How Much Difference Does Having a Big "Backside" Cache Make - A Performance Review

By Mike Juhaus

So How much of a difference does the size of your backside cache make? We were curious about this and the loan of two Sonnet Crescendo 300Mhz processor upgrade cards, one with 512K of backside cache and one with 1MB, gave us a chance to run some tests and find out. But first a word from the "mother-ship" about backside cache.....

Backside Cache -- Unleashing Processor Performance
By far the biggest boost to performance that the PowerPC G3 offers can be credited to its incorporation of an approach to level 2 cache memory known as backside cache. This approach effectively bypasses limitations on the speed at which transactions between the processor and the level 2 cache can occur. Earlier PowerPC processors used the system bus to access both the level 2 cache memory and the main memory, which could result in conflicts. For example, under the previous approach, at processor clock speeds above 200 megahertz, the CPU would often stall as it waited for data to arrive from the level 2 cache. To prevent such slowdowns, the PowerPC G3 processor features a new dedicated bus that handles only the CPU/cache transactions. This bus can operate at higher speeds than the system bus -- speeds that relate incrementally to the clock speed of the processor. This enables the more effective use of level 2 cache, because even the relatively large amounts of data they can store can be accessed by the processor rapidly and efficiently. In fact, as clock speeds increase, so does the performance value offered by the backside cache design.

- Apple Computer


....and how does it work inside your computer? We have created a few diagrams to give you and idea....

Cache Bus Scheme Before The G3 Processor

G3 Machine Cache Bus Scheme

What kind of performance boost will you get if you have a G3 processor with 1MB as opposed to 512K of backside cache? We have run MacBench 5.0 and some real world tests to give you an idea.....

MacBench 5.0 Results
(Longer bars are better)





"Real World" Tests
(Shorter bars are better)



Photoshop 4.0 Tests



Photoshop 4.0 Filter Results







Question: Would you pay the premium that 1MB of cache costs for this kind of performance? Reportedly the G4 machines will come with 2MB of cache or more how much of a speed boost do you think that will be? Let me know!

- Mike

Mike is a freelance writer and writes occasional articles for MacSpeedZone

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