4.3 Hardware Free Lunch?
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4.3 Hardware Free Lunch?

The major reason that concurrency has been receiving so much focus over the past few years is the end of Moore's-Law induced single-threaded performance increases (or ``free lunch'' [Sut08]), as shown in Figure on page . This section briefly surveys a few ways that hardware designers might be able to bring back some form of the ``free lunch''.

However, the preceding section presented some substantial hardware obstacles to exploiting concurrency. One severe physical limitation that hardware designers face is the finite speed of light. As noted in Figure on page , light can travel only about an 8-centimeters round trip in a vacuum during the duration of a 1.8 GHz clock period. This distance drops to about 3 centimeters for a 5 GHz clock. Both of these distances are relatively small compared to the size of a modern computer system.

To make matters even worse, electrons in silicon move from three to thirty times more slowly than does light in a vacuum, and common clocked logic constructs run still more slowly, for example, a memory reference may need to wait for a local cache lookup to complete before the request may be passed on to the rest of the system. Furthermore, relatively low speed and high power drivers are required to move electrical signals from one silicon die to another, for example, to communicate between a CPU and main memory.

There are nevertheless some technologies (both hardware and software) that might help improve matters:

1. 3D integration,
2. Novel materials and processes,
3. Substituting light for electrons,
4. Special-purpose accelerators, and
5. Existing parallel software.

Each of these is described in one of the following sections.

Subsections

- 4.3.1 3D Integration
- 4.3.2 Novel Materials and Processes
- 4.3.3 Special-Purpose Accelerators
- 4.3.4 Existing Parallel Software