In‑Order vs. Out‑of‑Order Command Queues: Which Minimizes Latency Spikes Under Concurrent Kernels?

Goal

Determine how the choice of command‑queue ordering affects per‑kernel latency when multiple kernels are launched concurrently on an OpenCL device.

Constraints & Uncertainty

In‑order queues serialize kernel enqueues, guaranteeing deterministic execution but potentially hiding latency spikes caused by resource contention. Out‑of‑order queues allow kernels to overlap, which can reduce overall throughput latency but introduces non‑deterministic scheduling and requires explicit event synchronization, possibly adding overhead.

OpenCL specifications do not mandate a uniform scheduling policy for out‑of‑order queues across vendors, so the observed latency behavior may vary by driver and device generation.

Unresolved Decision

Should applications favor deterministic in‑order queues to avoid unpredictable latency spikes, or adopt out‑of‑order queues to exploit overlap and reduce average latency, accepting the risk of race‑condition induced delays?

Specific Questions

1. Under what kernel workload patterns (e.g., compute‑heavy vs. memory‑bound) does an in‑order queue reduce latency spikes compared to an out‑of‑order queue?
2. Do vendor‑specific OpenCL extensions exist that expose scheduling hints for out‑of‑order queues, and how do they influence latency under concurrent launches?
3. What is the impact of explicit event synchronization (clWaitForEvents, clFinish) on overall latency when using out‑of‑order queues in a multi‑kernel scenario?