IA64 vs. Other 64-bit CPUs?

moZer asks: "There are countless reviews and comparisons between Intel's P4 and AMD's Athlon, but so far I haven't seen any benchmarks of IA64 versus other 64-bit CPUs. Is there anyone out there who has experience from working with the IA64 that can say something about its strengths and weaknesses?"

Comments

UltraSPARC I

[spinlocked]

UltraSPARC I was Sun first 64 CPU, back in 95 as I recall. Of course you needed to wait until Solaris 7 before you got a 64 OS and hence the ability to use a 64bit address space. USI chips are detected by most 64bit versions of Solaris and it reverts to 32bit mode (I have some pre-beta UltraSPARC I hardware). This can be overridden, but leaves you vulnerable to a user land hack, which can hang the box.

Running at 167MHz these chips were hotish for their time, but compared to USII (now at a maximum of 480MHz) or USIII (just recently 1050MHz) they are rather slow. Every three years or so Sun rework the SPARC design to have better pipelines, better prediction, more TLBs etc. and speed increases in-between odd number releases are just fabrication improvements. Sun is a chip design company not a chip fabrication company.

It's hard to compare Itanium with SPARC, PA-RISC, PowerPC and Alpha - as far as I know there are no benchmarks in which is performs very well against modern 64bit RISC chips, Integer and particulary FP performance is generally considered rather inferiour.

The true test of a server class CPU is how well it handles cache coherency and memory latency issues on machines designed to support 8 or more CPUs. Itanium has not been shown to scale to these numbers. This may of course be because it's not yet been used in a server platform which supports that number of CPUs.

What I find particularly intriging is how Intel's marketing department is going to handle the clock speed differences in their product range. They have always used MHz as a marketing tool, but now they're going to have to concede that their prestigious server CPU is almost half the clock speed than that of their desktop CPU.

IA-64 isn't the best choice

[acidblood]

To start off, there's an error in your question. There's no such thing as ``the IA-64 CPU.'' IA-64 is the instruction set architecture behind Itanium and the around-the-corner McKinley, and while I could list all the features and shortcomings behind it, it'd be a boring and technical explanation.

You can jump to conclusions, if you wish, by looking at Ace's Hardware SPECmine database, which contains all current SPEC2000 results. In case you don't know, those are the industry standard benchmarks. Here is a sample query of the SPEC2000 integer benchmarks, and here is a sample query of floating point results. As you can see, there are always better choices than the Itanium, and mostly from vendors who have been in the field of 64-bit processors for long, and probably with better prices too. You can never underestimate how important vendor reputation and experience is.

I alluded to the shortcomings of the instruction set of Intel's 64-bit offerings. Indeed, the poor performance can mostly be attributed to it (although the Itanium's poor design has helped a lot -- let's see whether McKinley fares better.) The truth is, Intel took VLIW and redressed it as EPIC; but VLIW has never been a panacea. Serial designs with out of order execution have been around longer, and worked great. The Itanium is a strict in-order processor, and the SPECint results show. And compiler technology isn't there yet; but Intel has acquired Alpha from Compaq and employed their compiler and MPU design team, widely reputed as the best in the field. Whether clever design will be able to mend the instruction set flaws, only time will tell. Indeed, the best strategy now is to wait a few years; seeing as how hardly a thousand Itanium system were actually purchased and paid for, most people seem to be taking this route. With McKinley the situation will be better for Intel, but not until the 3rd or 4th generation will the mass purchases begin, if ever.

Another common misconception is about the performance of Sun hardware. Just look at the values linked above from the SPEC benchmarks. Sun is known for scalable, reliable hardware, stuff you can depend on. But they're not the best performers by any account. The best designs come, undoubtedly, from the Alpha team; the upcoming EV8 would be the most advanced processor for a long time to come, if it weren't for Intel (who cancelled the EV8 project, obviously.) Unfortunately, Alpha is no longer a good buy, though not by technical merits. Having a vendor who will vanish sometime in the future is never a good strategy. Luckily, IBM's POWER series and HP-PA still remain, although the Precision Architecture will be discontinued some time in the future as well. IBM's Power4 design is the current king of the hill in SPEC scores, beating the closest competitors by a fair amount.

Finally, a great source of information is the Real World Tech forum, and the ``Silicon Insider'' columns by Paul DeMone, on the same website. (Paul also regularly reads the forum, and posts quite frequently too.)