IBM Says Hasta La Vista To Cell Processor

Long a mainstay in a few important supercomputers, and of course, the Sony PlayStation 3, the IBM Cell processor was a chip that always promised more than was shown.

We heard about the fantastic benchmarks it pulls off with ease, yet few things that touch our everyday lives ever were affected by it. IBM seems to do that quite often, don’t they? (Remember the Power 6? That should have become part of a Mac that took over the world!)

This week Ars Technica gives us the news that the life of the Cell processor is effectively over, but that some of its concepts will find their way into something new.

IBM has revealed that the Cell processor line is an evolutionary dead-end. Some of the ideas behind it will live on, but the Cell family itself will not. I guess we now know what probably won’t be in the PlayStation 4.

In an interview with Heise.de, IBM’s VP of Deep Computing, David Turek, confirmed that the Cell processor has reached the end of the line. Turek then put a more positive spin on the news by stating the obvious truth that heterogeneous multiprocessors, of which Cell was the first mass-market example of, are here to stay, so insofar as IBM continues to produce such chips, Cell’s basic concepts and ideas will live on in the company’s product line.

AMD and Intel are both working on these, AMD’s project is called Fusion. It is to be the first CPU plus GPU on a single die, and is AMD’s answer to low performance energy efficient computing for the office.

By many metrics, Cell has been a success for IBM, even if it didn’t live up to much of the hype that preceded its launch. In conjunction with x86 chips supplied by AMD processor, Cell has helped IBM’s RoadRunner take and, until recently, keep the top slot in the Top 500 Supercomputer List. And, as this older HPCWire article on rumors of Cell’s demise points out, Cell-based systems have been among the most power-efficient in the industry. IBM was able to build the specialized coprocessor in high enough volumes to keep its price down because the company was successful in selling it to Sony as a game console chip; the degree to which Cell, which gives the PS3 higher peak theoretical performance than the Xbox 360, has worked out for Sony is debatable. But IBM’s decision to call it quits on the line confirms suspicions that Cell’s overall commercial success has been limited, and there will be a number of cheaper, higher-performance, more widely supported alternatives to the processor starting in 2010.

Had the price come down a bit faster, we might have seen many more PS3s than Xbox 360s. The hacker community has done much with the Cell in the PS3; there are several Linux distributions that run on it. Had the PS3 gotten to commodity pricing sooner, there might have been many more used to play around with these distributions, instead of simply functioning as Blu-ray players

IBM is certainly correct that at least part of the future belongs to heterogeneous, single-chip multiprocessors (CMPs)—these are chips that contain at least two different types of cores, typically a mix of general-purpose cores and more specialized cores. In this respect, the heterogeneous CMP party really kicks off in 2010, when both Intel and AMD will launch processors that feature a CPU and GPU on the same die. These CPU/GPU combos will eventually give way, at least on the Intel side, to processors that look substantially like Cell.

At some point, Intel will integrate Larrabee, which is a GPU made up of multiple small x86 cores, onto the same die as a regular x86 core. The result will be at least one large, general-purpose processor core and some number of small general-purpose cores that consist heavily of SIMD floating-point hardware.

then the differences between what Cell is, and what is being produced by AMD and Intel -

The differences between Intel’s future CPU/Larrabee hybrid and IBM’s Cell may seem small, but they’re critical. Cell’s smaller floating-point cores are not general-purpose—they’re specialized and they implement their own instruction set. These small cores also don’t have cache coherency and a real virtual memory implementation. Rather, they have “local store” pools of programmer-managed memory that make them a huge pain to program for. So in terms of programmability, the difference between Cell and a CPU/Larrabee hybrid is night and day.

OK, it’s not exactly night-and-day; it’s more like “night and early evening,” because there are still significant hurdles to actually designing software that takes full advantage of higher levels of parallelism. So while it may be easier to write code for a heterogeneous x86 CMP, the higher-level challenge of breaking an application up into parallelizable tasks still remains.

As for comparisons between Cell and what AMD is doing, this is more difficult, because AMD isn’t dropping many hints about what it has in store for the final stages of its CPU/GPU “Fusion” plan. I’ve suggested that they won’t go the CPU/Larrabee route described above, but that remains to be seen.

Speaking of Larrabee, the oft-floated rumor that Intel’s upcoming GPU will form the brains of Sony’s PlayStation 4 hinges in part on the idea that Sony and IBM will abandon the Cell, and not double down on it. Now that we know IBM is ditching Cell, does this mean Larrabee is a shoo-in? Not quite. This removes one of the objections to Sony using Larrabee, but an even bigger objection remains: even in 2011, which is the earliest that we could expect a PS4, Larrabee still may not be as good at graphics as discrete GPUs from NVIDIA and AMD/ATI. There’s also an outside possibility that Sony could just use one or two of the last version of Cell in its console, and let Toshiba do the fabrication.

Sony would most likely stay with an IBM design, albeit an updated one, as many of its core audience became very upset when PS2 compatibility was dropped from the PS3. If Larrabee was used, chances are that once again, backwards compatibility would be dropped, and more unhappy campers would be created. Also, as the article attests, Larrabee is an unknown quantity, as it is still vaporware at this point.

I’m sure the government will be snapping up a few more of those PS3s before they changeover; they seem to be happy with the price/performance ratio when used to do password cracking.

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a new PS4 trial unit?

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