Processor Performance, 2002

Processor Frequency

Performance generally does not increase exactly in step with frequency. That is, a 10% frequency increase should not lead to a 10% performance gain. In order for performance to scale in step with frequency, the program must either run entirely within on-die cache, or non-cached memory accesses must be pre-fetched with sufficient lead time to completely hide the memory access latency. Memory access times, including signal delays, are on the order of 100ns. A 1GHz processor has a clock cycle time of 1ns, so completely hiding memory access latency is not a simple matter. Performance will not scale well with frequency if either memory bandwidth or access latency are limiting. A program that performs simple operations on a large memory set can be constrained by memory bandwidth. A program that depends on serialized memory accesses, where one memory access must be completed before the next memory access can be issued, will be performance bottlenecked by memory latency.

Figure 5 shows the Pentium III performance from 800MHz to 1000MHz. All processors have 256K L2 cache, 133MHz FSB and use the Intel D815EEA2 motherboard. Note the unusual GZIP results. The 866MHz and 1GHz results are better than expected based on the 800MHz and 933MHz results.

Figure 5. Pentium III performance from 800MHz to 1GHz, 256K, 133MHz FSB.

Figure 6 shows the Pentium 4 performance from 2.26GHz to 2.8GHz. All processors have 512K L2 cache, 533MHz FSB and use the Intel D850EMV2 motherboard, and the compiler is version 6.0.

Figure 6. Pentium 4 performance from 2.26GHz to 2.80GHz, 533MHz FSB.

Figure 7 below shows the Pentium III percentage performance gain from 866MHz to 1GHz, a frequency increase of 15.5%. EON, CRAFTY and PERLBMK performance gains are nearly instep with frequency increase. MCF has negligible performance gain with frequency. Overall performance gain is 0.63% per 1% frequency increase.

Figure 7. Pentium III performance gain from 866MHz to 1GHz.

Figure 8 shows the Pentium 4 percentage performance gain from 2.26GHz to 2.80GHz, a frequency increase of 23.5%. The Eon performance gain is 25.4%, which is more than what should be possible, unless there is some thing else happening. Overall performance gain is 0.67% per 1% frequency increase.

Figure 8. Pentium 4 performance gain from 2.26 to 2.80GHz, 533MHz FSB.

Some applications scale well with frequency (GZIP, CRAFTY, EON and PERLBMK). Others less so and MCF scales poorly.