Understanding the Cell Microprocessor
by Anand Lal Shimpi on March 17, 2005 12:05 AM EST- Posted in
- CPUs
Three very interesting things happened over the past couple of weeks here at AnandTech:
- Intel’s Spring IDF 2005 turned out to be a multi-core CPU festival, with Intel being even more open than ever before about future plans for their multi-core microprocessor architectures. Intel has over 10 multi-core CPU designs in the works, and they made that very clear at IDF.
- At GDC 2005, AGEIA announced that they had developed a Physics Processing Unit (PPU) that could be used to enable extremely realistic physics and artificial intelligence models.
- Johan De Gelas went one step further in his quest for more processing power earlier this week to find that there’s quite a lot of potential for multi-core CPUs in the gaming market, at the expense of increasing development times.
Cell, at a high level, isn’t too difficult to understand; it’s how the designers got there that is most intriguing. It’s the design decisions and building blocks of Cell that we’ll focus on here in this article, with an end goal of understanding why Cell was designed the way it was.
A joint venture between IBM, Sony and Toshiba, the Cell microprocessor is the heart and soul of Sony’s upcoming Playstation 3. However, this time around, Sony and Toshiba are planning to use Cell (or parts of it) in everything from consumer electronics to servers and workstations. If you don’t already have the impression, publicly, Cell has been given some very high aspirations as a microprocessor, especially a non-x86 microprocessor.
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scrotemaninov - Thursday, March 17, 2005 - link
#23: True, but I believe that when the SPE's access the outside memory they go through the cache. Sure it's a lower coherancy than we're used to but it's not much worse.Houdani - Thursday, March 17, 2005 - link
18: Top Drawer Post.20: Thanks for the links!
fitten - Thursday, March 17, 2005 - link
"Given the speed of the interconnect and the fact that it is cache-coherant,"Only the PPC core has cache. The individual SPEs don't have cache - they have scratchpad RAM.
#22: I believe the PPC core is a dual issue core that just happens to be 2xSMT.
AndyKH - Thursday, March 17, 2005 - link
Great article.Anand, Could you please clarify something:
I had the impression that the PPE was a SMT processor in the sense that it had to be executing 2 threads in order to issue 2 instructions per clock. In other words: I didn't think the PPE control logic could decide to issue 2 instructions from the same thread at any given clock tick, but rather that it absolutely needed an instruction from each thread to issue two instructions.
After reading the article, I don't assume my impression is right, but a comment from you would be nice.
As I come to think about it, my impression is rather identical to 2 seperate single thread in-order cores. :-)
Koing - Thursday, March 17, 2005 - link
Cell looks VERY interesting.Any of you guys seen Devil May Cry 3 on the PS2? Looks great imo same with T5 and GT4.
Cell at first will be tough like most consoles. BUT eventually THE developers will get around it and make some very solidly good looking games.
Lets hope they are innovative and not just rehashed graphics and nothing else.
Thanks for the great article.
Koing
scrotemaninov - Thursday, March 17, 2005 - link
I really hate just dumping loads of links, but this basically is the available content on the CELL.http://arstechnica.com/articles/paedia/cpu/cell-1....
http://arstechnica.com/articles/paedia/cpu/cell-2....
http://realworldtech.com/page.cfm?ArticleID=RWT021...
http://www.blachford.info/computer/Cells/Cell0.htm...
http://www.realworldtech.com/page.cfm?ArticleID=RW...
http://www.hpcaconf.org/hpca11/papers/25_hofstee-c...
http://www.hpcaconf.org/hpca11/slides/Cell_Public_... (slides)
mrmorris - Thursday, March 17, 2005 - link
Brilliant article, there are few places for in-depth hardcore technology presentations but Anandtech never fails.scrotemaninov - Thursday, March 17, 2005 - link
Real concurrency is hard to do for the programmers. It's a real pain to get it right and it's hard to debug. Systematic analysis just gets too complex as there are just too many states, you end up with a huge graph/markov-model and it's just impossible to solve it tractably.Superscalar and SMT just try to increase ILP at the CPU level without burdening the programmer or compiler-writer. However, we've pretty much come to the end of getting a CPU to go faster - at 5GHz, LIGHT travels 6cm between clocks, and an electic PD will travel slower. As it is, in the P4 pipeline, there are at least 2 stages which are simply there to allow signals to propogate across the chip. Clearly, going faster in Hz isn't going to make the pipeline go faster.
So the ONLY thing that they can do now is to put lots of cores on the same chip and then we're going to have to deal with real concurrency. IBM/Sony are doing it now with CELL and Intel will do it in a few years. It's going to happen regardless. What we need is languages which can support real concurrency. The Java Memory Model is an almost ideal fit for the CELL, but other aspects don't work out so well, maybe. We need Pi-calculus/Join-calculus constructs in languages to be able to really deal with these cpus efficiently.
Your comments about CELL not being general purpose enough are a little wrong. IBM /already/ has the CELL in workstations and are evaluating applications that will work well. Given the speed of the interconnect and the fact that it is cache-coherant, I think we'll be seeing super-computers based on many CELLs, it's an almost ideal fit (as it is, you've almost got ccNUMA on a single chip). Also, bear in mind that this is IBM's 5th (or 6th?) generation of SMT in the PPE - they've been at it MUCH longer than Intel - IBM started it in the mid-90s around the same time that the Alpha crew were working on the EV8 which was going to have 8-way thread-level parallelism (got canned sadly).
Also, if you look at IBMs heavy CPUs - the POWER5, that has SMT and dispatches in groups of 8 instructions, not the 3/4 that AMD/Intel manage.
What I'm saying here, is that sure, the SPEs don't have BPTs of BTBs, they're all 2-way dispatch and not greater, but, they all run REALLY fast, they have short pipelines (so the pain of the branch misprediction won't be so bad), and, IBM have had software branch prediction available since the POWER4, so they've been at it a few years and must have decided that compilers really can successfully predict branch directions.
Backwards compatibility doesn't matter. Sure, Microsoft took several years to support AMD64 but that didn't stop take up of the platform - everyone just ran Linux on it (well, everyone who wanted to use the 64bit CPU they'd bought). It'll only be a few months after the CELL is out that we'll have to wait until Linux can be built on it. 100quid says Microsoft will never support it.
Frankly, considering that it's far more likely to go into super-computer or workstation environments, no one there gives a damn about backwards compatibility or Windows support. No one in those environments /wants/ a damn paper clip.
Reflex - Thursday, March 17, 2005 - link
#14: Replace 'lazy developers' with 'developers on a budget' and you will have a true statement. Its not an issue of laziness, its an issue of having the budget to optimize fully for a platform.GhandiInstinct - Thursday, March 17, 2005 - link
Wow Super CPU and SUPER RAMBUS? AHHHH!This will replace my computer. PS3 that is.