Performance for Free: Unlocking Resizable Bar for unsupported AMD GPUs (Polaris, VEGA, Radeon VII)

Fantastic post! Excellent research! I've seen the benefits and Resizable Bar and the misses. As discussed it does depend on the application. It hasn't made a huge difference for me personally, but it is still nice to have. I'm happy that it can give a boost when needed.

 

just remember not all games scales well or with rebar. Some games may get overcommit issues and more frame-rate fluctuations. So you have to test on each game.

 

Is there some obvious way to test for this yet, like a HUD that plainly says how VRAM transfers are occurring? I want to know concisely if rBAR is directly causing a performance issue, vs some nonsense like dust on the motherboard or a game's anti-cheat not liking the weather that day.

I've only seen rBAR either improve or do nothing to performance for the games I play on Windows and Linux on a 3060 and 6600 XT; never worse performance.

Does this go against the BAR amount specified in the kernel logs? I've seen BAR reported in the kernel logs go from 256M to 8G (something 8000-ish M) with a RX 580 in the past.

 

It's not supposed to show up in the BIOS. You insert the module into your motherboard's BIOS, and then use Rebarstate executable in Windows, and enable it. Use the value 11.

 

What driver version are you on, and is Resizable Bar enabled in your motherboard's BIOS?

 

well if your game with rebar off uses 6-7GB of VRAM and with rebar on it uses 12 and you cannot load properly some resources mip-maps...

 

important to note that just increasing the bar size will not necessarily do anything, the graphics drivers have to actually use extra address space inorder to get the performance increase. There is a toggle with the opensource mesa drivers to enable those optimizations in the .drirc file, by default they are disabled on anything older than the rx 6000 series, but you can force enable it on any card that uses the amdgpu kernel module(r9 380/r9 fury or newer, with an experimental option to enable support for the hd 7000 and rx 200 series cards).

have no idea if it actually yields any performance improvement or not on older cards.
hypothetically if the bar size was set too large and the drivers tried to use the address space beyond what is supported in hardware , it could cause problems.

the patch i was referring to simply improves detection of supported bar sizes, which may fix problems with older cards. Doesn't look like it was actually merged however. (edit: upon further reading, it seems like some other less generalized solution to the issue was chosen for the rx 5600 xt, which is what sparked the patchset in the first place )

 

That would be pretty obvious, but none of my games exhibit this. D2R for example uses at the most either 7GB VRAM on a 8GB 6600 XT GPU, or 9GB VRAM on 12GB RTX 3060, and it does the same allocation seemingly without difference with or without rBAR on Windows and Linux.

 

Well, if I use 11 it does work but I can't control Resizable BAR feature in Radeon Settings. And I don't know it this is related but everytime I boot up an error shows up. Also KMD_RebarControlMode reset itself to "0".

 

I gotta say, that's quite odd. But in case you're using the modded drivers (!?) or Radeon Pro drivers, I am not sure if this tweak applies to you.

 

Yeah, I am using Nimez 22.10.2.

Edit: Stock driver doesn't work too.

 

You have to change the settings using RebarState it won't show up in BIOS settings

EDIT: nvm i didn't see that you figured it out. I think this looks like either a problem with drivers or Windows installation since that messages goes away when rebooting usually. The first time you boot with Resizable BAR you have to restart once to make it work but in your case that seems to not be working

 

Same case for me I most games have no improvement but some do like Cyberpunk 2077 having a 12% gain at high graphics. I never have any games run worse or have stuttering issues

On Ubuntu my RX 580 get's resized to 8GB BAR by the kernel driver. But it appears the Mesa driver doesn't take advantage of it for old GPUs unless you manually enable it.

 

Works on PRO driver

 

Now this was a year ago. It's possible things have changed with driver updates.



Apex Legends really didn't seem to like it. I haven't tested it myself.

**EDIT! See Romulus_ut3's post below! I should have looked harder**

 

Thank you for the report!

 

Why don't you take a look at a more recent testing done with REBAR?



The few games that scaled negatively were within the margin of error.

 

DOH! I should have looked harder. Thanks! Although there are still some games doesn't always give a boost. But it's mostly an edge case here and there.

 

There's a reason I am not showing more benchmarks. It's quite difficult to capture the difference using ReLive. I have found games such as Horizon Zero Dawn, Borderlands 3 benefit from having REBAR enabled on cards like Polaris. I also want the full size of REBAR unlocked for the Ivy Bridge platform I am using to have the point of comparison if having less than the full capacity of the REBAR allocation makes any difference to the performance, if at all.

When I have REBAR set to OFF, the GPU isn't taxed to its capacity, despite the usage claiming it's 100%. So, when capturing footage using ReLive, I get zero performance drops.

However, with REBAR enabled, the GPU gets taxed to near to its full capacity, and when using ReLive during situations like these, there's a performance penalty involved. That penalty can sometimes reduce the margin of the performance improvement to the point I can't provide you with the performance distinction. If I had a capture card, I'd have been able to do this properly.

I don't like posting still screenshots/images with just a performance figures, but if I don't have any other options left, I'll eventually have to resort to doing just that.

 

Little update: Changing PCI Bus driver to 22.20.0.0001 fixed my non-ending "hardware changed" error. I can fully use ReBAR on my system.