hn-classics/_stories/2008/12942732.md

---
created_at: '2016-11-13T10:39:44.000Z'
title: Why CPU Frequency Stalled (2008)
url: http://spectrum.ieee.org/computing/hardware/why-cpu-frequency-stalled
author: sajid
points: 51
story_text: 
comment_text: 
num_comments: 48
story_id: 
story_title: 
story_url: 
parent_id: 
created_at_i: 1479033584
_tags:
- story
- author_sajid
- story_12942732
objectID: '12942732'
year: 2008

---
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[![charts small
view](/img/31759-1372099381585.jpg)](/img/0408_data-xlrg-1372100315886.jpg)

 

Image: Intel; Charts: Michael Vella

Not so long ago, competitive sorts would boast of the cycle rate of
their PC’s central processing unit. But now it seems the only people who
talk it up are the overclockers—hobbyists who push their CPUs beyond
their specified limits. There are two reasons: CPU clock rates peaked a
few years ago \[see graph, top\], and they aren't a very useful key to
chip performance anyway.

The clock keeps a processor's parts working in unison, like rowers on a
galley ship. Other things being equal, the more ticks you have per
second, the more work will get done.

So why not push the clock faster? Because it's no longer worth the cost
in terms of power consumed and heat dissipated. Intel calls the
speed/power ­tradeoff a ”fundamental theorem of multicore
processors”—and that's the reason it makes sense to use two or more
processing areas, or cores, on a single chip.

Intel reports that ­underclocking a single core by 20 percent saves half
the power while sacrificing just 13 percent of the ­performance. That
means that if you divide the work between two cores running at an 80
percent clock rate, you get 73 percent better performance for the same
power. And the heat is dissipated at two points rather than one. So even
though the cutting-edge logic chip gulps ever more power \[see graph,
center\], it isn't about to melt its way through the floor.

That bodes well for Moore's Law, which predicts that about every two
years, ­manufacturers will double the number of ­transistors they cram
onto a given bit of silicon. The fundamental theorem says that we'll
still be able to make full use of those transistors for a good long
time. If once the whole choir of transistors had to sing to the beat of
a single metronome, now it can split up into sections—and harmonize.

**Count Paces? Or Measure The Distance Traveled?**

The rising power consumption of CPUs \[graph, center\] made it less
attractive to focus on cycles per second, so clock rates stalled
\[graph, top\]. A better gauge of performance, the number of
instructions performed per second \[graph, bottom\], continued to rise
without ­betraying any hint of the stall. That's because work once done
in a single ­processor is now divided among several processing
cores—four of them in the case of Intel's Quad-Core chip \[below\].

![data f1](/img/dataf1-1372099277050.jpg)

Image: Intel; Charts: Michael Vella

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