AltiVec and Dual Processor
Performance Scores For G4 ZIF Processor Upgrades
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.
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 on
2000 by MAXON.
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
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.
These scores are relative to the performance
of the slowest machine which receives a score of 100%. Higher
scores mean better performance.
Great Prices On Upgrades Check The Vendors Below
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.
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
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 donČt 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
These behaviors and limitations of the Velocity
Engine are confirmed by Apple's developer support
Copyright 1996-2003 by Cider Press Publishing LLC all rights reserved.
MacSpeedZone is not authorized, sponsored, or otherwise approved by Apple
Computer. Apple, the Apple logo, Macintosh, iPod, iBook, iMac, eMac, and
PowerBook are registered trademarks of Apple Computer, Inc. Additional
and product names may be trademarks or registered trademarks and are hereby