A Dynamic Tracing Mechanism For Performance Analysis of OpenMP Applications- Caubet, Gimenez, Labarta, DeRose, Vetter (WOMPAT 2001) - Presented by Anita Nagarajan
Introduction • OpenMP • Standard for shared memory parallel programming • Set of directives and library routines for Fortran and C/C++ • Performance Tools • Need: Analyse parallel behaviour. Determine causes for OpenMP application performance problems. • Properties: Minimize intrusion cost, maximize performance data captured
Introduction(Contd.)… • Dynamic Instrumentation • Instrument application while it is executing, recompilation not required. • Dynamic Probe Class Library(DPCL) • Developed at IBM, built on top of the Dyninst API. • Using DPCL, performance tool “attaches” to application, “inserts code patches” into the binary, “starts/continues” its execution • Program instrumentation can be done at “function entry points”, “exit points” and “call sites”.
DPCL • DPCL consists of • Client library • Runtime library • Daemon • Super-daemon
OMPtrace • Built on top of DPCL • IBM compiler translates OpenMP directives into function calls.
OMPtrace(Contd.)… • Hardware Counters • OMPtrace can access hardware counters, and provide statistics of the hardware events. • Eg.L1/L2 hits, L1/L2 misses, number of instructions • Paraver • Computes “Derived Metrics” from hardware events. • Eg. L1 misses per second
Case Study: Sweep3D • Multidimensional wavefront algorithm for “discrete ordinates” deterministic particle transport simulation.
Sweep3D(Contd.)… • diag - original version of Sweep3D • mkj – “do idiag” and “do jkm” loops replaced by a triple nested loop (“do m”, “do k”, “do j”) • ccrit - based on “mkj”, outer loop parallelized, synchronization implemented using the “CRITICAL” directive. • cpipe – based on “mkj”, outer loop parallelized, synchronization implemented using shared arrays and busy waiting.
Results from Experiments Elapsed time in seconds for the different OpenMP versions
Analysis of Results using Paraver • Ccrit • Not scalable • Overhead of mutex lock and unlock, contention Red: Trying to obtain lock Blue: Using lock Green – Release lock Light Blue – Execution outside critical section
Cpipe • Better performance than ccrit. • Poor locality because the “m” loop has an iteration count of 6.
Diag • Limited scalability due to high number of L2 misses Blue: Large values Green: Low values
Optimization • kjmi • Interchange loops • Good performance, better scalability
Conclusions • OMPtrace and Paraver form a useful tool for performance analysis and optimization of OpenMP applications.