AMD's Athlon XP 3000+: Barton cuts it close

AMD’s Cache Benefits vs. Intel’s Cache Benefits

All caches are not created equal and thus you should not expect AMD to benefit as much as Intel did from going to a 512KB L2 cache. Intel follows a much more conventional L1/L2 cache architecture that uses what is known as the inclusive principle; the inclusive principle states that the contents of the L1 cache are also included in the L2 cache. The obvious downside to this is that the L2 cache contains some data that is redundant that the CPU will never use (if it needs it, it will get it from the faster L1 cache). From the CPU's point of view, an inclusive cache just means it has less room to store its much needed data in, but from the standpoint of the rest of the system an inclusive cache does provide one advantage - if data is updated in main memory (e.g. through DMA), the memory controller only has to check the L2 cache to update data, and there is no need to check L1 for coherency. This is a small but important benefit to an inclusive cache architecture.

The opposite, obviously, is a cache subsystem that follows the exclusive principle - such as the Athlon XP's cache. In this case, the contents of the L1 cache are not duplicated in the L2 cache, thus favoring cache size over the added latency of checking for two levels of cache coherency in DMA situations. The exclusive approach makes much more sense for AMD, considering the Athlon XP has an extremely large 128KB L1 cache that would be very costly to duplicate in L2 (compared to Intel's 8KB L1 Data cache that is easily duplicated in L2).

Both architectures have their pros and cons, but are best suited for the particular CPU we are talking about. Recognizing the differences, however, helps us understand why AMD will benefit differently from Intel when it comes to the 256KB to 512KB cache leap, but this still isn't the full story.

In order to see the differences, we compared four CPUs - a 2.167GHz Barton to a 2.167GHz Thoroughbred, and a 2.00GHz Northwood to a 2.00GHz Willamette and compared the performance benefit of going to a 512KB L2 cache for both the Athlon XP and Pentium 4 in the chart below:

As you can see, the Pentium 4 consistently received a bigger performance boost from the move to a 512KB L2 cache. Intel seems to think that this is related to AMD's use of an exclusive cache architecture, but it also may have something to do with the fact that the Pentium 4 is penalized much more from having to wait to go to memory than the Athlon XP. What's very interesting to note is the performance improvement the Pentium 4 realizes in situations where a simple increase in cache size shouldn't boost performance that much, such as the encoding applications and 3D rendering apps.