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Date: -- (:)
From: Gerd Stolpmann <info@g...>
Subject: Re: [Caml-list] thousands of CPU cores

Am Donnerstag, den 10.07.2008, 23:01 -0400 schrieb Brian Hurt:
> 
> On Thu, 10 Jul 2008, Gerd Stolpmann wrote:
> 
> >
> > I wouldn't take this article too seriously. It's just speculation.
> 
> I would take the article seriously.
> 
> > Just open up your mind to this perspective: It's a big risk for the CPU
> > vendors to haven taken the direction to multi-core.
> 
> *Precisely*.  It also stands in stark contrast to the last 50 or so years 
> of CPU development, which focused around making single-threaded code 
> faster.  And, I note, it's not just one CPU manufacturer who has done this 
> (which could be chalked up to stupid management or stupid engineers)- but 
> *every* CPU manufacturer.  And what do they get out of it, other than 
> ticked off software developers grumbling about having to switch to 
> multithreaded code?
> 
> I can only see one explanation: they had no choice.  They couldn't make 
> single threaded code any faster by throwing more transistors at the 
> problem.  We've maxed out speculative execution and instruction level 
> parallelism, pushed cache out well past the point of diminishing returns, 
> and added all the execution units that'll ever be used, what more can be 
> done?  The only way left to increase speed is multicore.
> 
> And you still have the steady drum beat of Moore's law- which, by the way, 
> only gaurentees that the number of transistors per square millimeter 
> doubles every yeah so often (I think the current number is 2 years).  So 
> we have the new process which gives us twice the number of transistors as 
> the old process, but nothing we can use those new transistors on to make 
> single threaded code go any faster.  So you might as well give them two 
> cores where they used to have one.
> 
> At this point, there are only two things that can prevent kilo-core chips: 
> one, some bright boy could come up with some way to use those extra 
> transistors to make single-threaded code go faster, or two: Moore's law 
> could "expire" within the next 16 years.  We're at quad-core already, 
> another 8 doublings every 2 years, with all doublings spent on more cores, 
> gets us to 1024 cores.

Well, it is an open question whether this alternative holds. I mean
there is a market, and if the market says, "no we don't need that
multicore monsters", the chip companies cannot simply ignore it. Of
course, there are applications that would extremely benefit from them,
but it is the question whether this is only a niche, or something you
can make enough revenues to pay the development of such large
multicores.

In the past, it was very important for hardware vendors that existing
software runs quicker on new CPU generations. This is no longer true for
multicore. So unless there is a software revolution that makes it simple
to exploit multicore, we won't see 1024-cores for the masses.

> I think it'll be worse than this, actually, once it gets going.  The 
> Pentium III (single core) was 9.5 million transistors, while the Core Duo 
> was 291 million.  Even accounting for the 2 cores and some cost to put 
> them together, you're looking at the P-III to be 1/16th the size of a 
> Core.  If put on the same process so the P-III runs at more or less the 
> same clock speed, how much slower would the P-III be?  1/10th?  1/2?  90% 
> the speed of the Core?  So long as it's above 1/16th the speed, you're 
> ahead.  If your code can scale that far, it's worthwhile to have 32 P-III 
> cores instead of a the dual core Core Duo- it's faster.
> 
> Yes, there are limits to this (memory bandwidth springs to mind), but the 
> point is that more, simpler cores could be a performance win, increasing 
> the rate cores multiply faster than Moore's law would dictate.  If we 
> decide to go to P-III cores instead of Core cores, we could have 1024-core 
> chips comming out in 8 years (4 doublings, not 8, to go from 4x32=64 cores 
> to 1024 cores).  And remember, if Moore's law holds out for another 8 
> years after we hit 1K cores, that's another 4 doublings, and we're looking 
> at CPUs with 16K cores- literally, tens of thousands of cores.

Well, there is a another option for the chip industry. Instead of
keeping the die at some size and packing more and more cores on it, they
can also sell smaller chips for less. Basically, this alternate path
already exists (e.g. Intel's Atom). Of course, this makes this industry
more boring, and they would turn into more normal industrial component
suppliers.

At some point this will be unavoidable. The question is whether this
happens in the next years.

Gerd

> If Moore's law doesn't hold up, that's going to be a different, and much 
> larger and smellier, pile of fecal matter to hit the rotary air impeller.
> 
> Brian
> 
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