Okay - Having been through this painfull exercise of considering upgrade G4 processors with or without L3 cache - I feel I am well qualified to dispense my two cents.
Now before I get into it - let me just say that I'm no stranger to the advantages of L3 cache. I couldn't believe the performance difference between my orginal 800mhz G4 (without L3) and the 867mhz G4 (with 2mb L3 cache) that I upgraded it with. It was a real eye opener in terms of what an L3 cache provides - little things, like how converting music files from MP3 to AAC was significantly faster. As someone told me in another forum "Processor cache makes all the difference in the world. RAM (even fast CL2 RAM) will not compensate for lack of or tiny amounts of processor cache."
To understand why L3 cache is so important, you need to understand how L3 cache works.
Apple initially started adding L3 cache to it's G4 line when Motorola was having difficulty providing faster processors. It's an old trick - but a successful one. CPU's already have L1 and L2 cache. Generally, most operating systems only need to store a small amount of data that is frequently used by the CPU - and therefore there is a 80-90% possibility that the frequently used data the operating system needs will be in the L1 or L2 cache. If it's not, it will then access RAM. (And if it's not in RAM it will be on the hard drive.) However powerful modern applications (like Photoshop and games) have more than just a little amount of frequently accessed data - they have a sizable chunk, quite often bigger than a few MB's. When these modern programs start up, they load a large amount of data that it 'might' use into RAM. By adding a decent sized L3 cache you can cut out the high latency's (read time) that it takes the CPU to keep accessing this larger amount of frequently used data, because (instead of being in RAM) it is right there in the L3 cache. If there is no L3 cache, the CPU will have to be continually accessing RAM - which is much slower. So depending on the application or the type of processing an L3 cache can provide a significant boost in CPU productivity by either storing frequently used data or by continually keeping the CPU fed with the data it is processing.
Lets see what the Apple marketing department had to say:
"L3 cache keeps the PowerPC G4s engine stoked
L3 cache is high-speed memory. It provides fast access to data and application code through a dedicated bus to the processor. This dedicated bus provides throughput of up to 4 gigabytes per second (GBps), and is completely unhindered by any other data transfers. The high speed made possible by L3 cache, with its dedicated bus, allows the PowerPC G4 processor to receive data more than five times faster than it could from main memory (where a shared bus lowers speed). Because of this low latency, the G4 processor is constantly fed with data. So it doesnt just sit idle, waiting for the next set of data to arrive. And whether you choose a new Power Mac G4 with a single-processor or dual-processor configuration, you can be sure of one thing: each processor comes with its own dedicated L3 cache. In a dual-processor configuration, this enables both processors to share data seamlessly with each other without pausing to update main memory. The result? Accelerated processing. With up to 2MB of dedicated memory, the L3 cache can store large amounts of active application code and data. When you run an application, most of the active program code and user data remains in the L3 cache. This keeps the most important information instantly accessible to the processor. (Its analogous to caching web pages on your hard drive: when you click the Back button on your web browser, your computer uses the digital data you downloaded moments before, instead of downloading the same data again.) This is in marked contrast to the situation with PC architecture, where data travels from main memory to the processor through the system controller, elbowing its way through streams of data and instructions from other subsystems. The congestion caused by the merging of these various data streams results in slowdowns in the overall transfer rate and this in turn hampers application performance."
(Phew! - Glad I got that over with...)
I asked a guy at GigaDesigns to explain the difference between a CPU with L3 cache and one without. This is what he said: "I would like to talk a little more about the benefits etc. of L3 cache (or no L3). In our tests we have seen in a mixed application environment that not having a L3 Cache represents about a 10% penality. That is a Dual 1.8GHz w/o L3 will perform on par with a 1.6GHz with L3. Some applications exaggerate this as they rely more on L3 than others. Others like the OS do not seem to use L3 at all and therefore gain the full performance benefit."
Gigadesigns somewhere publicly stated that a 1.4GHz 7455 was equal to a 1.73GHz 7447. That gives you an good idea how the two different processors fair up. It was stated that the slower 7455 made up for the 18-19% speed increase because it had an L3 cache. From my research (from numerous sources) the general accepted percentage difference seems to be around 17% - that is, a processor with 1MB (or more) of L3 cache will be on average 17% faster than the same processor (at the same processor speed) without L3 cache.
So... If L3 cache is so important, why do people even bother with the 7447 processors which has no L3 cache? And why does Apple use the 7447 processor in many of its machines like the 'eMac'?
The answer to this is simple. The 7447 is a newer design that can operate at higher CPU speeds, has much lower power consumption (MUCH - read 40% less) and is therefore cooler operating. It's a bit of a trade off. By providing a slightly larger L2 cache and a faster system bus (the eMac that uses 7447 processors has a 167MHz system bus) you kind of make up for the lack of L3 cache. Furthermore, it's not just because the 7447 design doesn't support L3 cache - adding fast memory (L3 cache) to a processor is expensive and significantly increases production costs. Adding to the confusion is the fact that not all applications need or take advantage of an L3 cache. So, in those situations more MHz is much more desirable.
See this link: (
http://www.barefeats.com/g4up.html) - And notice these words at the conclusion. "PERFORMANCE INSIGHTS 3. Note that the Dual G4/1.4GHz is faster than the Dual G4/1.8GHz when running Photoshop MP actions and all four 3D Game tests. That's because the Dual G4/1.4GHz Power Mac has 2MB of L3 cache per processor. The Dual 1.8 only has 512K L2 cache per processor."
Anyway - I'm starting to ramble... The point being is that L3 cache (and whether it is important) is very subjective and really depends on what applications you use and how you use your computer. After doing all this research, I ended up buying a used dual 1GHz Quicksilver processor - and couldn't be happier. Sure there are times when I wish it was slightly faster - but there are also times when the L3 cache (combined with the dual processors) really kick in - and I go "Gee, this thing ROCKS!"
I would personally go the 1.2GHz with L3 cache...
