I just attended my first Apple Developer kitchen (workshop) last week,
"Optimizing applications for Mac OS X and the Power Mac G5". I came
away very impressed with the content, the facilities, and especially
the level of assistance provided by Apple personnel. Not only
productive, but fun, fun, FUN!
Apple spoils you by seating you in a room full of dual-processor 2 GHz
PowerMac G5s, each with 2 GB of RAM and a honkin' 23" Cinema display.
The G5s are FAST, gorgeous, surprisingly quiet, rugged machines with
brushed aluminum cases -- a wirehead engineer's dream. I was
unexpectedly stunned by the workmanship.
You're treated to informative, cut-to-the-chase presentations on how to
optimize your app's code to take advantage of the new G5 architecture
(floating point, integer and vector units, increased memory bandwidth,
etc.), how to move your project into Xcode (port resources into nibs,
customize the IDE, set optimum compiler flags), and how to use Apple's
excellent CHUD performance tools, including Shark and Spin Control.
Some really cool examples (especially the "Noble Ape" artificial
intelligence simulation, with inquisitive Simians roaming about an
island) are shown, which illustrate how to delve into your code and
tune up its performance. IIRC, the speed of the "ape thoughts"
(flashing brain wave patterns) was increased by a factor of about 16 by
multithreading and vectorizing their "coginitive processes". Ooh-ooh,
now that's worth howling about!
After the lectures, you work on the project(s) you brought along with
you. Apple lets you install any special software or compilers you need
on the G5s. (I loaded up IBM's xlf Fortran compiler and built MPICH.)
You make rapid progress because Apple's software engineers work along
beside you, right at your elbow, to help you port/optimize your code.
They are constantly asking, "Can we help you with something?" If you
run into a sticking point, they immediately phone upstairs and summon
the resident expert on that issue, who rushes down to hammer on your
problem until it's resolved. A number of attendees, who had been
toiling away for weeks trying to port their code from CodeWarrior/CFM
to Cocoa/Mach-O/Xcode, were astonished to have their projects completed
within the first 2-3 hours of lab time.
To top it all off, Apple provides breakfast, lunch and dinner -- all
for free! Plus a serving of multithreading sample code for dessert.
The top 10 most important points I came away with:
2. Profile before you optimize. First conduct a baseline benchmark.
Compile with the -g option (to obtain source code level analysis) then
use Shark to profile your app. Follow Shark's friendly
explanations/suggestions and begin tuning.
3. Thread your app to take advantage of multiple processors. Avoid
creation and destruction overhead -- set up pools and malloc first,
then spawn threads containing the most processor-intensive tasks.
4. Vectorize your app (AltiVec is a tres cool technology!) if possible
to attain a HUGE boost in performance. Apple's Advanced Computation
Group has developed some incredibly nice libraries (such as the vImage
Image Processing Framework) to make it easy and fun.
5. Because of the G5's deeper 23-stage pipelines (vs. the G4's 7-stage
pipelines), avoid if-branches in your "hot loops".
6. You can achieve a big improvement in performance by unrolling loops
(-funroll-loops compiler option) but this can cause "code bloat" (large
memory footprint) so only use on computationally intensive portions of
your app.
7. Avoid type casting between floats and ints. Stay in one domain
(integer or floating point) within your "hot loops".
8. Try the -fast compiler option (see 'man gcc') but use with caution.
Check results for numerical accuracy.
9. Take advantage of the G5's hardware floating-point fast square-root
function by using the -mpowerpc-gpopt option.
10. Load fewer, larger chunks of memory. "Maximize locality" (use more
local variables). "Globals are evil." It is better to copy global data
into local variables and perform loops on them.
A big thanks to Apple's software engineers (including one wildly
animated Frenchman, Mr. Xcode, and a certain unnamed Schizophrenic
Optimization Scientist) and to all the folks at Apple Developer
Connection for creating such a useful, friendly workshop!
Craig Mattocks, PhD/President
Scientific Software Solutions, Inc.
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