[Bug 97879] [amdgpu] Rocket League: long hangs (several seconds) when loading assets (models/textures/shaders?)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=97879

--- Comment #68 from Marek Olšák <maraeo at gmail.com> ---
The long hangs are spent in an upload/decompression thread the game uses. The
thread doesn't use any GL calls.

My theory that's now confirmed was that the game maps buffers in VRAM and does
read-modify-write on that memory in that thread. We know that direct VRAM
access is uncached and super slow if it's not a series of sequential writes
that is write-combined on the CPU, which is like a write-only cache. Games
typically upload data using a sequential copy/fill, which is why we had not
seen this issue before.

If the game used the MAP READ flag, the driver would return a mapping in RAM
with caching enabled. You'll also get that if you set the CLIENT STORAGE bit in
glBufferStorage.

For completeness, you can also get a mapping in RAM that's uncached. The CPU
and GPU performance is somewhere between the VRAM and cached RAM. We use it for
streaming because the GPU has faster access to RAM uncached by the CPU.

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