Dynamic Execution Tracing of Physical Simulations Jonathan Cohen

Dynamic Execution Tracing of Physical Simulations

Jonathan Cohen

Jonathan CohenSIGGRAPH 2007 San Diego

The Motivation

• Typical bugs in physical simulation are hard to find!

• Complex code path & data flow

• gdb/printf ill-suited to the task – where's the breakpoint go? Insert complex debugging routines into the code?

• Need: trace aggregate data flow, gather statistics, look for anomalies, profiling, etc.

• Want: programmable, no recompiling, minimal impact


Anatomy of an sstrace Script

• A script is made of probes – like breakpoint, triggered by name

MODULE:FUNCTION:EVENT

or

MODULE:CLASS@METHOD:EVENT


Examples of probe names

• sim:IncompressibleSolver@Advance:enter

• sim:IncompressibleSolver@*:enter

• sim:*:enter

• sim:PoissonSolver@Solve:iterations

• sys:main:enter



• Following each probe is a block of code that is executed when probe is hit

MODULE:FUNCTION:EVENT{

command -option arg -option argcommand -option arg -option arg

}


Some Commands

• print – display text to a log file

• timer – start, stop, pause a stopwatch

• histogram – add sample points, display

• indent – set indentation level of print out

• counters – basic math



• Command arguments can reference variables

MODULE:FUNCTION:EVENT{

print $TOTAL_COSTcounter -name TOTAL_COST -add $ARG0

}


Special Variables

• Any timer can be read as variable ($total_time)

• Arguments passed from code ($ARG0, $ARG1, etc.)

• Other special variables ($TIME, $PROBE, $PROCESSMEM, etc.)


Example: Execution Stack Tracing

*:*:enter{ print -var $PROBEBASE indent -add 2}

*:*:exit{ indent -add -2}

sim:SphereSim@step sim:SphereWorldBase@AddParticleSim sim:SphereWorldBase@AddParticleSim sim:ContSphereSolverGuendelman@Advance sim:ContSphereSolverGuendelman@TakeStep sim:ContSphereSolverGuendelman@DoCollisionImpulses sim:ContSphereMultiCollision@Insert sim:ContSphereUniformGridCollision@Insert sim:ContSphereUniformGridCollision@Insert sim:ContSphereUniformGridCollision@GetCollisions sim:ContSphereUniformGridCollision@GetCollisions sim:ContSphereSolverGuendelman@DoCollisionImpulses sim:ContSphereUniformGridCollision@GetCollisions sim:ContSphereUniformGridCollision@GetCollisions<snip>


sim:FluidSim@step:enter{

timer -reset mytimertimer -start mytimer

}

sim:FluidSim@step:exit{

histogram -name myhist -add $mytimer}

sys:main:exit{

histogram -print myhist}

Example: Analyze FluidSim Timing


Example: Sphere-Sphere collision

• Complex collision data structure:

– KD-Tree

– Leaf nodes are either flat lists or uniform grids

– Heuristic recursively decides whether to make leaf (which type?) or keep splitting

– How does the heuristic work in practice?


Video – First spheresim


FRAME 240--------------------------

Type: <top level> Type:ContSphereUniformGridCollision (X,Y,Z) = 9,14,15 efficiency 46.8783 Type:ContSphereUniformGridCollision (X,Y,Z) = 8,13,13 efficiency 60.5769Acceleration data structure memory usage: 122812Total memory usage: 3.03354e+08

Trace Output


dem:*SphereSim@step:enter{ print -msg "\nFRAME " -var $ARG0 print -msg "--------------------------\n"}

Per-Frame Callback

FRAME 240--------------------------



Memory Statistics

dem:*SphereWorld@MakeAccelValid:memusage{ print -msg "Acceleration data structure memory usage: " -var $ARG0 print -msg "Total memory usage: " -var $PROCESSMEM}

FRAME 240--------------------------



dem:*SphereMultiCollision@Insert:child?type{ print -msg "Type:" -var $ARG0}

dem:*SphereMultiCollision@Insert:enter{ indent -add 2}

dem:*SphereMultiCollision@Insert:exit{ indent -add -2}

Tree Structure

FRAME 240--------------------------



dem:*SphereUniformGridCollision@Insert:dimension{ print -msg " (X,Y,Z) = " -var $ARG0, $ARGV1, $ARGV2}

dem:*SphereUniformGridCollision@Insert:efficiency{ print -msg " efficiency " -var $ARG0}

Uniform Grid Statistics

FRAME 240--------------------------



Video – second spheresim


FRAME 240--------------------------

Type: <top level> Type:ContSphereUniformGridCollision (X,Y,Z) = 23, 38, 87 efficiency 3.92304 Type:ContSphereUniformGridCollision (X,Y,Z) = 41, 440, 62 efficiency 0.164151Acceleration data structure memory usage: 2.86844e+07Total memory usage: 3.25038e+08

Trace Output – Second try


FRAME 240--------------------------

Type: <top level> Type:ContSphereUniformGridCollision (X,Y,Z) = 23, 38, 87 efficiency 3.92304 Type:ContSphereUniformGridCollision (X,Y,Z) = 41, 440, 62 efficiency 0.164151Acceleration data structure memory usage: 2.86844e+07Total memory usage: 3.25038e+08

Trace Output – Second try


Found a bug in the Heuristic!

• Splitting threshold was set so that it could never succeed

• Code was never splitting

• Validate (with another sstrace script):

– cumulative time: 107.553 sec (old)


Found a bug in the Heuristic!

• Splitting threshold was set so that it could never succeed

• Code was never splitting

• Validate (with another sstrace script):

– cumulative time: 107.553 sec (old)

– cumulative time: 82.6091 sec (new)


Type: <top level> Type:ContSphereUniformGridCollision (X,Y,Z) = 23, 38, 87 efficiency 3.96249 Type:ContSphereMultiCollision Type:ContSphereUniformGridCollision (X,Y,Z) = 32, 18, 24 efficiency 8.99161 Type:ContSphereMultiCollision Type:ContSphereUniformGridCollision (X,Y,Z) = 26, 150, 20 efficiency 0.387179 Type:ContSphereMultiCollision Type:ContSphereMultiCollision Type:ContSphereBruteForceCollision Type:ContSphereBruteForceCollision Type:ContSphereMultiCollision Type:ContSphereBruteForceCollision Type:ContSphereBruteForceCollisionAcceleration data structure memory usage: 5.63038e+06Total memory usage: 3.10231e+08

Trace Output – Third try


sstrace Run-Time

• In static constructor, loads sstrace script based on environment variable

• Parse each probe, build master list of active probes and stores pre-parsed commands

• Each probe associated with command list

• Commands triggered at run-time


Implementation

• Triggers are C++ macros, may be disabled with compile-time option

• Total overhead for probes ~5%

• Probe binding during static constructor

– Pro: minimize overhead when probes disabled

– Con: Can't change trace script after program launch


Implementation

#define T_PROBE_FUNCTION1(BASENAME, ARG0) \ static THookToProbe *__hook = TInitializeProbe(BASENAME); \ TProbeFunctionRef __probe_function; \ if (__hook) __probe_function = \ new TProbeFunction(__hook, TDataList(ARG0))

FluidSim::Step(float dt) { T_PROBE_FUNCTION1(“sim:FluidSim@Step”, dt); ...}


Implementation


• __hook is bound statically

• If BASENAME is never referenced in the script, TInitializeProbe

returns NULL


Implementation


• TProbeFunctionRef is a smart pointer

• Basically like std::auto_ptr


Implementation


• Since __hook is bound statically, one test per function call

• TProbeFunction triggers “enter” and “exit” probes in its constructor and

destructor

• Passes ARG0 down to the sstrace run-time as $ARG0


Related Work

• Solaris DTRACE

– Same concept, but probes operate at kernel level (e.g. break on file IO, thread start, timers, etc.)

– DTRACE does everything sstrace can do & more

• Aspect Oriented Programming (AOP)

– “aspects” not “objects” - dynamic emergent entities

– “code weaver” interleaves aspects with rest of code


Future

• Get a real interpreter

• Hook up 2D/3D visualization tools

• Allow control to pass to user (like gdb breakpoint) for interactive queries


An idea...

• What if this were a standard library?

• All 3rd party applications could use the same trace language

• Maybe Sun DTRACE team will do this?

– User-level DTRACE for compatibility with non-Solaris/BSD applications


Thanks

• Entire Sandstorm development team

• Spider-Man 3 sand team

• DTRACE team for the idea

Documents

Dynamic Execution Tracing of Physical Simulations Jonathan Cohen