[mythtv-users] "Cell Microprocessor Simultaneously Decoding 48 MPEG-2 Streams"

David Wood obsidian at panix.com
Mon May 2 20:49:03 UTC 2005


Please correct me if I have this wrong: the changes in cell are more than 
just skin deep - more, even, than just adding multiple functional units 
and interconnects. I think it's appropriate to say that cell represents a 
kind of philosophical shift.

There are some readable background papers out now, for instance:

http://www.blachford.info/computer/Cells/Cell0.html

As an example, the cell approach to caching is radically different.

Your CPU does a lot of complex run-time guesswork with hard-wired 
heuristics about how to best organize your code in the different tiers of 
memory resources available. Cache misses can stall the CPU for long 
periods; effective utilization of various cache levels is instrumental to 
performance, yet your interaction with caching mechanisms is weak or 
non-existent. We typically rely on the CPU to know what to do, or at best 
give it hints. Many important performance gains in the PC are tied to this 
"magic CPU" approach. We always expect new machines to run old code (and 
for that matter, old binaries) faster.

The cell, at last, switches directions. Probably of necessity. Grid 
computing makes these problems perhaps too difficult to want to solve with 
such a great reliance on the hardware. The "APU" has no cache in the 
traditional sense. It has a "local memory," and your code (or your 
compiler, or your OS, or some interesting combination thereof) manages 
that.

Virtual Memory hardware, as you know it, isn't present in cell. There is 
something, of course, but it's different. Apparently, it's simpler, and 
thus faster.

Though in many ways it's just a continuation of the RISC approach of 
pushing complexity from the silicon to the software. Software has to do 
more work, especially the compiler, but the changes reverberate all the 
way up the stack. On the other hand, if you are willing to do the work, 
you can win more performance.

On a $50 million video game project, they are willing to do the work.  :)

On Mon, 2 May 2005, Devan Lippman wrote:

> The CELL is a RISC CPU, why would you want to emulate x86?  What I was
> saying tho was that its based off the power 5 instruction set if I'm
> not mistaken and shouldn't take much to port to (it'll prolly get done
> faster than I can get my hands on one).
>
> -- 
> Thanks,
> Devan Lippman <devan at lippman.net>
>
>
> On 5/2/05, Robert Johnston <anaerin at gmail.com> wrote:
>> On 5/2/05, Devan Lippman <devan.lippman at gmail.com> wrote:
>>> I hope I can get one of these as the CPU for my machine!  Been a while
>>> since I read that explination but I think that its a series of power
>>> CPUs arranged similar to a beowulf cluster all on one chip which means
>>> it shouldn't be too far off from existing CPUs that linux will run on
>>> in the final presentation to the system.
>>> Interesting that they have a screenshot of the demo being replayed in
>>> windows media player and not the original demo... still I LOVE SMP and
>>> this should have the potential to kill current SMP, AMD and INTEL
>>> Multicore all in one hit.
>>
>> Well, yes, but think of the cost.
>>
>> Chances are running Dual Dual-core Opteron will be much better "Bang
>> for your buck" than getting a Cell system and having to write an x86
>> emulator, or porting the entire OS to Cell's instruction set
>> --
>> Robert "Anaerin" Johnston
>>
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