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.

Blueprint for a High Performance per Transistor CPU
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  • microbrew - Thursday, March 17, 2005 - link

    "System on a Chip (SoC)"

    What will make or break the Cell is the tools available, especially the operating system and libraries.

    I would like to see what they're doing in terms of marketing the chip to consumer electronics, telecom, military and other embedded applications. I could see the Cell as a viable alternative to the usual mixures of PowerPcs, ARMs and DSPs.

    I also agree with Final Words; I don't see the Cell breaking into the consumer PC market any time soon either.
    Reply
  • Locut0s - Thursday, March 17, 2005 - link

    #17 Yeah that was a bit too harsh I agree. Reply
  • Eug - Thursday, March 17, 2005 - link

    I'm just wondering how well a dual-core PPE-based 4+ GHz chip would do in general purpose (desktop) code.

    And I also wonder how cool/hot such a chip would be. The Xbox 2's CPU is probably a 3-core PPE, but it runs at 3 GHz, and we don't have power specs for it anyway.
    Reply
  • Filibuster - Thursday, March 17, 2005 - link

    #11 (well, everyone should if they haven't before) read the Arstechnica article on PS2 vs PC - static applications vs dynamic media. Cell is taking it to the next level.

    http://arstechnica.com/articles/paedia/cpu/ps2vspc...

    Very nice article Anand!
    Reply
  • Googer - Thursday, March 17, 2005 - link

    Besides a release date, is there any news or knowledge of a Linux Kit for Playstation 3 like there was for PS2? Does anyone KNOW OF Either? Reply
  • Illissius - Thursday, March 17, 2005 - link

    Damn. Awesome article. If I hadn't known the site and author beforehand, I would've guessed Ars and Hannibal. Seems he isn't the only one with a talent for these kinds of articles ;)
    You should do more of them.
    Reply
  • scrotemaninov - Thursday, March 17, 2005 - link

    #22: This is just a guess so don't rely on this. The POWER5 has 2way SMT. Each cycle it fetches 8 instructions from the L1I cache. All instructions fetched per cycle are for the same thread so it alternates (round robin). It also has capabilities for setting the thread priority so that you effectively run with 1 thread and it just fetches 8 instructions per cycle for the one running thread.

    I would expect the PPE to be similar to this, fetching 2 instructions for the same thread each cycle. The POWER5 has load balancing stuff in there too - if one thread keeps missing in L2 then the other thread gets more instructions decoded in order to keep the CPU functional unit utilisation up. I've no idea whether this kind of stuff has made it over into the PPE, I'd be a little surprised if it has, especially seeing as this is in-order anyway so it's not like you're going to be aiming for high utilisations rates.
    Reply
  • 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. Reply
  • Houdani - Thursday, March 17, 2005 - link

    18: Top Drawer Post.
    20: Thanks for the links!
    Reply
  • 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.
    Reply

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