Texture pipeline in ExtraFPS has wrong texture bias calculation, causing unnecessary texture scaling.

[canny]

2.) Texture bias calculation is bugged and can lead to constant up- and downscaling of textures: If an over-budget of VRAM is detected, the discard level is increased. When memory is available again, the viewer slowly reduces discard level until an over-budget is detected. Then the cycle starts again and the viewer will never stop and reach a stable state. This is accompanied by the CPU constantly ramping up and spikes in frame times every 4-5 seconds, making it an awful and unpleasant experience. Here is a video of the problem when observing the texture console: https://gyazo.com/57cb4d19a9cd24eea22a4a8f0e52fcd2

3.) Bug #2 in texture bias calculation (in LLViewerTexture::updateClass()): While downscaling textures by increasing the discard bias, the viewer waits 1 second after a downscaling before another possible downscaling happens. However, if memory isn't low anymore, the decrement of discard bias and upscaling of textures happens every frame.

https://secondlife.canny.io/admin/board/bug-reports/p/texture-pipeline-in-extrafps-still-uses-too-much-vram-and-the-bias-is-wrong

[canny]

This issue has been linked to a Canny post: Texture pipeline in ExtraFPS still uses too much VRAM and the bias is wrong. 🎉

[akleshchev]

At the moment it's either 'we are low, increase bias' or 'we are not low, decrease bias', a middle state of 'we are not low, but have little space to spare, do nothing' would provide a place for bias to reach equlibrium at.