Lecture 13: Does this Code Make My Method Look Hot ?

LECTURE 13:DOES THIS CODE MAKE MY METHOD LOOK HOT?

Computer Science 313 – Advanced Programming Topics

Tom Cargill

The first 90% of the code accounts

for thefirst 90% of the

development time. The

remaining 10% of the code

accounts for the other 90% of the

development time.

Let’s Get Coding

Who will write next lab in assembly? Can only use assembly code for entire lab

pushl %ebp movl %esp, %ebp subl $24, %esp movl 8(%ebp), %eax movl %eax, -4(%ebp) cmpl $0, -4(%ebp)

Let’s Get Coding


During the first pass, apply standard optimizations


Let’s Get Coding



Every method should be tuned to maximize branches


Let’s Get Coding



Every method should be tuned to maximize branches

Necessary to achieve top performance


Promise of Perfect Debugging Programs just λ-calculus equations Use Java, but instead prove program

correct Translate program into λ-calculus Specify theorem & prove program

correctness

Fewest Lectures Ever

What if guaranteed A for student doing this? Would still need to submit assignment

on time Working assembly needed (with

optimizations) Code should work, but proof must be

perfect

Any takers?

Fewest Lectures Ever

What if guaranteed A for student doing this? Would still need to submit assignment

on time Working assembly needed (with

optimizations) Code should work, but proof must be

perfect

Any takers? Why not?

Why Isn’t This Done?

Almost never use these techniques Low-level device drivers use some C (or

similar) Rare for partial proof of correctness to

be used Simple result from easy cost-benefit

analysis Techniques requires massive amounts of

time Time is money; these are very, very

expensive The provided benefits just not worth it

What’s The Secret?How do I know

when to optimize? I

need to know. Please, please,

please?

Problem is Universal

Frequent need to make optimal decision Using reagents during chemical

synthesis Take power plant offline & perform

maintenance Fastest torrent from which to download

Cannot optimize everything Instead we only focus efforts where it

matters

Problem is Universal

Frequent need to make optimal decision Using reagents during chemical

synthesis Take power plant offline & perform

maintenance Fastest torrent from which to download

Cannot optimize everything Instead we only focus efforts where it

matters

But how can I know what is important?

Are Graphics Important?

Rewrite Swing to improve its performance?


Rewrite Swing to improve its performance? Hells, yes. We

need 60 fps for new games.


Rewrite Swing to improve its performance? Hells, yes. We

need 60 fps for new games.

%#$ no. What does it do for search speed?

Critical Paths

All that is important is code along critical path Determines time needed and not just

the fluff Graphics are not critical for Google

Major graphics improvements are nice… …but have zero affect on search times

Bungie’s critical path is graphics package Games performance limited by graphic

times Would hate GPU for search – slows them

down

Guess the Critical Path

$ java -Xprof edu.canisius.ann.NetworkFlat profile of 16.4 secs (956 total ticks): main Compiled + native Method35.6% 282 + 58 Neuron.getWeightedError21.1% 187 + 15 Neuron.updateWeights13.0% 124 + 0 Network.trainNetwork 9.5% 0 + 91 java.lang.StrictMath.exp 9.5% 91 + 0 Neuron.compute 4.5% 22 + 21 Network.setNetworkInput 3.6% 34 + 0 Network.runOneInput 2.0% 0 + 19 java.util.ArrayList.<init> 0.8% 8 + 0 Network.main99.6% 748 + 204 Total compiled

Still a Cost-Benefit Trade

Great. More useless geekery.

When will it make a difference?

When is it worth my time?

Amdahl’s Law

Determines how much faster program can go Use this to answer question that matters

Speedup calculated by this equation Speedup of 2 means program twice as

fast Need 1/10 the time, 10 is speedup factor of

approach When speedup is 1, time taken

unchanged Speedup of ½ == twice as long as past

approach

Amdahl’s Law






approach(But usually say slowdown factor of 2)

Amdahl’s Law






approach(But usually say slowdown factor of 2)(Or hide report that shows this sucks)

Amdahl’s Law Equation

%changed of time in methods to be optimized Expressed as decimal between 0 - 1

All other time from run is %unchanged Another decimal from 0 – 1

If %changed + %unchanged ≠ 1, you FAIL

Applying Amdahl’s Law

Made getWeightedError twice as fast Speedupchanged = 2, since now twice as fast Took 16.4 seconds before, what will it take

now?

Get execution times using java –Xprof

35.6% 282 + 58 Neuron.getWeightedError

%changed = 0.356%unchanged= 1 - 0.356 =

0.644

Amdahl’s Law

Speedup = 1.21

Applying Amdahl’s Law

Move Neuron.compute off critical path? Removes it from program execution time Speedupchanged = ∞

Get execution times from java –Xprof

9.5% 91 + 0 Neuron.compute

%changed = 0.095%unchanged= 1 - 0.095 =

0.905

Amdahl’s Law

Speedup = 1.10

20%

43%

56%

64%

69%

73%

76%

79%

81%

83%

84%

85%

0%10%20%30%40%50%60%70%80%90%

5%10%25%50%75%90%

%Faster Optimized Section Runs

Ove

rall

Redu

ctio

n In

Tim

eWhat Amdahl’s Law Means

For Next Class

Lab #3 available on Angel Asks you to implement Decorator

Pattern Have time Friday, but may want help

profiling For Friday, read pages 109-122 in

book Allocation sucks; can we do anything

about it? Why would we care about improving

allocation? When would this help write better (good)

code?

Lecture 13: Does this Code Make My Method Look Hot ?

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