[Bug 110781] Radeon: heavy r300 performance drop regression between 11.x and 19.x

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=110781

--- Comment #54 from Richard Thier <u9vata at gmail.com> ---
> The GPU has a scaler that can scale a smaller image to the native timing.  Many > LCD monitors also have a scaler built in to provide the same functionality.

I see, but still I see a big performance rise when I change the resolution
smaller and let this scaler do its work in the last phase.

I have no idea if there is a real speed difference between this two:
- having 640x480 on a 4:3 aspect screen without a scaler
- Having hardware pixels of 1024x768, but using the scaler in the end as
640x480 software.
- Z tests run for smaller amount of pixels
- Stencil and Z buffers are smaller just like the backbuffer
- etc.

Memory usage on the GPU should be still smaller, pixel shaders still run rarer
times, post processing things still run on smaller amount of pixels etc... The
only reason it can be slower if for some reason the scaler circuit would be so
slow while doing its job that it cannot keep its FPS, but I think this is a
highly specialized hardware not doing anything else so I hoped it is just
"fast" and even if there is some loss I guess it is barely measurable - but
tell me if I am wrong as that would be new to me if this has a really
measurable overhead.

In any ways I think it is better to use a same aspect ratio, but smaller
resolution than using the biggest possible native size. Actually on my machine
I get faster frame rate even if I run a game in 640x480 window on the top left
corner the same way as if I turn the screen into 640x480 itself and even then I
barely see a difference despite this case I feel there should be some overhead
against using directly a video mode for that..

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