Thursday, August 14, 2008

How Many Cores Will You Have?

My last post generated  number of posts from both sides of the "hardware divide" (that'd be the have's and have-not's).  I think everyone at least grasps that developer time is finite and features have to get prioritized at the cost of other features, even if not everyone agrees about what we should be coding.

I think the term "hardware divide" is the right one, because the hardware market has changed. Years ago when I bought myself a nice shiny new Dell (back when that wasn't an idiotic idea) a medium-priced Dell had medium-priced hardware.  Not only did I get a decently fast CPU (for the time), but I got a decent AGP bus, decent motherboard, etc.  The machine wasn't top-end, but it scaled.

When you look at any computer market, you need to consider what happens when consumers can no longer accept "more" and instead want "the same for cheaper".  This change in economics turns an industry on its head, and there are always winners and losers.  (I have claimed in the past that operating systems have turned that corner from "we want more" to "we want cheaper", a shift that is very good for Linux and very bad for Microsoft.)

Desktop computers hit this point a while ago, and the result is that a typical non-gamer computer contains parts picked from the lower end of the current hardware menu.  You're more likely to see:
  • Integrated graphics/graphics by the chipset-vendor.
  • System memory used for VRAM.
  • Slower bus speeds, or no graphics bus.
  • GPU picked from the lowest end (with the fewest number of shader units).
  • CPUs with less cache (this matters).
Someone commented a few days ago that computers would get more and more cores, and therefore multi-core scalability would be very important to fully utilizing a machine.  I agree. 

But: how many cores are those low-end PCs, aimed for general use (read: email, the web, text editing) going to have?

My guess is: not that many.  Probably 2-4 at most.

These low end PCs are driven by one thing: price - the absence of VRAM or dedicated graphics hardware is all about bringing the hardware costs down - a $25 savings matters!  In that situation, box-builders will want the cheapest CPU, and the cheapest CPUs will be the physically smallest ones, allowing for more chips on a wafer.  A low-end PC will get no benefit from more than 4 cores - the intended use probably doesn't even use one.*

Multiple cores are great because they give us a new way to benefit from smaller transistors (that is, by packing more cores on a chip, rather than clocking it faster, which has real limitations).  But I think you'll start to see the same kinds of gaps in CPU count that you see now with GPUs.

(In fact, the mechanics are very similar.  The main differences between high-end and low-end GPUs of the same family are the number of parallel pixel pipelines - the low-end chip is often a high-end chip with a few defective pipelines disabled.  Thus you can have a 4x or 8x performance difference due to parallel processing between siblings in a GPU family.  Perhaps we'll see the same idea with multi-core chips: build an 8-core chip, and if 4 of the cores fail, cut them out with the laser and sell it as a low-end chip.)

* One advantage of multiple cores is that they can take the place of dedicated hardware.  For example, there is no penalty for doing CPU-based audio mixing (rather than having a DSP chip on the sound card) if the mixing happens on a second core.  Being able to replace a dedicated component with a percentage of a core is a win in getting total hardware cost down, particularly if you were going to have the second core already.

No comments: