Performance Analysis & Code Profiling It’s 2:00AM -- do you know where your program counter is?

Code is tooooo slooooooowwwwww.... Real world: performance matters. Often an app killer. Perceived performance usually bottom line for consumer apps. “In academia, the constant in the O() is nothing. In industry, it’s the only thing.” Good algorithms/data structures are crucial starting points After that, different implementations can have huge impact

Where’s the bottleneck? Assuming same data structs/algs, why is one implementation slower than another? Where is code spending most of its time? Related question: where is all of the memory? Are objects going away when they should? Rule of thumb: 80/20 rule In most programs, 80% of the time is spent in 20% of the code Problem: humans are very bad at finding the 20%! Even worse at predicting where the 20% will be when writing/designing program!

Non-solutions Blame the language. Write in C/FORTRAN/etc. Some languages do impose runtime penalties Mostly, small compared to choice of alg, etc. Use foreign language calls (assembly, C, etc.) Sub-case of above Can be useful for critical chunks of code Still stuck with -- which chunks? (80/20) Micro-optimize while writing code Lot of pain Makes code hard to read/follow Typically doesn’t help

More non-solutions Wait for HW to get faster Solution used in practice :-P Encourages sloppy design/programming Some problems won’t go away with time Let the compiler handle it Good for micro-optimizations (esp. block local) Compiler is smarter than you, usually Doesn’t handle design choices, overuse of function calls, poor data structs, indescribable invariants, data-dependent performance, etc.

Watching the code run... Ultimate answer: look and see Instrument and monitor code; see where most time is being spent Must load program under real data conditions! You’ve already done some of this Program timing Counting get()/put()/remove() calls, etc. In principle, could get everything you need that way Massive pain in the rear... 80/20 rule strikes again

Profiling tools to the rescue... Automated instrumentation of code Use external tool to monitor execution Run under realistic conditions Post-mortem examine results for critical 20% Typically work by: Rewriting compiled executable (gcc -p) Monitoring the runtime system/JVM (java - Xrunhprof)

Hprof: the Java profiler Runs JVM in special mode; watches code as it runs. Tracks: Subroutine calls/stacks Object allocations Thread execution CPU usage Invoke with: java -Xrunhprof:[hprof opt list] ClassToProf Produces text summary of run, post-mortem Note: Javasoft demo tool; NOT professional quality, industrial strength tool (but free!)

Hprof options file=fname : set output/dump file cpu=samples|times : set profiling method for CPU utilization, method calls, stack trace, etc. heap=dump|sites|all : set tracing of heap (dynamically allocated) objects depth=# : set depth of stack traces to report (max # of nested calls) thread=y|n : report thread IDs? Example: java -Xrunhprof:file=hprof.txt,cpu=samples,depth=6 \ Analyzer -u 8 -a 4 -x 842 -r results.txt -m model.dat

Problems with hprof Unreadable output (ugh) Static analysis Only gives you snapshot of results at end of run Uses sampling Only checks state of JVM periodically Can miss very short/infrequent calls Doesn’t check many things Dynamic heap state; memory leaks (yes, even in Java) File I/O Strange data access patterns Multi-thread accesses