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Understanding the Cell Microprocessor

 by Anand Lal Shimpi on March 17, 2005 12:05 AM EST
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70 Comments

Blueprint for a High Performance per Transistor CPU

Given that Cell was designed with a high performance per transistor metric in mind, its architecture does serve as somewhat of a blueprint for the technologies that result in the biggest performance gains, at the lowest transistor counts.   Now that we’ve gone through a lot of the Cell architecture, let’s take a look back at what some of those architectural decisions are:

1.      On-die memory controller

We’ve seen this with the Athlon 64, but an on-die memory controller appears to be one of the best ways to improve overall performance, at minimal transistor expenditure.   Furthermore, we also see the use of Rambus’ XDR memory instead of conventional DDR, as the memory of choice for Cell.   High frequencies and high bandwidth are what Cell thrives on, and for that, there’s no substitute but Rambus’ technology.

2.      SMT

On-die multithreading has also been proven to be a good way of extracting performance at minimal transistor impact.   Introducing Hyper Threading to the Pentium 4’s core required a die increase of less than 5%, just to give you an idea of the scale of things.   The performance benefits to SMT will obviously vary depending on the architecture of the CPU.   In the case of the Pentium 4, performance gains ranged from 0 - 20%.   In the case of the in-order PPE core of Cell, the performance gains could be even more.   Needless to say, if implemented well, and if proper OS/software support is there, SMT is a feature that makes sense and doesn’t strain the transistor budget.

3.      Simpler, in-order, narrow-issue core - but lots of them

This next design decision is more controversial than the first two, simply because it goes against the design strategies of most current generation desktop microprocessors that we’re familiar with.   By making the PPE and SPEs 2-issue only, each individual core still remains a manageable size.   Narrower cores obviously sacrifice the ability to extract ILP, but doing so allows you to cram more cores onto a single die - highlighting the ILP for TLP sacrifice that the Cell architects have made.

Getting rid of the additional logic and windows needed for an out-of-order core helps further reduce transistor count, but at the expense of making sure that you have a solid compiler and/or developers that are willing to deal with more of the architecture’s intricacies to achieve good performance.

Looking at Intel’s roadmap for Platform 2015, the type of microprocessors that they’re talking about are eerily Cell-like - a handful of strong general purpose cores surrounded by smaller cores, some of which are more specialized hardware.

However, the time frame that Intel is talking about to introduce those Cell-like architectures is much further away than today; the first Cell-like architectures don’t appear on roadmaps until the 2009 - 2015 range.

In the past, when Intel dictated a major architectural shift, it didn’t happen until they said so.  It is yet to be seen if Cell is an exception to the rule or if it will be another architecture well before its time.

Cell's Dynamic Logic Final Words
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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.
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