threads in cPP

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Threads in C++

Gabor CzilliUniversity of Erlangen-Nuremberg - Advanced C++ Seminar

July 11, 2011

Gabor Czilli (CS Masters Student) Threads in C++ July 11, 2011 1 / 26
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Table of Contents

1 Introduction & Motivation

2 Pthreads (Posix Threads)Pthread example

3 Boost ThreadsThread ManagementSynchronizationThread Pool

4 OpenMPThread ManagementSynchronization

5 Boost vs. OpenMP

6 Summary / Conclusion

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Introduction & MotivationWhy everyone wants Threads

Processors do not get (much) faster anymoreBut the number of processing units increase on a single chip(2,4,8,16 core machines)

As well as the number of simultaneous operations per clock cycleWe need to solve bigger and bigger problemsIn reasonable execution times

So...Computational work has to be done in parallel,

to take advantage of modern processors

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PthreadsPosix Threads

Posix: Portable Operating System Interface for UnixStandard API for Unix systems not available on WindowsC thread library

i n t p t h r e a d c r e a t e ( p t h r e a d t t , c o n s t p t h r e a d a t t r t a t t r , v o i d(s t a r t r o u t i n e ) ( v o i d ) , v o i d ar g ) ;

i n t p t h r e a d j o i n ( p t h r e a d t t , v o i d r e t v a l ) ;

i n t p t h r e a d m u t e x i n i t ( p t hr e ad m u te x t mutex , c o n s t

p t h re a d m u t ex a t t r t a t t r ) ;

i n t p t h r e a d m u t e x d e s t r o y ( p t h r e ad m u t e x t mutex ) ;

i n t p t h r e a d m u t e x l o c k ( p t h r e ad m u t e x t mutex ) ; // a nd same u n l o c kf u n c t i o n

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PthreadsExample

// l i n k w i th p t h r e a d# i n c l u d e < p t h r e a d . h ># i n c l u d e < i o s t r e a m >

# d e f i n e NTHREADS 4

s t a t i c p t h r e a d m u t e x t m ut ex 1 ;

i n t main () {

p t h r e a d m u t e x i n i t (& mu te x1 , 0 ) ;pt hr ea d t th re ad [NTHREADS] ;

f o r ( i n t i = 0 ; i < NTHREADS; ++i )p t h r e a d c r e a t e ( t h re a d + i , 0 , t h r e a d s t a r t , ( v o i d ) i ) ;

f o r ( i n t i = 0 ; i < NTHREADS; ++i )p t h r e a d j o i n ( t h r e a d [ i ] , 0 ) ;

p t h r e a d m u t e x d e s t r o y (& mu te x1 ) ;

r e t u r n 0 ;}

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PthreadsExample

s t a t i c v o i d t h r e a d s t a r t ( v o i d ar g ) {i n t i n d e x = ( i n t ) ar g ;

p t h r e a d m u t e x l o c k (& mut ex1 ) ;s t d : : cou t


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Boost ThreadsGeneral

Part of the next C++ standard

Portable threading library (all systems boost supports)Uses e.g. pthreads in Linux and Windows-Threads in WindowsC++ API with support for RAII

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Boost ThreadsThread Management

c l a s s boos t : : th re ad ;

For launching and managing threadsConstructor takes class with function call operatormember f u n c t i o n b o o s t : : t h r e a d : : j o i n ( )

Wait for thread to nish executionb o o st : : t h i s t h r e a d : : g e t i d ( )

Non-member function, returns unique id representing the currentlyexecuting thread# i n c l u d e < boos t / th re ad . hpp >

s t r u c t MyThread {v o i d o p e r a t o r ( ) ( ) {

// new t h re a d s s t a r t h er e}

} ;

i n t main () {boost : : thre ad thr ead (MyThread () ) ;t h re ad . j o in ( ) ;

r e t u r n 0 ;}Gabor Czilli (CS Masters Student) Threads in C++ July 11, 2011 9 / 26
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Boost ThreadsSynchronization

c l a s s bo os t : : mutex ;

Implements Lockable interface ( for lock(), unlock(), try lock())Should be used with a boost::lock for RAII resource management

c l a s s b o o s t : : l o c k g u a r d ;

Calls lock() in its constructor and unlock in destructorc l a s s b o o s t : : u n i q u e l o c k ;

Constructor can wait for locking or try to lock, when additionallypassing boost::try to lock functions: owns lock, timed lock, unlock,

release more exiblec l a s s b o o s t : : s h a r e d l o c k ;

Used when only read access is needed for a threadUnderlying mutex can be shared e.g. with a unique lock

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Boost ThreadsSynchronization example

# i n c l u d e < boos t / th rea d . hpp >

s t a t i c boos t : : sha red mutex mutex ;

s t a t i c v oi d read ( ) {b o o s t : : s h a r e d l o c k < boost : : shared mutex > lo ck (mutex ) ;// do r e a d o n l y o p e r a t i o n s h e r e i n p a r a l l e l

}

s t a t i c v oi d w r i t e ( ) {b o o s t : : u n i q u e l o c k < boost : : shared mutex > lo ck (mutex ) ;// do w r i t e o p e r a t io n s h e re

}

i n t main () {boos t : : t h rea d th rea d1 ( r ead ) ;boos t : : t h rea d th rea d2 ( r ead ) ;boos t : : t h rea d th rea d3 ( r ead ) ;boos t : : t h read th rea d4 ( wr i t e ) ;

t h rea d1 . j o in ( ) ;t h rea d2 . j o in ( ) ;t h rea d3 . j o in ( ) ;t h rea d4 . j o in ( ) ;

r e t u r n 0 ;}

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boos t : : l ock ( Lockab le1 , Lockab le2 , . . . ) ,

boos t : : l oc k ( beg in , end )

Non-member functions, acquire multiple locks in a way that avoidsdeadlocksc l a s s b o o st : : b a r r i e r ;boos t : : b a r r i e r : : wa i t ( ) ;

Barriers are created for n threads, asthreads reach the barrier (call wait())they must wait until all n threads have

arrived.c l a s s b oo st : : c o n d i t i o n v a r i a b l e ;b o o st : : c o n d i t i o n v a r i a b l e : : w a i t ( l o c k ) , n o t i f y o n e , n o t i f y a l l ;

Provide a mechanism for one thread to wait for notication from

another thread. Atomically unlocks the lock and starts sleeping.Gabor Czilli (CS Masters Student) Threads in C++ July 11, 2011 12 / 26
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b o os t : : c o n d i t i o n v a r i a b l e c on d ;boo st : : mutex mutex ;v o l a t i l e bo ol d a t a r e a d y ;

v o i d c r e a t e d a t a ( ) {// . . .d a ta r e ad y = t r u e ;c on d . n o t i f y o n e ( ) ;// . . .

}

v o i d w a i t f o r d a t a t o p r o c e s s ( ) {// . . .b o o s t : : u n i q u e l o c k < bo os t : : mutex > lo ck (mutex) ;

w h i l e ( ! d a t a r e a d y ) { // mutex i s l o c k ed h e r econd . wai t ( loc k ) ; // a t o m i c a l l y w a it and u n lo c k// mute x l o c k e d a g a i n

}

p r o c e s s d a t a ( ) ;// . . .

}

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Boost ThreadsThread Pool

Concept of a thread pool

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Boost ThreadsThread Pool

Concept of a thread pool

There is no implementation of a thread pool in Boost, but a verygood implementation based on Boost(http://threadpool.sourceforge.net)

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OpenMPGeneral

API for multi-threaded, shared memory parallelismBased on compiler support makes heavy use of compiler directives(#pragma omp)

Portable to all platforms with supporting compilers, even whencompiler ignores directives(among others: Intel-, Microsoft-, GNU-, IBM- and Cray-Compilers)Number of threads determined by: enviroment variable, explicitfunction call or automatically by the number of processors (atruntime)Higher level multithreaded programming

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OpenMPExample

Matrix-Matrix multiplication:f o r ( i n t i = 0 ; i < n ; ++i )

f o r ( i n t k = 0 ; k < n ; ++k )f o r ( i n t j = 0 ; j < n ; ++j )

out [ in+j ] += a [ i n+k ] b [ kn+j ] ;

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OpenMPExample

Matrix-Matrix multiplication:f o r ( i n t i = 0 ; i < n ; ++i )f o r ( i n t k = 0 ; k < n ; ++k )

f o r ( i n t j = 0 ; j < n ; ++j )out [ in+j ] += a [ i n+k ] b [ kn+j ] ;

Parallel with OpenMp:# i n c l u d e < omp.h >

// . . .

#pragma omp p a r a l l e l f o r

f o r ( i n t i = 0 ; i < n ; ++i )f o r ( i n t k = 0 ; k < n ; ++k )f o r ( i n t j = 0 ; j < n ; ++j )

out [ in+j ] += a [ i n+k ] b [ kn+j ] ;

// . . .

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OpenMPManagement Directives

#pragma omp p a r a l l e l{ / b l o c k / }

Start team of threads, execute the block in parallel, join threads atthe end

#pragma omp f o r{ / b l o c k / }

Partition loop iterations, to be executed in parallel

#pragma omp s e c t i o n s{

#pragma omp se c t io n{ / b l o c k / }#pragma omp se c t io n

{ / b l o c k / }}

Execute sections in parallel, each section once, by one thread


O MP
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OpenMPSynchronization Directives

#pragma omp c r i t i c a l ( name){ / b l o c k / }

Critical section, cannot be executed in parallel, identied by name

#pragma omp b a r r i e r

A thread will wait at the barrier until all other threads have reachedthis pointData scope attributes:sh a re d ( l i s t )p r i v a t e ( l i s t )

Declare data to be shared between threads, or to give each thread alocal copy


B O MP
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Boost vs. OpenMPPerformance comparison

On Linux, Boost is implemented with the help of pthreads...OpenMP tooPerformance comparison:

How well do the implementations scale on a calculation intensiveproblem? (like matrix multiplication)Synchronization is often unaviodable, which locking mechanisms are

implemented more efficiently? (when facing high lock contention)Problem 1: multithreaded calculation of a matrix-matrixmultiplicationProblem 2: like problem 1, but locking a critical section in eachiteration of the second loopHardware: 4x Intel Xeon CPU E7340 @ 2.40GHz server 16 cores,32GB RAMSoftware: g++ 4.4.5, options: -O3 -march=native -mtune=native-mfpmath=sseBoost version 1.42 (no change since 1.41)


B t O MP
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Boost vs. OpenMPResults Problem 1

0

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t i m e

i n m s

matrix siz e

OpenMPBoost

Single threaded


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OpenMPBoost


Boost vs OpenMP
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0

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Boost vs OpenMP
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Boost vs. OpenMPResults Problem 2, threads: 4

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t i m e

i n m s

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OpenMPBoostSingle threaded


Summary / Conclusion
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Summary / Conclusion

Pthreads:

low level, medium portable interfacecan be of high performancebut a lot of manual resource management neededtherefore error-prone

Boost threads

medium level, highly portable interfacemedium performance, when lockingRAII style management of resourcesstill full exibility

OpenMP

high level, highly portable interfacehigh perfomance, if it ts to your applicationcompletely different approach to multithreadingdoes not save the programmer from thinking about deadlocks, raceconditions, synchronization and efficient parallel algorithms

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Boost.Boost C++ Libraries , July 2011.

http://www.boost.org .Boost.Boost C++ Libraries , July 2011.http://www.boost.org/doc/libs/1_46_1/doc/html/thread.html .

OpenMP.Open Multi-Processing , July 2011.http://openmp.org .

OpenMP.Open Multi-Processing , July 2011.https://computing.llnl.gov/tutorials/openMP .

http://www.boost.org/http://www.boost.org/doc/libs/1_46_1/doc/html/thread.htmlhttp://www.boost.org/doc/libs/1_46_1/doc/html/thread.htmlhttp://www.boost.org/doc/libs/1_46_1/doc/html/thread.htmlhttp://openmp.org/https://computing.llnl.gov/tutorials/openMPhttps://computing.llnl.gov/tutorials/openMPhttp://openmp.org/http://www.boost.org/doc/libs/1_46_1/doc/html/thread.htmlhttp://www.boost.org/doc/libs/1_46_1/doc/html/thread.htmlhttp://www.boost.org/http://find/


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Thank you for your attention.

Questions?

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