Host-Device Synchronization
Host-device synchronization is the coordination points where CPU waits for GPU completion or GPU waits for host-side readiness - while necessary for correctness at boundaries, excessive synchronization destroys overlap and throughput.
What Is Host-device synchronization?
- Definition: Explicit or implicit barriers that align execution state between host and accelerator.
- Common Triggers: Blocking API calls, device-to-host reads, and debug operations requesting immediate results.
- Correctness Role: Required before consuming GPU outputs on CPU or enforcing strict operation order.
- Performance Cost: Frequent barriers serialize otherwise parallel work and increase idle time.
Why Host-device synchronization Matters
- Overlap Preservation: Reducing sync frequency allows compute and I/O pipelines to run concurrently.
- Throughput Stability: Lower synchronization overhead improves step-time consistency.
- Debug Awareness: Many hidden sync points come from convenience calls in logging or metrics collection.
- Scalability: Large multi-GPU systems amplify penalty of unnecessary host-device barriers.
- Resource Efficiency: Avoiding redundant waits keeps both CPU and GPU better utilized.
How It Is Used in Practice
- Barrier Audit: Identify and remove accidental sync calls in hot training loops.
- Event-Based Coordination: Use stream events for fine-grained dependencies instead of global synchronize.
- Deferred Logging: Batch metric extraction to reduce frequent device-to-host synchronization points.
Host-device synchronization is a necessary but expensive control point in GPU pipelines - disciplined barrier placement preserves correctness without sacrificing concurrency.