How Much Difference Does Having a Faster "Backside"
Cache Make - A Performance Review
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.
Mike is a freelance writer and writes occasional
articles for MacSpeedZone
The Performance Edge Index