prog-ompss

Programming with OmpSs

Alejandro Duran

Barcelona Supercomputing Center

28th May 2011

1 Installation

2 Tasking in OpenMPWaiting for tasksThe dependency clauses

3 Other OpenMP useful concepts

Alex Duran (BSC) Programming with OmpSs 28th May 2011 2 / 35

Installation

Outline

1 Installation




Installation

Installation

You need to install the following:MercuriumNanos++Decide a TARGET installation directory

TARGET=$HOME/soft


Installation

Installing Nanos++

1 git clone http://pm.bsc.es/git/nanox.git

2 cd nanox

3 autoreconf -f -i

4 ./configure --prefix=$TARGET

5 make install


Installation

Installing Mercurium

1 git clone http://pm.bsc.es/git/mcxx.git

2 cd mcxx

3 autoreconf -f -i

4 ./configure --prefix=$TARGET --enable-ompss

5 make install


Installation

Ready to use

1 Add to pathPATH=$TARGET/bin:$PATH

2 Compile your applicationmcc --ompss -o app app.c

3 RunOMP_NUM_THREADS=4 ./app


Tasking in OpenMP

Outline

1 Installation




Tasking in OpenMP

What is a task in OpenMP?

Tasks are work units whose execution may be deferredthey can also be executed immediately

Tasks are composed of:code to executea data environment

Initialized at creation time

Threads cooperate to execute them


Tasking in OpenMP

Creating tasks

The task construct

#pragma omp task [ c lauses ]s t r u c t u r e d block | func t ion−dec l | f unc t ion−def

Where clauses can be:Data-sharing clauses:

sharedprivatefirstprivatedefault

if(expression)

untied


Tasking in OpenMP

Data-sharing clauses

Each variable in OpenMP has a data-sharing attribute:shared The task shares the variable of its parentprivate Each task has its own copy of the variable with undefinedvaluefirstprivate The private copy is initialized with the parent variable

This is value is captured when the task is created

This is the default for tasks


Tasking in OpenMP


void foo ( ){

i n t a , b , c ;

a = 1;b = 2;c = 3;

#pragma omp task shared ( a ) private ( b ) firstprivate ( c ){

p r i n t f ("%d\n" , a ) ;p r i n t f ("%d\n" , b ) ;p r i n t f ("%d\n" , c ) ;

}

a++;b++;c++;

}

prints 1 or 2. Requires synchronization

prints whatever. Undefined valueprints 3


Tasking in OpenMP


void foo ( ){

i n t a , b , c ;

a = 1;b = 2;c = 3;



}

a++;b++;c++;

}

prints 1 or 2. Requires synchronization

prints whatever. Undefined value

prints 3


Tasking in OpenMP


void foo ( ){

i n t a , b , c ;

a = 1;b = 2;c = 3;



}

a++;b++;c++;

}

prints 1 or 2. Requires synchronizationprints whatever. Undefined value

prints 3


Tasking in OpenMP

List traversal

void t r a v e r s e _ l i s t ( L i s t l ){

Element e ;for ( e = l −> f i r s t ; e ; e = e−>next )

#pragma omp taskprocess ( e ) ;

}e is firstprivate


Tasking in OpenMP

Task clauses

untied Allows the task to be stolen once startedif Allows to create a task conditionally


Tasking in OpenMP

Tree traversal

void po s to rd e r _ t r a ve r sa l ( Node ∗ n , i n t depth ){

#pragma omp task i f ( d < MAX_DEPTH) untiedpo s to r de r _ t r ave r sa l ( n−> l e f t , d +1 ) ;

#pragma omp task i f ( d < MAX_DEPTH) untiedpo s to rde r _ t r ave r sa l ( n−>r i g h t , d +1) ;

process ( n−>data ) ;}

Helps with load balancingRegular if works betterwith fine-grain tasks

This could be executed before children


Tasking in OpenMP

Tree traversal





Helps with load balancing

Regular if works betterwith fine-grain tasks



Tasking in OpenMP

Tree traversal





Helps with load balancing

Regular if works betterwith fine-grain tasks



Tasking in OpenMP

Tree traversal





Helps with load balancingRegular if works betterwith fine-grain tasks



Tasking in OpenMP Waiting for tasks

Waiting for children

The taskwait construct

#pragma omp taskwait

Suspends the current task until all children tasks are completedJust direct children, not descendants



Taskwait

Example





}

All tasks guaranteed to be completed here



Taskwait

Example





}All tasks guaranteed to be completed here



Tree traversal


#pragma omp task i f ( d < MAX_DEPTH) untiedpo s to r de r _ t r a ve r sa l ( n−> l e f t , d +1 ) ;

#pragma omp task i f ( d < MAX_DEPTH) untiedpo s to r de r _ t r a ve r sa l ( n−>r i g h t , d +1) ;

#pragma omp taskwaitprocess ( n−>data ) ;

}

Children completed here



Tree traversal


#pragma omp task i f ( d < MAX_DEPTH) untiedpo s to r de r _ t r a ve r sa l ( n−> l e f t , d +1 ) ;

#pragma omp task i f ( d < MAX_DEPTH) untiedpo s to r de r _ t r a ve r sa l ( n−>r i g h t , d +1) ;

#pragma omp taskwaitprocess ( n−>data ) ;

}Children completed here


Tasking in OpenMP The dependency clauses

Clauses

We have four clauses:input(lvalue-expression)

specifies that the task will block until its inputs are fulfilledoutput(lvalue-expression)

specifies that the task will release tasks waiting on its outputsinout(lvalue-expression)

combination of the above

concurrent(lvalue-expression)

Variables specified as depedencies are sharedCorrectness depends on the programmerExpressions are evaluated at runtime!



Example

x = 0;#pragma omp task input ( x )

p r i n t f ("x = %d\n" , x ) ;

#pragma omp task inout ( x )x++;

#pragma omp task output ( x )x = 2 ;

#pragma omp task input ( x )p r i n t ("x = %d\n" , x ) ;

#pragma omp task input ( x )w r i t e ( x ) ;



Extended expressions

OmpSs extends the valid expressions in the context of dependencyclauses

Array sections. Two syntaxes:[ lower : upper ]

A[3:5] <=> A[3], A[4], A[5]A[ : upper ] <=> A[ 0 : upper ]A[ lower : ] <=> A[ lower : elements of (A) - 1 ]A[ : ] <=> A <=> A[ 0 : elements of (A) - 1 ]

[ lower ; size ]A[3;5] <=> A[3],A[4],A[5],A[6],A[7]

Shaping expressionsAllow to define the shape of the memory behind a pointerMultiple dimensions can be specified

[N][N] p


Right now the sections must overlap perfectly!


How the graph is build

The graph is built in sequential orderEnsures that no-deadlock can occur

For each dependency:1 Evaluate the expression2 If input, lookup for a previous writer on the evaluated address3 If output/inout, lookup for a previous task on that address4 If found, connect5 If output/inout, become last writer



SparseLU

The problemPerform a LU factorization on blocked sparse matrix

lu0 fwd fwd fwd

bdiv bmod bmod bmod

bdiv bmod bmod bmod

bdiv bmod bmod bmod



SparseLU

for ( kk =0; kk<NB; kk ++) {lu0 (A [ kk ] [ kk ] ) ;for ( j j =kk +1; j j <NB; j j ++)

i f (A [ kk ] [ j j ] != NULL)fwd (A [ kk ] [ kk ] , A [ kk ] [ j j ] ) ;

for ( i i =kk +1; i i <NB; i i ++)i f (A [ i i ] [ kk ] != NULL)

bdiv (A [ kk ] [ kk ] , A [ i i ] [ kk ] ) ;for ( i i =kk +1; i i <NB; i i ++)

for ( j j =kk +1; j j <NB; j j ++)i f (A [ kk ] [ j j ] != NULL) {

i f (A [ i i ] [ j j ]==NULL) A [ i i ] [ j j ]= a l l oca te_c lean_b lock ( ) ;bmod (A [ i i ] [ kk ] , A [ kk ] [ j j ] , A [ i i ] [ j j ] ) ;

}}



SparseLUWith dependences

for ( kk =0; kk<NB; kk ++) {#pragma omp task inout (A [ kk ] [ kk ] )

lu0 (A [ kk ] [ kk ] ) ;for ( j j =kk +1; j j <NB; j j ++)

i f (A [ kk ] [ j j ] != NULL)#pragma omp task input (A [ kk ] [ kk ] ) inout (A [ kk ] [ j j ] )

fwd (A [ kk ] [ kk ] , A [ kk ] [ j j ] ) ;for ( i i =kk +1; i i <NB; i i ++)

i f (A [ i i ] [ kk ] != NULL)#pragma omp task input (A [ kk ] [ kk ] ) inout (A [ i i ] [ kk ] )

bdiv (A [ kk ] [ kk ] , A [ i i ] [ kk ] ) ;for ( i i =kk +1; i i <NB; i i ++)

for ( j j =kk +1; j j <NB; j j ++)i f (A [ kk ] [ j j ] != NULL) {

i f (A [ i i ] [ j j ]==NULL) A [ i i ] [ j j ]= a l l oca te_c lean_b lock ( ) ;#pragma omp task input (A [ i i ] [ kk ] ,A [ kk ] [ j j ] ) inout (A [ i i ] [ j j ] )

bmod (A [ i i ] [ kk ] , A [ kk ] [ j j ] , A [ i i ] [ j j ] ) ;}

}#pragma omp taskwait



SparseLUGenerated graph



Waiting on some data

#pragma taskwait on ( expression )

Expressions allowed are the same as for the dependency clausesBlocks the encountering task until the data is available


#pragma omp taskwait on ( x )p r i n t f ("%d\n" , x ) ;



Waiting on some data

#pragma taskwait on ( expression )

Expressions allowed are the same as for the dependency clausesBlocks the encountering task until the data is available


#pragma omp taskwait on ( x )p r i n t f ("%d\n" , x ) ;



The concurrent clause

Sometimes the dependencies are too restrictive because of someknow property

E.g., reductionsThe concurrent clause is less-restrictive inout clause:

Concurrent dependencies are ordered with respect to outputsbeforeConcurrent dependencies are ordered with respect to outputsafterConcurrent dependencies are not ordered between themselvesThe task may require additional synchronization becauso of thisordering



The concurrent clause

#pragma omp task output ( [ n ] vec )void generate_vector ( i n t ∗vec , i n t n )

#pragma omp task input ( [ n ] vec ) concurrent ( ∗ r e s u l t s )void sum_task ( i n t ∗vec , i n t n , i n t ∗ r e s u l t s ){

i n t i ;i n t local_sum =0;

for ( i = 0 ; i < n ; i ++)local_sum += vec [ i ] ;

#pragma omp atomic∗ r e s u l t s += local_sum ;

}



Dependencies & nesting

Dependencies are only computed between siblingsThere are multiple graphs in practice

Each task must specify the coarse-grain depedenciesIts children can specify a subset of those

Depedencies across branches of tasks result in undefinedbehavior


Other OpenMP useful concepts

Outline

1 Installation





Syncronization

#pragma omp atomicx++

#pragma omp critical{

foo ( ) ;}


Only works on SMP


Loop worksharings

#pragma omp for schedule ( STATIC )for ( i = 0 ; i < N; i ++ )

f ( i ) ;

Iterations distributed among threadsDependencies can be applied to the whole loop



Loop dependencies

#pragma omp for output ( a )for ( i = 0 ; i < N; i ++ )

a [ i ] = 0 ;

#pragma omp task input ( a )for ( i = 0 ; i < N; i ++ )

p r i n t f ("%d\n" , a [ i ] ) ;

Implies a nowait clause



Loop dependencies

#pragma omp for output ( a )for ( i = 0 ; i < N; i ++ )

a [ i ] = 0 ;

#pragma omp task input ( a )for ( i = 0 ; i < N; i ++ )

p r i n t f ("%d\n" , a [ i ] ) ;

Implies a nowait clause



API Calls

omp_num_threads Returns number of total threadsomp_get_thread_num Returns id of the threadomp_get_wtime Returns wall time


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