Originally posted by mindbend
My gut stills tells me not much, it's clearly an attempt to distract us from the chasm known as MHZ gap.
To talk about processor cache, let step away from the Mac/PowerPC arena for a minute. I think a good real world example would be my SGI Indy running with an R4600SC (secondary cache, in this case 512k) at 133 MHz (hey, you know this must be a really slow system looking at the MHz, strange that it's main use for me is video capture). I had a friend who saw and like my Indy so much that he bought one that he thought was the same because it was a 133 MHz system. He could understand why some of the things that seemed quite fast on my system where not as fast on his. As it turned out, he had gotten an R4600PC (primary cache only) system which runs 15-30% slower than mine. At another point in time I thought about getting an R5000 upgrade (same type of processor used in the early O2 systems), and the R5000PC/150 MHz looked good until I saw that it wasn't really any faster than my system. Then I saw that the R4400SC/200MHz was selling at about half the price of the R5000SC/180MHz (which was because the R5000SC/180 was quite a bit faster than the R4400SC/200, again with the wonderful MHz not matching real system speed).
Lets now take a look at the second generation of PowerPC processors for a minute (seeing as you brought them up). The 603/604 series were not design to use cache effeciently. Add to that that the L2 cache was on the mother board and used the system bus, and the performance boost would not be worth the effort.
Then came the G3. Have you ever asked yourself "why was the G3 so much faster than the 603/604 processors at any given MHz?" The answer to that question is how it takes advantage of L2 cache and how a dedicated cache only bus was created between the processor and the L2 memory (usually at some fraction of the actual processor speed). The only problem with this was that you couldn't get more than one processor's cache to play nicely within a system... until the G4 that is. The nexted big step was having a large (256k) on board/at speed L2 cache (data bus is now inside the processor) and having a larger L3 cache on a dedicated bus with the processor(s).
So what difference does this make? Lets look at the Pentium series of processors for a moment. The Pentium II is (MHz for Mhz) faster than the Pentium III. Why? Because in order to move forward in the
MHz gap they had to take performance short cuts (which the higher MHz mostly made up for). This happened again in the transition from the Pentium III to the Pentium 4. Now lets look at the PowerPC line. One branch moves along from the 601 to the 603 to the G3, and the other from the 601 to the 604 to the G4. The move from the 601 to the 603 was a step backwards for performance, but a step forwards for MHz (and heat which is why that line ended up on early PowerPC processors). The move from the 603 to the G3 was both a big jump forwards in performance (a 233 MHz G3 was much faster than a 300 MHz 603e) and heat (the first time in history that a processor debuted in both desktop and laptop systems). In the jumps from 601 to 604 to G4, we got both a performance increase and a MHz increase (though as you noted, the MHz numbers didn't jump up as fast as in the Pentium/x86 lines).
So where does this
"gap" show up again... why in Intels new Itanium processor (running at 800 MHz, compared to 2.2 GHz for the cheeper Pentium 4). The fact of the matter is, once you get past the consumer market (where people don't know any better), MHz doesn't tell you much about a system, but a systems use of cache can be very telling (some SGI's have been at 4 MB for quite some time now, we should have match them with that mark by now). Mind you, believe what you like, you really should check these types of things out for yourself. And remember that there are more than just Macs and PC in this world to take into account when studying relative performance between processors.
