Page 167 - A Practical Guide from Design Planning to Manufacturing

140 Chapter Five

1500

Itanium 2 Increasing
1200
3250 performance
2500
SpecFP2000/GHz 600 PA-RISC 1000 1750 Opteron
UltraSparc III
900

Specfp2000 = 250
300
Pentium 4

0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Frequency (GHz)
Figure 5-9 SPECfp2000 results.

instructions to be executed more efficiently, and higher performance
buses can provide data to the processor more quickly, so that it spends
fewer cycles waiting. Despite these variations the initial microarchitec-
ture design of the processor is the most important factor affecting IPC.
This is apparent in Figs. 5-8 and 5-9, where each processor tends to stay
in a narrow vertical band of IPC compared to larger changes in frequency.
Intel’s Pentium 4 has the longest pipeline of the processors shown in
Figs. 5-8 and 5-9. By dividing its pipeline into very small steps this
processor achieves very high frequencies. However, this comes at a price;
its performance per cycle is low. The long pipeline causes many pipeline
breaks and low IPC. At the other end of the spectrum the Itanium 2
achieves very high IPC in both integer and floating-point programs. Its
EPIC architecture allows more reordering and parallelism to be exposed
by the compiler; its microarchitecture is also designed to support simul-
taneous issue of many instructions. Although the Itanium 2 supports
many parallel pipelines, the pipeline depth itself is relatively short,
containing a large amount of logic in each pipestage. This results in the
Itanium 2 being limited to lower frequencies than the Pentium 4. The
Opteron processor takes a more balanced approach with frequencies
and performance per cycle in between the Pentium 4 and Itanium 2.
These graphs show how different processor designs have chosen to
focus on either frequency or IPC in the pursuit of higher performance.
Eventually successful ideas from either approach end up being incorporated