[Mesa3d-dev] Correct behavior of GL_DYNAMIC_DRAW in glBufferData?

[Zou, N. <nan...@in...>]

Hi,
         I have notice that in current driver implementation.
When GL_DYNAMIC_DRAW flag is set in glBufferData, driver will not upload data to
Graphics memory.
Instead it will upload the _entire_ vertex/index buffer when glDrawElements/glDrawArray was called.

This sometimes brings a huge bottleneck ,
e.g. in Skia.

Skia allocate a 256k vertexdata buffer and a 64k indexdata buffer as pools, and use glBufferData to upload the pools.

Most of the Skia draw operations will only use a few vertexes in the pool, but driver will upload the _entire_
buffer each operation.

This becomes No 1 bottleneck for Skia bench with GPU.

I have tested if reduce the pool size to 1k, many tests in Skia bench get 3x to >10x performance improvement...

Per my understanding. Even GL_DYNAMIC_DRAW is set in glBufferData, driver should not copy the entire buffer
When glDrawElements/glDrawArray is called, it should copy only used vertex/index.

Otherwise this is too heavy in Skia like usage scenario.

Thanks
Zou Nanhai