Open-Source AMD OpenGL Driver Drops Support For Smart Access Memory / ReBAR

[phoronix]

Phoronix: Open-Source AMD OpenGL Driver Drops Support For Smart Access Memory / ReBAR

Since late 2020 there had been work by AMD Linux engineers on adding Smart Access Memory (Resizable BAR) support to RadeonSI as the Gallium3D OpenGL driver and improved since that point in the name of performance. However, for this OpenGL driver now they've come to realize the benefits haven't necessarily panned out and the developers went ahead and disabled this SAM/ReBAR support followed by removing the support from this driver...

https://www.phoronix.com/news/RadeonSI-Drops-ReBAR-SAM

[vipor29]

i almost thought they were dropping it for the intel driver i was like NO!! lol i have a arc 770 and that thing needs that to even be playable.

[ms178]

Why not leave the feature inside Mesa and disable it by default? This needlessly regresses functionality for unaffected users. There was even a drirc config option which let me use it on a non-validated platform and as far as I can tell it used to work fine with my X99 motherboard and my Vega (for the few OpenGL games I still play). I've seen issues on AM4 with the "Above 4G decoding" BIOS feature myself and the reported performance issue sounded very familiar, other people also were affected by this rather weird BAR issue and it made me switch platforms even back then. It seems AMD and its partners couldn't be bothered with analyzing and fixing this properly up to this date.

[V1tol]

I think it is because the way OpenGL is designed. And old games/software abusing read backs from GPU making ReBAR hurt performance. And I am not sure ReBAR even helped for modern OpenGL software.

[mbriar]

Why not leave the feature inside Mesa and disable it by default? This needlessly regresses functionality for unaffected users. There was even a drirc config option which let me use it on a non-validated platform and as far as I can tell it used to work fine with my X99 motherboard and my Vega (for the few OpenGL games I still play). I've seen issues on AM4 with the "Above 4G decoding" BIOS feature myself and the reported performance issue sounded very familiar, other people also were affected by this rather weird BAR issue and it made me switch platforms even back then. It seems AMD and its partners couldn't be bothered with analyzing and fixing this properly up to this date.

Did you ever get any measurable improvement from it in OpenGL games?

[pinguinpc]

In my case rebar give more performance or personally that seems in dead rising 2

no rebar



rebar



Actually my system use rebar and run most titles very good in my little UHD 630 CFL-GT2

And back to neptunia runs ok in my case using opengl (yeah neptunia is opengl dont directx, rare remember me apocalyptica from konami, this game use opengl too)

OpenGL





Zink





Almost forget @Michael maybe can test rebar on igps for see appear improvement or not

[piotrj3]

I think it is because the way OpenGL is designed. And old games/software abusing read backs from GPU making ReBAR hurt performance. And I am not sure ReBAR even helped for modern OpenGL software.

It is something in MESA that makes it bad.

"The legacy proprietary drivers from AMD seem to be completely unaffected by this so this is most likely a mesa-specific bug.

Both radeonsi and zink seem to stumble on a huge CPU bottleneck with this game.

If you set Gallium HUD (or better MangoHud) to show all CPU cores, you should able to see one of them pegged at 100% most of the time."

[marek]

If you have a case where the radeonsi version of SAM improves performance, please let me know.

SAM isn't really removed. The behavior that's removed is an aggressive use of VRAM for everything when SAM is enabled in the BIOS, which has slower CPU access.

SAM is still used if an app specifically asks for VRAM and then demands full CPU access to it. The kernel handles that optimally if SAM is enabled in the BIOS.

[DanielG]

Thanks for the clarification, marek !

Michael you should update the newspost with the clarification, right now it seems to be wrong

[Danny3]

I wish this was not removed in Mesa 22.3.7 so I could still test some Linux-native games with this option both on ad off and see if any of them has any improvements.