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