Memory performance
Since the big change we're examining today is a faster front-side bus, memory performance is key. SiSoft Sandra's synthetic memory bandwidth tests will give us a peek at how effective the higher bus speeds are in delivering more throughput.

Sure enough, the 533MHz bus delivers a good chunk more bandwidth than the "old" 400MHz bus, especially with dual-channel RDRAM. Although the 400MHz bus theoretically was fast enough to accommodate 3.2GB/s, in practice, it was a bottleneck.

Speaking of bottlenecks, the Athlon XP 2100+ delivers almost no more memory bandwidth here, with DDR333 memory on a KT333 chipset, than it did last time out, when we used DDR266 on a KT266A. The limiting factor is clear: The Athlon XP's 266MHz bus. The Athlon XP's bus is now effectively half the speed of Pentium 4's new 533MHz bus (although AMD's bus is probably a little more efficient). Unless and until AMD raises the speed of the Athlon XP's front-side bus, the Athlon XP will be at a disadvantage in scenarios where memory or bus bandwidth is critical.

We'll use Linpack to illustrate memory performance with a little more precision. Check out the funky graph:

You can see here how the L1 and L2 caches of the processors help performance when handling small data matrices, but once we get past about 512K, main memory access is the name of the game. The Athlon XP is fastest when it's crunching numbers stored in its 64K L1 cache. After that, though, it's all Pentium 4. The P4's big, fast L2 cache delivers more peak performance than the Athlon XP, and the P4 never relinquishes the lead.