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clSVMAlloc.adoc

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clSVMAlloc

Allocates a shared virtual memory (SVM) buffer that can be shared by the host and all devices in an OpenCL context that support shared virtual memory.

void * clSVMAlloc(cl_context context,
                  cl_svm_mem_flags flags,
                  size_t size,
                  cl_uint alignment)

Parameters

context

A valid OpenCL context used to create the SVM buffer.

flags

A bit-field that is used to specify allocation and usage information. The following table describes the possible values for flags.

cl_svm_mem_flags Description

CL_MEM_READ_WRITE

This flag specifies that the SVM buffer will be read and written by a kernel. This is the default.

CL_MEM_WRITE_ONLY

This flag specifies that the SVM buffer will be written but not read by a kernel.

Reading from a SVM buffer created with CL_MEM_WRITE_ONLY inside a kernel is undefined.

CL_MEM_READ_WRITE and CL_MEM_WRITE_ONLY are mutually exclusive.

CL_MEM_READ_ONLY

This flag specifies that the SVM buffer object is a read-only memory object when used inside a kernel.

Writing to a SVM buffer created with CL_MEM_READ_ONLY inside a kernel is undefined.

CL_MEM_READ_WRITE or CL_MEM_WRITE_ONLY and CL_MEM_READ_ONLY are mutually exclusive.

CL_MEM_SVM_FINE_GRAIN_BUFFER

This specifies that the application wants the OpenCL implementation to do a fine-grained allocation.

CL_MEM_SVM_ATOMICS

This flag is valid only if CL_MEM_SVM_FINE_GRAIN_BUFFER is specified in flags. It is used to indicate that SVM atomic operations can control visibility of memory accesses in this SVM buffer.
size

The size in bytes of the SVM buffer to be allocated.

alignment

The minimum alignment in bytes that is required for the newly created buffer’s memory region. It must be a power of two up to the largest data type supported by the OpenCL device. For the full profile, the largest data type is long16. For the embedded profile, it is long16 if the device supports 64-bit integers; otherwise it is int16. If alignment is 0, a default alignment will be used that is equal to the size of largest data type supported by the OpenCL implementation.

Notes

If CL_MEM_SVM_FINE_GRAIN_BUFFER is not specified, the buffer can be created as a coarse grained SVM allocation. Similarly, if CL_MEM_SVM_ATOMICS is not specified, the buffer can be created without support for SVM atomic operations (refer to the OpenCL C++ kernel language and IL specifications).

Calling clSVMAlloc does not itself provide consistency for the shared memory region. When the host can’t use the SVM atomic operations, it must rely on OpenCL’s guaranteed memory consistency at synchronization points.

For SVM to be used efficiently, the host and any devices sharing a buffer containing virtual memory pointers should have the same endianness. If the context passed to clSVMAlloc has devices with mixed endianness and the OpenCL implementation is unable to implement SVM because of that mixed endianness, clSVMAlloc will fail and return NULL.

Although SVM is generally not supported for image objects, clCreateImage may create an image from a buffer (a 1D image from a buffer or a 2D image from buffer) if the buffer specified in its image description parameter is a SVM buffer. Such images have a linear memory representation so their memory can be shared using SVM. However, fine grained sharing and atomics are not supported for image reads and writes in a kernel.

If clCreateBuffer is called with a pointer returned by clSVMAlloc as its host_ptr argument, and CL_MEM_USE_HOST_PTR is set in its flags argument, clCreateBuffer will succeed and return a valid non-zero buffer object as long as the size argument to clCreateBuffer is no larger than the size argument passed in the original clSVMAlloc call. The new buffer object returned has the shared memory as the underlying storage. Locations in the buffer’s underlying shared memory can be operated on using atomic operations to the device’s level of support as defined in the memory model.

Errors

Returns a valid non-NULL shared virtual memory address if the SVM buffer is successfully allocated. Otherwise, like malloc, it returns a NULL pointer value. clSVMAlloc will fail if:

  • context is not a valid context.

  • flags does not contain CL_MEM_SVM_FINE_GRAIN_BUFFER but does contain CL_MEM_SVM_ATOMICS.

  • Values specified in flags do not follow rules described for supported values in the table above.

  • CL_MEM_SVM_FINE_GRAIN_BUFFER or CL_MEM_SVM_ATOMICS is specified in flags and these are not supported by at least one device in context.

  • The values specified in flags are not valid i.e. don’t match those defined in the table above.

  • size is 0 or > CL_DEVICE_MAX_MEM_ALLOC_SIZE value for any device in context.

  • alignment is not a power of two or the OpenCL implementation cannot support the specified alignment for at least one device in context.

  • There was a failure to allocate resources.