I'm trying to investigate a huge performance drop affecting some low-end cards.

I currently suspect:

• a bug in the driver itself.
• something stupid done on our side similar to the way our skybox code was iterating over GPU memory directly:
  Sky performance cost #849 (comment)
• a combination of the twos.

Here is the Thunder map, lowest preset, 640×480 resolution, default spectator scene (1024 -3456 448 -90 0), 3fps:

Here is the Vega map, lowest preset, 640×480 resolution, default spectator scene (1472 -1792 176 -180 0), 5fps:

This configuration is expected to achieve 20~30fps 60fps.

The testbench is a very low-end config, one of my favorites for identifying bottlenecks:

• CPU: AMD Athlon 64 X2 4400+, dual core 2.3GHz.
• RAM: 8Go DDR2 800MHz , 4×2Go Hynix HYMP125U64CP8-S6, dual channel.
• GPU: ATi Radeon X550 HyperMemory, RV370 (r300, tiny ALU), 128Mo 64-bit DDR, PCIe x16 1.0.
• OS: Ubuntu 23.10 mantic, Linux 6.5.0.
• Driver: Mesa 23.2.1 (stock) or Mesa 24.0.7 (self built).

In the Thunder map, I suspect the bottleneck comes from rendering something in that room and/or the outside. It can be the electric arc effect on those pillars, and/or the lightning effect on the top of the outside of the building, as this room as huge windows on the outside. Leaving that room and losing sight with the outside brings back performance, leaving that room without losing sight with the outside keeps the performance low.

In the Vega map, I suspect the bottleneck comes from rendering that globe or something around that globe. From the 4 sides of the room, I recover performance by turning back to that globe.

I identified places were just turning around the view enables/disables the performance loss in both maps.

Thunder, -30 -1497 10 -20 0:

Thunder, -30 -1497 10 112 0:

Vega, 667 -1792 176 0 0:

Vega: 667 -1792 176 -180 0:

As you see, just turning around the head without moving from the place you are enables/disables the cause of the bottleneck.