Hardware Reference
In-Depth Information
Phenom II X6 1055T as reported by CPU-Z.
Figure 3.3. Two examples of six-core processors with L1, L2, and L3 cache from Intel (a) and
AMD (b).
Cache Performance and Design
Just as with the L1 cache, most L2 caches have a hit ratio also in the 90% range; therefore, if you look
at the system as a whole, 90% of the time it runs at full speed (233MHz in this example) by retrieving
data out of the L1 cache. Ten percent of the time it slows down to retrieve the data from the L2 cache.
Ninety percent of the time the processor goes to the L2 cache, the data is in the L2, and 10% of that
time it has to go to the slow main memory to get the data because of an L2 cache miss. So, by
combining both caches, our sample system runs at full processor speed 90% of the time (233MHz in
this case), at motherboard speed 9% (90% of 10%) of the time (66MHz in this case), and at RAM
speed about 1% (10% of 10%) of the time (16MHz in this case). You can clearly see the importance
of both the L1 and L2 caches; without them the system uses main memory more often, which is
significantly slower than the processor.
This brings up other interesting points. If you could spend money doubling the performance of either
the main memory (RAM) or the L2 cache, which would you improve? Considering that main memory
is used directly only about 1% of the time, if you doubled performance there, you would double the
speed of your system only 1% of the time! That doesn't sound like enough of an improvement to
justify much expense. On the other hand, if you doubled L2 cache performance, you would be
doubling system performance 9% of the time, which is a much greater improvement overall. I'd much
rather improve L2 than RAM performance. The same argument holds true for adding and increasing
the size of L3 cache, as many recent processors from AMD and Intel have done.
The processor and system designers at Intel and AMD know this and have devised methods of
improving the performance of L2 cache. In Pentium (P5) class systems, the L2 cache usually was
found on the motherboard and had to run at motherboard speed. Intel made the first dramatic
improvement by migrating the L2 cache from the motherboard directly into the processor and initially
running it at the same speed as the main processor. The cache chips were made by Intel and mounted
next to the main processor die in a single chip housing. This proved too expensive, so with the
Pentium II, Intel began using cache chips from third-party suppliers such as Sony, Toshiba, NEC, and
Samsung. Because these were supplied as complete packaged chips and not raw die, Intel mounted
