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

By Mike Juhaus

Last time I took a look at how much of a speed improvement a 1MB Backside cache gives you over a 512K one. Now I examine cache speed as opposed to size. Below you'll find a brief performance comparison between two PowerLogix 300Mhz cards both with 1MB of Backside cache, but on one the cache is running at 150Mhz and on the other the cache is running full processor speed or 300Mhz. Unfortunately we did not have the cards ourselves to test and could not preform some real world test (perhaps we will be able to do this in the future). The bench mark results for the two cards were provided by the manufacturer.

G3 Machine Cache Bus Scheme

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

On the first card below the cache is running at 150Mhz and on the second at 300Mhz. Longer bar equals better performance.


And the speed improvement if you have both a larger and faster cache ....

Actual MacBench 4.0 Scores

CPU Upgrade Card

MacBench Processor Score
NewerTechnology MAXpower G3/300/150/512K


PowerLogix PowerForce G3/300/150/1MB


PowerLogix PowerForce G3/300/300/1MB


How much do you pay for that extra performance?


Question: It would appear that you pay a 84% premium for a 31% performance gain, at least at Infinity Micro. I know time is money but is that premium justified in your opinion? Does anyone know what the L2 cache setup will be on the G4 machines? How about, when are the G4s suppose to arrive in your favorite outlet? I'm thinking of getting one. Let me know!

Mike is a freelance writer and writes occasional articles for MacSpeedZone

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