an absolute killer, outperforming the Opteron quite considerably especially on floating point [3]. There are some interesting scientific benchmarks [4] from last year which show just how erratic benchmark results can be: in some cases the Itanium 2 is shows a clear lead, in others it's the Opteron clearly in front.

Unfortunately, the 970 isn't listed on SPEC but it generally seems to do comparatively badly in unofficial SPEC tests. The 970 is listed in the scientific benchmarks but again the story is less than clear. That said, where gcc 3.3 is used the 970 does badly, while where the IBM compiler is used it does well. Compilers can have a big effect on performance so the figures for all the CPUs could have changed considerably by now.

If there is an overall performance leader in the desktop market right now I'd put my money on AMD's Opteron, though the top 3 desktop processors (Pentium 4, AMD 64, G5) all have relative strengths and weaknesses in different areas, so none of them can be said to have a commanding lead.

The moral of the story is that no one test or set of tests tells the whole story; it's best to run your own tests based on what you intend to run or at least look at tests specific to your application field. Independent tests are best of all, vendor-supplied benchmarks should be taken with a pinch of salt - it's not called "benchmarketing" for nothing! That said, all the top CPUs are fast, but not in the same area.

But could this be about to change? The 970 was based on IBM's high-end POWER4 CPU. POWER5 has since been released and rumours have long hinted at a 9x0 CPU based on it.

POWER5
With new enhanced cores and improved memory and cache systems, IBM's recently released POWER5 has shown itself to be an absolutely stellar performer in a number of different areas; in some cases nearly doubling its performance over the previous POWER4+ model [5].

The POWER5 is better than the POWER4 in a number of ways. Improved cores now offer simultaneous multi threading (SMT) and in order to keep the threads moving the pool of rename registers has been increased.

The POWER5 also has a slightly larger on-die cache which has a better design than before, the same goes for the 36MB external L3 cache. In addition to the improvements in the design of the caches, the L3 cache has been moved so it is now connected behind the L2 cache, reducing its latency considerably. The memory controller has also now been integrated on-die and its bandwidth increased to nearly 20GBytes per second.

A lot of CPUs spend an awful lot of time sitting around waiting for data. A lot of design effort in the POWER5 seems to have been concentrated on getting data to the CPU cores faster and this has paid off as shown by the improved benchmarks.

In addition to these improvements, internal changes mean the cores run at a higher frequency than the previous generation.

All in all, the POWER5 is a very impressive processor, especially given that it is implemented in a previous-generation (130nm) silicon process and IBM use a deliberately conservative design process.

If rumours are to believed, IBM are working on a desktop version of this CPU. The question is can IBM translate these sorts of performance gains to the desktop? We can't expect performance like POWER5 but what can we expect?

Table of contents
  1. "G6 Power, Page 1/3"
  2. "G6 Power, Page 2/3"
  3. "G6 Power, Page 3/3"
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