Detailed Description

Limitations of using OpenGL fences for VAO and buffer upload readiness.

This document explains why glFenceSync() is unsuitable for determining the readiness of individual VAOs or buffer objects during asynchronous data uploads. The behavior follows directly from the OpenGL execution and memory model.

Global Timeline Semantics

A fence created via glFenceSync() represents a global timeline marker. The fence becomes signaled only after all GPU commands issued prior to the fence in the same context have completed, regardless of which resources those commands operate on.

As a consequence, a fence cannot be associated with a specific VAO or buffer. Any unrelated long-running operation (e.g., a slow upload to a different buffer) delays the fence, even if the resources relevant to the intended VAO are already fully uploaded and ready for use.

Draw-Call Resource Dependencies

In contrast to fences, draw commands perform resource-based dependency tracking. A draw call waits only for GPU writes that affect the specific buffers, textures, and state objects it will read. Unrelated uploads or operations on other VAOs do not delay the draw.

This means a draw call may execute correctly even when a fence inserted after the upload of its associated buffers remains unsignaled due to unrelated earlier commands.

Implications for Upload Synchronization

Fence readiness does not imply VAO readiness.
Fence non-readiness does not imply VAO non-readiness.
Fences cannot be used to determine whether a specific VAO's buffers have completed uploading.
Fences are affected by all prior commands in the context, including uploads to unrelated buffers.

Recommended Practice

Fences should not be used as a per-VAO or per-buffer readiness mechanism. Instead, readiness must be tracked at the granularity of the specific buffer operations (e.g., completion of the copy into the device-local buffer), using explicit synchronization associated with those operations only.

Summary

Fences enforce global timeline ordering and are not resource-aware.
Draw calls enforce resource-based ordering and wait only for required data.
Therefore, fences are unsuitable for determining VAO or buffer upload readiness in asynchronous upload pipelines.