// the host has a (big) malloced hostArray, on which OpenCl should work.
- (void)makeBufferOfSize: (size_t)arraySize from: (void *)hostArray
{
_clArray = clCreateBuffer( context, CL_MEM_READ_WRITE |
CL_MEM_USE_HOST_PTR, arraySize, hostArray, NULL );
_hostArray = hostArray;
}
for(;;)
{
// the host tells OpenCL to run a kernel which modifies _clArray
// then the host needs some of the data which OpenCL has modified:
- (void)theHostWantsDataFrom: (NSUInteger)sta to: (NSUInteger)end
{
clEnqueueReadBuffer( commands, _clArray, CL_TRUE, sta, end -
sta, _hostArray + sta, 0, NULL, NULL );
}
}
This works fine. clEnqueueReadBuffer is a no-op for DEVICE_HOST_UNIFIED_MEMORY
and is reasonably fast else (i.e. for an external GPU).
But now for an exercise in gcl:
- (void)makeBufferOfSize: (size_t)arraySize from: (void *)hostArray
{
_clArray = gcl_malloc( arraySize, hostArray, CL_MEM_READ_WRITE |
CL_MEM_USE_HOST_PTR );
_hostArray = hostArray;
}
- (void)theHostWantsDataFrom: (NSUInteger)sta to: (NSUInteger)end
{
if ( DEVICE_HOST_UNIFIED_MEMORY )
{
// nothing needs to be done here; just wait till all work
has been done
dispatch_group_wait( group, DISPATCH_TIME_FOREVER );
}
else // very bad code here
{
NSUInteger bytesToCopy = end - sta;
uint8 *tempBuff = malloc( bytesToCopy );
dispatch_sync( queue, ^void{ gcl_memcpy( tempBuff, _clArray +
sta, bytesToCopy ); } );
memcpy( _hostArray + sta, tempBuff, bytesToCopy ); // (BAD)
free( tempBuff );
};
}
This works, but the double copy at (BAD) does not look very efficient.
There clearly must be a better way. But how?
Gerriet.
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