Saturday, August 09, 2008

Pixel Shaders and Moore's Law

In my post on 64-bit computing and X-Plane, there's a point that's implicit: there is a cost (in development time) to adopting any new technology, and it takes away from other things. I've been slow to work on 64-bit X-Plane because it would take away from things like texture paging and generic instruments. Similarly, there is a cost every time we do a build to supporting more configurations, so we pay for 64-bit continuously, by supporting six platforms instead of 3 (3 operating systems x 2 "bit-widths" of 32 and 64 bits).

We have a similar problem with graphics hardware, but it's even more evil. Moore's Law more or less says that in a given period of time, computer technology gets twice as fast. In the case of graphics cards, each generation of cards (coming out about every 12-18 months) is twice as fast as the last.

This has some scary implications for X-Plane. Consider these stats for video cards (taken from Wikipedia):
Card      Date        fill rate       Bus         Memory bw
GF3 01Q4 1920 MT/S 4x 8 GB/S
GF4 Ti 03Q1 2400 MT/S 8x 10 GB/S
GF5950 03Q4 3800 MT/S 8x 30.4 GB/S
GF6800 04Q2 7200 MT/S PCIe16 35.2 GB/S
GF7900 06Q1 15600 MT/S PCIe16 51.2 GB/S
GF8800 06Q4 36800 MT/S PCIe16 86.4 GB/S
GF9800 08Q2 47232 MT/S PCIe16/2 70.4 GB/S
Let's assume we support any video card in the last 5 years (in truth we support more than that). The difference between the best card and the oldest in w006 was 13,680 MT of fill rate.

Now in 2008 the difference is 43,432 megatexels per second!

In other words, the gap between the best and worst cards we might support is over 3x larger in only 3 years!

This is no surprise - since cards get twice as fast with every revision, the gap for a given number of generations also gets twice as wide.

What this means for us, programming X-Plane, is that coming up with a single simulator that runs on the very best and worst hardware is becoming increasingly more difficult, as the performance gains at the high end run away.

1 comment:

Anonymous said...

and now there is OpenGL 3.0 - what will you make out of this?