Memory Bandwidth
Figure 15 shows the Pentium III 1.0GHz performance with a 100MHz and 133MHz FSB, both with 256K L2 cache, the same motherboard and complier. Figure 16 shows the Pentium 4 2.4GHz performance with the 400MHz and 533MHz FSB. Both systems use the same motherboard and compiler versions.
Figure 15. Pentium III 1.0GHz with 100MHz and 133MHz FSB.
Figure 16. Pentium 4 2.0GHz with 400MHz and 533MHz FSB.
Figure 17 shows the Pentium III performance gain from 100 to 133MHz FSB. Figure 18 shows the Pentium 4 performance gain from 400MHz to 533MHz FSB. In both cases, the relative increase in FSB bandwidth is 33%. The Pentium III performance shows a 12% gain while the Pentium 4 shows 4%. This consistent with the hypothesis that the Pentium III is more memory bandwidth constrained than the Pentium 4.
Figure 17. Pentium III 1.0GHz with 100MHz and 133MHz FSB.
Figure 18. Pentium 4 2.40GHz with 400MHz and 533MHz FSB.
Chipset, RDRAM and SDRAM
Figures 19 and 20 shows the Pentium III 800MHz and 1.0GHz performance with the Intel 820 RDRAM based chipset and the 815 SDRAM based chipset. Both platforms use the version 5.0 compiler.
Figure 19. Pentium III performance with 820 RDRAM and 815 SDRAM chipsets.
Figure 20. Pentium III performance with 820 RDRAM and 815 SDRAM chipsets.
Figures 21 and 22 shows the relative performance change from the 820 to 815 chipsets.
Figure 21. Pentium III 800MHz performance change from 820 to 815 chipsets.
Figure 22. Pentium III 1.0GHz performance change from 820 to 815 chipsets.
The Pentium III platform does not benefit in a meaningful manner from RDRAM. The Pentium III system bus cannot effectively utilize the higher bandwidth capability of RDRAM. RDRAM does have slightly higher latency than SDRAM. The net result is that some applications show a small benefit, while others have a small negative effect.