Understanding the Cell Microprocessor
by Anand Lal Shimpi on March 17, 2005 12:05 AM EST- Posted in
- CPUs
Final Words
Concluding anything about Cell requires a multifaceted look at the architecture and the platform as a whole.First from the perspective of the game industry, more specifically Playstation 3:
Cell’s architecture is similar to the next version of Microsoft’s Xbox and upcoming PC microprocessors in that it is heavily multithreaded. The next Xbox will execute between 3 and 6 threads simultaneously, while desktop PC microprocessors will execute between 2 - 4. The problem is that while Xbox 2/360/Next and the PC will be using multiple general purpose cores, Cell relies on more specialized hardware to achieve its peak performance. Cell’s SPEs being Altivec/VMX derived is a benefit, which should mean that the ISA is more familiar to developers working on any POWER based architecture, but the approach to development on Cell vs. development on the PC will literally be on opposite ends of the spectrum, with the new Xbox somewhere in between.
The problem here is that big game development houses often develop and optimize for the least common denominator when it comes to hardware, and offer ports with minor improvements to other platforms. Given Cell’s architecture, it hardly looks like a suitable “base” platform to develop for. We’d venture to say that a game developed for and ported from the PC or Xbox Next would be under-utilizing Cell’s performance potential unless significant code re-write time was spent.
Console-only development houses, especially those with close ties to Sony, may find themselves able to harness the power of Cell much more efficiently than developers who ascribe to the write-once, port-many process of cross-platform development. Given EA’s recent acquisition and licensing-spree, this is a very valid concern.
With Cell, Sony has effectively traded hardware complexity for programmer burden, but if anyone is willing to bear the burden of a complicated architecture, it is a game developer. The problem grows in complexity once you start factoring in porting to multiple platforms in a timely manner while still attempting to achieve maximum performance.
As a potential contender in the PC market, Cell has a very tall ladder to climb before even remotely appearing on the AMD/Intel radars. The biggest strength that the x86 market has is backwards compatibility, which is the main thing that has kept alternative ISAs out of the PC business. Regardless of how much hype is drummed up around Cell, the processor is not immune to the same laws of other contenders in the x86 market - a compatible ISA is a must. And as Intel’s Justin Rattner put it, “if there are good ideas in that architecture, PC architecture is very valuable and it will move to incorporate those ideas.”
Once again, what’s most intriguing is the similarity, at a high level, of Intel’s far future multi-core designs to Cell today. The main difference is that while Intel’s Cell-like designs will be built on 32nm or smaller processes, Cell is being introduced at 90nm - meaning that Intel is envisioning many more complex cores on a single die than Cell. Intel can make that kind of migration to a Cell-like design because their microprocessors already have a very large user base. IBM, Sony and Toshiba can’t however - Cell must achieve a very large user base initially in order to be competitive down the road. Unfortunately, seeing a future for Cell far outside of Playstation 3 and Sony/Toshiba CE devices is difficult at best.
The first thing you have to keep in mind is that Cell’s architecture is nothing revolutionary, it’s been done before. TI’s MVP 320C8X is a multi-processor DSP that sounds a lot like Cell: http://focus.ti.com/docs/military/catalog/general/general.jhtml?templateId=5603&path=templatedata/cm/milgeneral/data/dsp_320c80&familyId=44. So, while Cell is the best mass-market attempt at a design approach that has been tried before, it doesn’t have history on its side for success beyond a limited number of applications.
Regardless of what gaming platform you’re talking about, Cell’s ability to offer an array of cores to handle sophisticated physics and AI processing is the future. AGEIA’s announcement of the PhysX PPU (and the fact that it’s been given the “thumbs up” by Ubisoft and Epic Games) lends further credibility to Cell’s feasibility as a high performance gaming CPU.
The need for more realistic physics environments and AI in games is no illusion; the question is will Intel’s forthcoming dual and multi-core CPUs (with further optimized SIMD units) offer enough parallelism and performance for game developers, or will the PPU bring Cell-like architecture to the desktop PC well ahead of schedule? The answer to that question could very well shape the future of desktop PCs even more so than the advent of the GPU.
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Houdani - Friday, March 18, 2005 - link
I think I missed something fundamental.Can the SPEs be addressed directly by software, or do they have to be fed all of their instructions by the PPE?
If they DO have to be fed be the PPE, I fail to see how the PPE can possibly feed them enough to keep them all working concurrently.
Someone throw me a bone here.
suryad - Friday, March 18, 2005 - link
I thought the G5 was a POWER5 proc. But I could of course be wrong. All I can say is the Cell definitely intriguing as it may be will have a rough road ahead of it and I am quite surprised that these large corporations invested so much in it, cutting edge though it might be. And as for the current forseeable future, I think when multi-core FX processors from AMD comes out, I do not believe there will be anything more devastating than that. Especially once they hit the 3 Ghz barrier with multi-cores enabled and faster DDR2-3 or even RAMBUS memory capabilities.tipoo - Thursday, December 3, 2015 - link
No, G5 was 970 based.Questar - Friday, March 18, 2005 - link
#50,Yes the G5 is a POWER4 derivitive.
Since you were wrong on that, don't think that you know what is significant about the design of POWER5. There were major architechture changes made to the processor.
fitten - Friday, March 18, 2005 - link
The only things new about Cell is its target market and being a single chip. The article mentions the TI DSP chip, but there were other similar architectures as well. One example that I'm familiar with is the MAP1310 board by CSPI. Back then, processes weren't good enough to put all the cores on a single chip but the basic architecture is the same - a PPC core to do the 'normal' stuff and two quad-core DSPs (SHARC) to do the 'work'. This board wasn't successful because it was considered too hard to program to get the performance it promised.... and this opinion is from people who live/breathe real-time systems and multiprocessing codes.The only thing new about Cell is that a) it's all on one chip now and b) the target market is a general marketplace and not a niche.
scrotemaninov - Friday, March 18, 2005 - link
#48. OK, I was under the impression that the G5 was based on the POWER5. You're saying it's based on the POWER4 instead?And the POWER4 and POWER5 aren't really "completely different chips" in the same way that the P4 and P3 are different chips, or in the way that the P4 and the Opteron are different chips. I can give you a list of the differences if you want. Start at http://www.elet.polimi.it/upload/sami/architetture...
The POWER5 is designed to not only be completely compatible with the POWER4 but to also to support all the optimisations from the POWER4. The only things of significance they've done is a) move the L3 cache controller on chip; b) change the various branch predictors to bimodal instead of 1-bit; c) increase the associativity and size of the caches.
Anyway, this is going off topic now...
Jacmert - Friday, March 18, 2005 - link
Rofl. Computer engineering and VLSI design. Gotta love those NMOS/PMOS transistor circuits.I never thought that I'd see stuff from my textbook explained on anandtech.com
saratoga - Friday, March 18, 2005 - link
"#38. You're right that the G5 is a derivative of the POWER5. The POWER5 is dual core, each core with 2way SMT giving a total of 4 'visible' cpus to the OS. The G5 is simply a single core version of the same thing."Err no its not. POWER4 != POWER5. Hence the different names ;)
They're completely different chips.
"Well scrotemaninov I am not disputing that the POWER architecture by IBM is brilliantly done. IBM is definitely one of those companies churning out brilliant and elegant technology always in the background.
But my problem with the POWER technology is from what I understand very limitedly, is that the POWER processors in the Mac machines are a derivative of that architecture right? Why the heck are they so damn slow then?
I mean you can buy an AMD FX 55 based on the crappy legacy x86 arch and it smokes the dual 2.5 GHz Macs easily!! Is it cause of the OS? Because so far from what I have seen, if the Macs are any indication of the performance capabilities of the POWER architecture, the Cell will not be a big hit.
I did read though at www.aceshardware.com benchmark reviews of the POWER5 architecture with some insane number of cores if I recall correctly and the benchmarks were of the charts. They are definitely not what the Macs have installed in them..."
There are slow memeory systems and then theres the one used on the G5. I've heard that you can put 8 Opterons together and still get average access times across all 8 cores that are better then a single G5. Thats probably a good part of the reason the G5 was so much slower then many people thought it would be. The rest is mainly IBM's trouble making them, and their inability to ramp clock speed like they planned on.
scrotemaninov - Friday, March 18, 2005 - link
#38. You're right that the G5 is a derivative of the POWER5. The POWER5 is dual core, each core with 2way SMT giving a total of 4 'visible' cpus to the OS. The G5 is simply a single core version of the same thing.As for the performance, Opteron is pretty much unbeatable for integer-bound applications. Itanium2 is unbeatable for FP applications. POWER5 is somewhere in the middle.
Most desktop applications are going to be integer bound. So it's not at all surprising that you find the G5 'slow' in that respect in comparison to the FX55. Plus, and this is the whole problem with the CELL, there's no point putting dual CPUs in there unless you can utilise them properly. If you have one process going flat out trying to run a heavy application and it's single threaded then you're only using about 1/4 of the CPUs you've bought for that application (for a dual G5 2.5), whereas the Opterons and FX55 stuff is more designed around quick, single threaded applications.
dmens - Friday, March 18, 2005 - link
psuedo-pmos wtf? That's domino logic, it's been around forever, and it's definitely not efficient in terms of power. Oh, and it takes forever to verify timing.