Understanding the Cell Microprocessor
by Anand Lal Shimpi on March 17, 2005 12:05 AM EST- Posted in
- CPUs
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.
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ceefka - Thursday, March 17, 2005 - link
Rambus'RevengeLocut0s - Thursday, March 17, 2005 - link
Great article Anand!! Yeah I actually get to bring my Comp150 knowledge to bear in reading this article! If this had come out 6 months ago I would have been totally lost. It will indeed be interesting to see what headway Cell can make, however unfortunately as Anand alludes to the x86 architecture is just too heavily entrenched for anything to budge it except the Big 2 (AMD and Intel). I can't wait to see what type of power the Playstation 3 will have though, and especially how that power will be utilized in games. I bet there will be some jaw dropping graphics awaiting us there. That is if Cells limitations don't hold back lazy game developers and lead to a string of mediocre games punctuated by a few amazing titles made by independent developers who really care to utilize the architecture. Didn't the Playstantion 1 suffer something similar?knitecrow - Thursday, March 17, 2005 - link
The real world technology article on the cell, states that it gives up single thread performance in favour of runing many parallel threads. That sounds like a terrible difficult processor to development games for.I for one think it will be easier to put the burden on the hardware rather than on the software side.
Can we see another repeat of PS2? Technically impressive, but hard to code for.
JarredWalton - Thursday, March 17, 2005 - link
11 - I think the point is that games tend to use certain functions of a CPU much more frequently, while general business/office applications make use of a wider range of generic operations. I understand your complaint, as office applications generally don't need a lot more power than about 1.5 GHz at most. However, the key of the statement was the "general purpose microprocessor" and not the "very powerful" part.AnandThenMan - Thursday, March 17, 2005 - link
WAIT. What the flock does this mean?"Performance in business/office applications requires a very powerful, very fast general purpose microprocessor, but performance in a game console, for example, does not."
WHAT??????? Hello?? So an office app like Word needs a very powerful processor, but a game console does not? I beg to differ. I suppose it depends on how you define "business/office application" but I think that statement is WAY off. I know several current office applications that will limp along on a pentium 133, but no current game has any hope on the same CPU.
tipoo - Wednesday, July 30, 2014 - link
It was clear to me that meant console CPUs didn't have to be as general purpose and brute force powerful in every regard - they can get away with being more specialized, and suck at general work, but still fast for game specific code.Googer - Thursday, March 17, 2005 - link
When are they coming out? Anyone know of a release date?jeffbui - Thursday, March 17, 2005 - link
#4, I do. Heh.I've been waiting for this article forever.. thanks!
JarredWalton - Thursday, March 17, 2005 - link
Interesting stuff. The Playstation has always been something of a pain in the rear to program. PS1 went it's own way, and PS2 did the same. PS3 and Cell seem ready to pave new roads into the "OMG this is really complex" land of programming. I'm glad I've given up serious programming.... :)Googer - Thursday, March 17, 2005 - link
In soviet russia cell processor controls your mind.