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.....
"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)
Processor
Graphics
"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?
- Mike
Mike is a freelance writer and writes
occasional articles for MacSpeedZone
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