The importance of cache

[Kynan Shook]

OK, I found this interesting, and very much in-line with Apple's 
decisions; the L3 cache on my 17" 1 GHz PowerBook doesn't affect speed 
particularly much, at least in certain benchmarks.  If you have the 
developer tools installed (including the CHUD tools), you can test 
these things for yourself, YMMV.  Anyway, I see very little difference 
between L2 and L3 cache enabled and just L2 cache enabled in 
CacheBasher or SkidmarksGT (using the Hardware system preference 
installed by the CHUD tools to selectively enable or disable the 
caches).  Using L3 alone (with no L2) does make a huge difference over 
using no L2 and no L3, however.  Anyway, the bottom line: many people 
are going to have improved performance from the increase in L2 cache 
and the loss of all L3.  Only a few applications will work better with 
the third level.
FWIW, I got up to about 12 GBps of bandwidth in CacheBasher, by doing a 
load of vector data, with a stride of 16.  With no cache, this drops to 
about 700 MBps.  With no cache using integer data, it's only 170 MBps.
I guess this is probably more interesting to me than to the average 
person; in one of my classes right now, we are discussing how to build 
a memory system, including how to make a cache.  Our final project for 
the class (we started about 2 weeks ago, we have another month and a 
half to finish it) is to build a 16-bit microprocessor.  For me, that's 
LOT of fun.  For the vast majority of my friends, that would probably 
be worse than chinese water torture.  ;-)

Happy Halloween, everybody!