Three Gigahertz Forever

Monday, 9 Mar 2015

I hear people occasionally mention that CPU clock rates have been stuck around 3 GHz for a while. They say that multicore instead of higher clock rates is a trend. But it seems few realise how far this while has stretched.

The last time CPU clock rates went up was when the Pentium 4 Prescott LGA 570J came out and hit 3.8 GHz – in 2005.

It’s been a decade that CPU clock rates have been frozen. Exponential increases in single-threaded performance are well and truly over.

We now live in the titular era: the era of Three Gigahertz Forever.

Meanwhile, Moore’s Law has continued to apply, so we have still been getting ever faster chips despite the new situation. But the gains have moved sideways into multi-threaded performance. That’s not a “trend” any more, it’s the status quo.

Even with that, we have arrived on the outskirts of the era of Dark Silicon. An end to Moore’s Law itself is within sight. While it’s unclear how far off it really is – it’s hard to tell the distance of something you don’t know the size of –, we now find the eventual end of Moore’s Law to be a tangible inevitability.

What’s more, power consumption has increasingly gained importance as an optimisation target. Now of course optimising for power consumption largely has the same basic shape as optimising for speed – try to achieve the goal with less work, try to avoid needing to achieve the goal at all. Where power consumption differs is that it can be sensitive to wasted work even in non-hotspot areas of the code.

So not only can the “just wait for hardware to catch up” approach no longer cover for slow code, but power consumption now demands caring about wasteful work even where it previously never mattered.

The free lunch is over.