I have a for
loop to be parallelized with OpenMP, but there are multiple computational errors, probably due to my lack of understanding of the concept of multithreading with OpenMP:
for ( int i = -X/2; i < X/2; ++i )
{
base.y = anchor + i*rho_step;
temp = some_function( base );
if( temp > response )
{
buffer.y = base.y;
response = temp;
}
}
This works fine, then I made the following changes:
#pragma omp parallel for shared (buffer, response) private(base, temp)
for ( int i = -X/2; i < X/2; ++i )
{
base.y = anchor + i*rho_step;
temp = some_function( base );
if( temp > response )
{
buffer.y = base.y;
response = temp;
}
}
In this code, neither buffer.y
nor response
will have the correct values. In my understanding, every single thread should have an own copy of base.y
and temp
, they are only temporary variables for the computation, and buffer
and response
must be shared (they will store the computed data), but this does not work as I would expect.
The only version that is perfect is the following, but obviously, there is no performance increase:
omp_lock_t writelock;
omp_init_lock(&writelock);
omp_set_num_threads (4);
#pragma omp parallel for
for ( int i = -X/2; i < X/2; ++i )
{
omp_set_lock(&writelock);
base.y = anchor + i*rho_step;
temp = some_function( base );
if( temp > response )
{
buffer.y = base.y;
response = temp;
}
omp_unset_lock(&writelock);
}
omp_destroy_lock(&writelock);
What can be the problem? ( anchor
and rho_step
are constants in this loop)
In order to get your code to deal with the trans-thread of the buffer
and response
variables, you'll need to use some per-thread local variables for them, and perform a final reduction with them to update their shared counterparts.
Here is what it would look like (not tested):
#pragma omp parallel firstprivate( base )
{
auto localResponse = response;
auto localBuffer = buffer;
#pragma omp for
for ( int i = -X/2; i < X/2; ++i )
{
base.y = anchor + i * rho_step;
auto temp = some_function( base );
if ( temp > localResponse )
{
localBuffer.y = base.y;
localResponse = temp;
}
}
#pragma omp critical
{
if ( localResponse > response )
{
buffer.y = localBuffer.y;
response = localResponse;
}
}
}
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