Cache associativity isn't necessarily the only problem. Also possibly relevant, from http://drdobbs.com/184405848 http://drdobbs.com/184405848 (Mike Abrash):

Cache associativity isn't necessarily the only problem. Also possibly relevant, from http://drdobbs.com/184405848 http://drdobbs.com/184405848 (Mike Abrash): "The other cause of erratic performance—and by far the worst—was 64K aliasing. On most Pentium 4s, if two data items in the L1 cache are at the same address modulo 64K, then a massive slowdown often occurs. Intel's documentation explains this by saying that the instruction that accesses the second piece of 64K-aliasing data has to wait until the first piece is written from the cache, which is to say until the first instruction is completely finished. As you might expect, this can wreak havoc with out-of-order processing. It was 64K aliasing that caused the aforementioned four-times slowdown in our large-triangle drawing benchmark. "The truly insidious thing about 64K aliasing is that it is very hard to reliably eliminate it. Sure, it helps to avoid obvious problems. For example, pixel buffers and 32-bit z buffers are accessed at the same intrabuffer offsets for any given pixel, so if they encounter 64K aliasing at all, it will happen constantly; to avoid this, Pixomatic makes sure the two buffers never start at the same address modulo 64K (and also not at the same address modulo 4K, to avoid cache contention). But time after time, we've tweaked our data to get rid of 64K aliasing, only to have it pop up again a few days later after more changes were made. About all you can do is run VTune periodically to see if the number of 64K-aliasing events has spiked and, if it has, start backing out changes to figure out what variables or buffers need to be tweaked so they're not 64K aligned. "On newer Pentium 4s, 64K aliasing changes to 1M aliasing, and in the most recent versions to 4M aliasing, which should reduce the problem considerably. Older Pentium 4s that have 64K aliasing also have 1M aliasing, which introduces an additional penalty on top of the cost of 64K aliasing, so beware of 1M aliasing even more than 64K. "By the way, on Pentium 4s that have 1M aliasing but not 64K aliasing, the aliasing event counter still fires on 64K-aliasing events, even though they don't affect performance on those chips, so VTune will report incorrect results in such cases. On newer P4s that only have 4M aliasing, VTune will correctly report only 4M-aliasing events."