CL_QUEUE_OUT_OF_ORDER_EXEC_MODE_ENABLE: latency reduction versus implicit kernel reordering for sporadic workloads

Direct Answer

With CL_QUEUE_OUT_OF_ORDER_EXEC_MODE_ENABLE (OpenCL 1.1+), the runtime preserves kernel ordering only when you enforce it explicitly. The specification permits arbitrary reordering unless commands are connected by event wait lists, clEnqueueBarrierWithWaitList, clEnqueueMarkerWithWaitList, or clFinish. For a sporadic workload with implicit dependencies, out-of-order mode does not lower per-command enqueue latency; it only enables overlap of independent work.

Confirmed Facts vs. Likely Explanation

Confirmed by the OpenCL specification:

• Event wait lists, barriers, markers, and clFinish are the only portable ordering mechanisms in an out-of-order queue.
• Memory-object flags such as CL_MEM_READ_ONLY or CL_MEM_WRITE_ONLY do not create implicit ordering edges between kernels.
• SVM flags (CL_MEM_SVM_FINE_GRAIN_BUFFER, CL_MEM_SVM_ATOMICS) control memory consistency and host–device synchronization, not command-queue scheduling.

Likely explanation for your latency concern: if enqueue overhead dominates, out-of-order mode will not amortize it. The driver still serializes each clEnqueueNDRangeKernel call. Throughput gains require independent kernels that can overlap, or explicit pipelining across multiple queues.

SVM Flags and Reordering

SVM allocation flags do not alter queue ordering semantics. A fine-grained buffer may change when writes become visible, but it does not insert an implicit dependency between a producer and consumer kernel. You must still pass the producer event into the consumer kernel's event wait list.

Profiling Reordering vs. Merging

Use clGetEventProfilingInfo with CL_PROFILING_COMMAND_START and CL_PROFILING_COMMAND_END. If the second kernel's start timestamp precedes the first kernel's end timestamp and no event dependency exists, the runtime has overlapped or reordered them.

Kernel merging or fusion is not a profiling-visible state in standard OpenCL. Event timestamps expose execution intervals only; they cannot reliably distinguish driver-level fusion from genuine concurrent execution. Proving fusion requires vendor-specific tooling, not portable metrics.

Verification Steps

1. Enqueue a write kernel and a dependent read kernel in an out-of-order queue without events. Run repeatedly; incorrect output confirms the driver does not infer dependencies.
2. Record CL_PROFILING_COMMAND_START and CL_PROFILING_COMMAND_END for both kernels. Overlap without an event dependency proves reordering.
3. Repeat with CL_MEM_READ_ONLY / CL_MEM_WRITE_ONLY and with SVM fine-grained buffers. Ordering behavior should remain unchanged.
4. Add an explicit event dependency and verify the profiling timestamps become strictly ordered.

Recommendation

If your kernels share data and enqueue overhead dominates, keep the in-order queue or use explicit events; out-of-order mode adds risk without latency benefit. If the kernels are independent, out-of-order mode plus multiple queues can improve throughput, but it will not reduce per-command enqueue latency.