Difference between USE_HOST, ALLOC_HOST, COPY_HOST pointer

OpenCL
1

What are the major differences between those three? Can someone provide examples if necessary?

2

See Creating buffers maybe that might explain it more.

3

These are the only valid combinations (ignoring the read-write flags which I consider othogonal) of the Buffer host-pointer memory flags:

  1. none - no host-pointer flags specified
  2. COPY_HOST_PTR
  3. ALLOC_HOST_PTR
  4. ALLOC_HOST_PTR | COPY_HOST_PTR
  5. USE_HOST_PTR

Based upon these combinations and “Table 5.3 List of supported cl_mem_flags values” especially the definition of ALLOC_HOST_PTR as “host accessible memory” I have interpreted the combinations as follows:

  1. none - possibly not “host accessible memory”
  2. COPY_HOST_PTR - possibly not “host accessible memory”
  3. ALLOC_HOST_PTR - “host accessible memory”
  4. ALLOC_HOST_PTR | COPY_HOST_PTR - “host accessible memory”
  5. USE_HOST_PTR - “host accessible memory” (implied by definition, but not specifically stated in the specification)

In addition based upon “5.2.3 Mapping Buffer Objects”, which states for CL_MAP_FAILURE: “This error cannot occur for buffer objects created with CL_MEM_USE_HOST_PTR or CL_MEM_ALLOC_HOST_PTR.”, I have interpreted the phrase “host accessible memory” to also mean “mappable”, and the phrase ‘possibly not “host accessible memory”’ to also mean “possibly non-mappable”.

  1. none - possibly non-mappable
  2. COPY_HOST_PTR - possibly non-mappable
  3. ALLOC_HOST_PTR - mappable
  4. ALLOC_HOST_PTR | COPY_HOST_PTR - mappable
  5. USE_HOST_PTR - mappable

Therefore, Buffers may use the following enqueue commands:

  1. none - can always use clEnqueue[Read|Write]Buffer, may possibly use clEnqueue[Map|Unmap]Buffer, but may received CL_MAP_FAILURE
  2. COPY_HOST_PTR - can always use clEnqueue[Read|Write]Buffer only, may possibly use clEnqueue[Map|Unmap]Buffer, but may received CL_MAP_FAILURE
  3. ALLOC_HOST_PTR - may use either clEnqueue[Read|Write]Buffer or clEnqueue[Map|Unmap]Buffer
  4. ALLOC_HOST_PTR | COPY_HOST_PTR - may use either clEnqueue[Read|Write]Buffer or clEnqueue[Map|Unmap]Buffer
  5. USE_HOST_PTR - may use either clEnqueue[Read|Write]Buffer or clEnqueue[Map|Unmap]Buffer

Therefore I make the following enqueue command recommendations, but these are probably implementation-specific and should be verified with performance/tuning evaluations.

  1. none - use clEnqueue[Read|Write]Buffer
  2. COPY_HOST_PTR - use clEnqueue[Read|Write]Buffer
  3. ALLOC_HOST_PTR - use clEnqueue[Map|Unmap]Buffer
  4. ALLOC_HOST_PTR | COPY_HOST_PTR - use clEnqueue[Map|Unmap]Buffer
  5. USE_HOST_PTR - use clEnqueue[Map|Unmap]Buffer

So when telling people about OpenCL Buffers these terms are appropriate (e.g. “host accessible memory”, “mappable”) along with these enqueue command recommendations in general.