Correcting Threading Errors with Intel® Parallel Inspector

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 2 Intel® Parallel Inspector Objectives After successful completion of this module you will be able to… Use Parallel Inspector to detect and identify a variety of threading correctness issues in threaded applications Determine if library functions are thread-safe

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 3 Intel® Parallel Inspector Agenda What is Intel® Parallel Inspector? Detecting race conditions Detecting potential for deadlock Checking library thread-safety

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 4 Intel® Parallel Inspector Motivation Developing threaded applications can be a complex task New class of problems are caused by the interaction between concurrent threads Data races or storage conflicts More than one thread accesses memory without synchronization Deadlocks Thread waits for an event that will never happen

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 5 Intel® Parallel Inspector Debugging tool for threaded software Plug-in to Microsoft* Visual Studio* Finds threading bugs in OpenMP*, Intel® Threading Building Blocks, and Win32* threaded software Locates bugs quickly that can take days to find using traditional methods and tools Isolates problems, not the symptoms Bug does not have to occur to find it!

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. Intel® Parallel Inspector Features Integrated into Microsoft Visual Studio.NET* IDE 2005 & 2008 Editions Supports different compilers Microsoft* Visual* C++.NET* Intel Parallel Composer View (drill-down to) source code for Diagnostics One-click help for diagnostics Possible causes and solution suggestions 6 Intel® Parallel Inspector

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 7 Intel® Parallel Inspector Parallel Inspector: Analysis Dynamic as software runs Data (workload) -driven execution Includes monitoring of: Thread and Sync APIs used Thread execution order Scheduler impacts results Memory accesses between threads Code path must be executed to be analyzed

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 8 Intel® Parallel Inspector Parallel Inspector: Before You Start Instrumentation: background Adds calls to library to record information Thread and Sync APIs Memory accesses Increases execution time and size Use small data sets (workloads) Execution time and space is expanded Multiple runs over different paths yield best results Workload selection is important!

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 9 Intel® Parallel Inspector Workload Guidelines Execute problem code once per thread to be identified Use smallest possible working data set Minimize data set size Smaller image sizes Minimize loop iterations or time steps Simulate minutes rather than days Minimize update rates Lower frames per second Finds threading errors faster!

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 10 Intel® Parallel Inspector Building for Parallel Inspector Compile Use dynamically linked thread-safe runtime libraries ( /MDd ) Generate symbolic information ( /ZI ) Disable optimization ( /Od ) Link Preserve symbolic information ( /DEBUG ) Specify relocatable code sections ( /FIXED:NO )

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 11 Intel® Parallel Inspector Binary Instrumentation Build with supported compiler Running the application Must be run from within Parallel Inspector Application is instrumented when executed External DLLs are instrumented as used

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. Starting Parallel Inspector Build the Debug version of the application with appropriate flags set 12 Intel® Parallel Inspector

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. Starting Parallel Inspector Select Parallel Inspector from the Tools menu 13 Intel® Parallel Inspector You can choose to look for Memory Errors Threading Errors

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. Starting Parallel Inspector The Configure Analysis window pops up 14 Intel® Parallel Inspector Select the level of analysis to be carried out by Parallel Inspector The deeper the analysis, the more thorough the results and the longer the execution time Click Run Analysis

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. Starting Parallel Inspector The initial (raw) results come up after analysis 15 Intel® Parallel Inspector Click the Interpret Results button to filter the raw data into more human consumable formats

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. Starting Parallel Inspector The analysis results are gathered together in related categories 16 Intel® Parallel Inspector Double-click a line from the Problem Sets pane to see the source code that generated the diagnostic

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. Starting Parallel Inspector The source lines involved in a data race can be shown 17 Intel® Parallel Inspector

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 18 Intel® Parallel Inspector Activity 1a - Potential Energy Build and run serial version Build threaded version Run application in Parallel Inspector to identify threading problems

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 19 Intel® Parallel Inspector Race Conditions Execution order is assumed but cannot be guaranteed Concurrent access of same variable by multiple threads Most common error in multithreaded programs May not be apparent at all times

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 20 Intel® Parallel Inspector Solving Race Conditions Solution: Scope variables to be local to threads When to use Value computed is not used outside parallel region Temporary or “work” variables How to implement OpenMP scoping clauses ( private, shared ) Declare variables within threaded functions Allocate variables on thread stack TLS (Thread Local Storage) API

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 21 Intel® Parallel Inspector Solving Race Conditions Solution: Control shared access with critical regions When to use Value computed is used outside parallel region Shared value is required by each thread How to implement Mutual exclusion and synchronization Lock, semaphore, event, critical section, atomic… Rule of thumb: Use one lock per data element

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 22 Intel® Parallel Inspector Activity 1b - Potential Energy Fix errors found by Parallel Inspector

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 23 Intel® Parallel Inspector Deadlock Caused by thread waiting on some event that will never happen Most common cause is locking hierarchies Always lock and un-lock in the same order Avoid hierarchies if possible DWORD WINAPI threadA(LPVOID arg) { EnterCriticalSection(&L1); EnterCriticalSection(&L2); processA(data1, data2); LeaveCriticalSection(&L2); LeaveCriticalSection(&L1); return(0); } DWORD WINAPI threadB(LPVOID arg) { EnterCriticalSection(&L2); EnterCriticalSection(&L2); EnterCriticalSection(&L1); EnterCriticalSection(&L1); processB(data2, data1) ; processB(data2, data1) ; LeaveCriticalSection(&L1); LeaveCriticalSection(&L1);LeaveCriticalSection(&L2); return(0); return(0);} ThreadA: L1, then L2 ThreadB: L2, then L1

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 24 Intel® Parallel Inspector Deadlock Add lock per element Lock only elements, not whole array of elements void swap (shape_t A, shape_t B) { lock(a.mutex); lock(b.mutex); // Swap data between A & B unlock(b.mutex); unlock(a.mutex); } typedef struct { // some data things SomeLockType mutex; } shape_t; shape_t Q[1024]; swap(Q[986], Q[34]); Thread 4 swap(Q[34], Q[986]); Thread 1 Grabs mutex 34 Grabs mutex 986

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 25 Intel® Parallel Inspector Windows* Critical Section Lightweight, intra-process only mutex Most useful and most used New type CRITICAL_SECTION cs; Create and destroy operations InitializeCriticalSection(&cs) DeleteCriticalSection(&cs);

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 26 Intel® Parallel Inspector Windows* Critical Section CRITICAL_SECTION cs ; Attempt to enter protected code EnterCriticalSection(&cs) Blocks if another thread is in critical section Returns when no thread is in critical section Upon exit of critical section LeaveCriticalSection(&cs) Must be from obtaining thread

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 27 Intel® Parallel Inspector Example: Critical Section #define NUMTHREADS 4 CRITICAL_SECTION g_cs; // why does this have to be global? int g_sum = 0; DWORD WINAPI threadFunc(LPVOID arg ) { int mySum = bigComputation(); EnterCriticalSection(&g_cs); g_sum += mySum; // threads access one at a time LeaveCriticalSection(&g_cs); return 0; } main() { HANDLE hThread[NUMTHREADS]; InitializeCriticalSection(&g_cs); for (int i = 0; i < NUMTHREADS; i++) hThread[i] = CreateThread(NULL,0,threadFunc,NULL,0,NULL); WaitForMultipleObjects(NUMTHREADS, hThread, TRUE, INFINITE); DeleteCriticalSection(&g_cs); }

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 28 Intel® Parallel Inspector Activity 2 - Deadlock Use Intel® Parallel Inspector to find and correct the potential deadlock problem.

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 29 Intel® Parallel Inspector Thread Safe Routines All routines called concurrently from multiple threads must be thread safe How to test for thread safety? Use OpenMP and Parallel Inspector for analysis Use sections to create concurrent execution

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 30 Intel® Parallel Inspector Thread Safety Example Check for safety issues between Multiple instances of routine1() Instances of routine1() and routine2() Set up sections to test all permutations Still need to provide data sets that exercise relevant portions of code #pragma omp parallel sections { #pragma omp section routine1(&data1); #pragma omp section routine1(&data2); #pragma omp section routine2(&data3); }

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 31 Intel® Parallel Inspector It is better to make a routine reentrant than to add synchronization Avoids potential overhead Two Ways to Ensure Thread Safety Routines can be written to be reentrant Any variables changed by the routine must be local to each invocation Don’t modify globally shared variables Routines can use mutual exclusion to avoid conflicts with other threads If accessing shared variables cannot be avoided What if third-party libraries are not thread safe? Will likely need to control threads access to library

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 32 Intel® Parallel Inspector Activity 3 – Thread Safety Use OpenMP framework to call library routines concurrently Three library calls = 6 combinations to test A:A, B:B, C:C, A:B, A:C, B:C

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 33 Intel® Parallel Inspector Intel® Parallel Inspector What’s Been Covered Threading errors are easy to introduce Debugging these errors by traditional techniques is hard Intel® Parallel Inspector catches these errors Errors do not have to occur to be detected Greatly reduces debugging time Improves robustness of the application

Copyright © 2006, Intel Corporation. All rights reserved. Intel and the Intel logo are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States or other countries. *Other brands and names are the property of their respective owners. 34 Intel® Parallel Inspector