correcting threading errors with intel® parallel inspector

Correcting Threading Errors with Intel® Parallel Inspector

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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


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Agenda

What is Intel® Parallel Inspector?Detecting race conditions Detecting potential for deadlockChecking library thread-safety


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Motivation

Developing threaded applications can be a complex taskNew 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


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Debugging tool for threaded software• Plug-in to Microsoft* Visual Studio*Finds threading bugs in OpenMP*, Intel® Threading Building Blocks, and Win32* threaded softwareLocates 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!


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Intel® Parallel Inspector Features

Integrated into Microsoft Visual Studio .NET* IDE• 2005 & 2008 EditionsSupports different compilers• Microsoft* Visual* C++ .NET*• Intel Parallel ComposerView (drill-down to) source code for DiagnosticsOne-click help for diagnostics• Possible causes and solution suggestions


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Parallel Inspector: Analysis

Dynamic as software runs• Data (workload) -driven executionIncludes 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


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Parallel Inspector: Before You Start

Instrumentation: background• Adds calls to library to record information

• Thread and Sync APIs• Memory accesses

• Increases execution time and sizeUse small data sets (workloads)• Execution time and space is expanded• Multiple runs over different paths yield best results

Workload selection is important!


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Workload Guidelines

Execute problem code once per thread to be identifiedUse 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!


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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)


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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


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Starting Parallel Inspector

Build the Debug version of the application with appropriate flags set


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Select Parallel Inspector from the Tools menu

You can choose to look forMemory ErrorsThreading Errors


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The Configure Analysis window pops up

Select the level of analysis to be carried out by Parallel InspectorThe deeper the analysis, the more thorough the results and the longer the execution time

Click Run Analysis


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The initial (raw) results come up after analysis

Click the Interpret Results button to filter the raw data into more human consumable formats


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The analysis results are gathered together in related categories

Double-click a line from the Problem Sets pane to see the source code that generated the diagnostic


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The source lines involved in a data race can be shown


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Activity 1a - Potential Energy

Build and run serial version Build threaded versionRun application in Parallel Inspector to identify threading problems


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Race Conditions

Execution order is assumed but cannot be guaranteed• Concurrent access of same variable by multiple threadsMost common error in multithreaded programsMay not be apparent at all times


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Solving Race Conditions

Solution: Scope variables to be local to threadsWhen to use• Value computed is not used outside parallel region• Temporary or “work” variablesHow to implement• OpenMP scoping clauses (private, shared)• Declare variables within threaded functions• Allocate variables on thread stack• TLS (Thread Local Storage) API


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Solving Race Conditions

Solution: Control shared access with critical regionsWhen to use• Value computed is used outside parallel region• Shared value is required by each threadHow to implement• Mutual exclusion and synchronization• Lock, semaphore, event, critical section, atomic…• Rule of thumb: Use one lock per data element


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Activity 1b - Potential Energy

Fix errors found by Parallel Inspector


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Deadlock

Caused by thread waiting on some event that will never happenMost 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(&L1); processB(data2, data1) ; LeaveCriticalSection(&L1);LeaveCriticalSection(&L2);

return(0);}

ThreadA: L1, then L2

ThreadB: L2, then L1


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Deadlock

Add lock per elementLock 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


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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);


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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 sectionUpon exit of critical section

LeaveCriticalSection(&cs)• Must be from obtaining thread


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Example: Critical Section#define NUMTHREADS 4CRITICAL_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);}


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Activity 2 - Deadlock

Use Intel® Parallel Inspector to find and correct the potential deadlock problem.


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Thread Safe Routines

All routines called concurrently from multiple threads must be thread safeHow to test for thread safety?• Use OpenMP and Parallel Inspector for analysis

• Use sections to create concurrent execution


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Thread Safety Example

Check for safety issues between• Multiple instances of routine1()• Instances of routine1() and

routine2()

Set up sections to test all permutationsStill 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);

}


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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


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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


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Intel® Parallel InspectorWhat’s Been CoveredThreading errors are easy to introduceDebugging these errors by traditional techniques is hardIntel® Parallel Inspector catches these errors• Errors do not have to occur to be detected• Greatly reduces debugging time• Improves robustness of the application


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correcting threading errors with intel® parallel inspector

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