AltiVec and Dual Processor
Performance Scores For XLR8 G4 ZIF Processor Upgrades
According
to Apple, these applications,
to some extent, take advantage of the Velocity Engine (AltiVec
Instructions) of the G4 processor. So you should see some performance
improvement running these applications if you have a G4 machine
or upgrade. How much improvement depends on what parts of the
application have been written to utilize the Velocity Engine.
The AltiVec instruction set was developed primarily to speed
up multimedia and graphics performance. Please see the note
below from Glenn Fisher of Apple Performance Product Marketing
for more detailed information on the benefits and limitations
of the G4's Velocity Engine.
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The benchmark scores below are, for the most part, estimates.
Though based on real numbers, we did not test each individual
machine, but relied instead on what we know about the performance
of G4 upgrade cards, and the relative performance of each
Macintosh model. The dual G4 processor scores are based solely
on the claim by MAXON
that their Cinebench
product will run 80% faster on a dual processor machine over
a single processor one - we hope to put this claim to a test
soon.
In deriving these numbers we tried to be conservative
in our estimates, so that, if you are surprised, you will
be pleasantly so. That said in evaluating the results keep
in mind that they are "in the ballpark". Also only
certain applications take advantage of the AltiVec instructions
of the G4 and it is only certain functions of these applications
that are sped up (for instance certain Photoshop filters get
a performance boost from the G4 and other filters show only
the same performance that you would get from a similarly clocked
G3).
At the moment dual processor aware applications
are even fewer. This should change once OS X is released and
more OS X savvy applications come on market.
For the results below we based the scores on
the knowledge that a G4 running at the same speed as a G3
is 43% faster at rendering images with Photoshop AltiVec enhanced
filters, 128% faster when encoding in QuickTime (Sorenson)
and 38% faster using SoundJam to encode MP3.
We do not, as of yet, have any performance scores
for what the G4 in dual processor configuration will do in
terms of performance improvement, but hope to add those scores
soon.
These scores are relative to the performance
of the slowest machine which receives a score of 100%. Higher
scores mean better performance.
A note from the MotherShip
MSZ : Is there a limit to what the AltiVec instructions
can accelerate? In other words could someone rewrite an application
to make all of it AltiVec savvy or can only certain portions
of an application be accelerated? If only certain parts can
be accelerated what is this limited to.
GF: Yes and yes. AltiVec can only provide acceleration
based on it's ability to handle complex instructions and up
to 4 words of data at a time; theoretically, if you were able
to parallelize all your data, and it was all half-word size,
you would get 8x [performance]. AltiVec also has a few instructions
that essentially do two instructions in one cycle, so you
can theoretically get 2 instructions per cycle speed up over
non-AltiVec code. However, this assumes your code can be rewritten
to take advantage of AltiVec. Many things can't. AltiVec was
designed to speed up multimedia-type operations, such as transparency,
en- and de-coding, graphics operations, 3D, etc. What deveopers
generally find is they can accelerate the part of their code
that does these things anywhere from 1.5 to 16x. But there's
a lot of 'overhead' code that can't be accelerated, so the
final acceleration ends up being substantially less. For example,
reading or writing files to disk cannot be accelerated--it's
dependent on the file system and disk performance.
Glenn Fisher
Performance Product Marketing Apple Computer
MSZ writes: We are doing some
benchmarking using Cinema 4D running on machines upgraded
with G4s. Our understanding is that Cinema is AltiVec savvy,
however we are not seeing any improvement over a similarly
clocked G3. We are using 3 of the sample files and batch rendering
them. What parts of Cinema are accelerated for the G4? Can
you offer us any advice on how to gauge the performance of
the G4 when using your product?
Maxon Computer: Yes, XL6 is optimized for the
use of the velocity engine of the G4 processor. However, if
you try to compare XL6 on a G3 and a G4 at same clockspeed,
you might get different results, ranging from no acceleration
to up to 50% faster rendering.
Why? The velocity engine is mainly designed
to speed up certain mathematical functions, which are mostly
designed for speeding up playing back video, encoding and
decoding MPEG (DVD video, MP3), and also for game acceleration.
All these tasks require only a limited accurancy of 64 Bit.
A raytracer like CINEMA 4D needs a much higher precision and
therefore calculates mostly with 256 Bit accurancy. The velocity
engine is unfortunately no help here. But there are certain
areas in the software, which donåt need this high precision,
where 64 Bit calculation is enough (e.g. the noise in lights
and some other things).
It depends very much on what you have inside
your test scene. The size of the rendered image, the size
and format of the textures, models, if your objects are parametric
or polygonized, and lots more have an influence on the amount
of acceleration.
These behaviors and limitations of the Velocity
Engine are confirmed by Apple's developer support
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According
to Apple, these 
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