NVIDIA Resizable BAR Performance - A Big Boost For Some Linux Games

[CochainComplex]

I think it is good, that we have them both available for use on Linux.
The different approach in the driver strategy is no problem, as long as everyone can choose what fits his needs.

I totally support this statement - even if I think that the OpenSource approach is the more sustainable and benefical one for the users.
But AMD deserves more Credit for pushing Ideas which then are adopted by their competitor (which is ok because thats part of FOSS too). None the less low Level Api started with Mantle. BAR, ReBar or SAM was also the idea of AMD. Generously they pushed it as OpenSource (transformed Mantle into the base of Vulkan) or at least made i usable by their competitors too. Everyone profits.

On the other hand if you have Nvidia and CUDA you can clearly see the downside. You have to have a Nvidia Product otherwise you will struggle. There is no advantage for the users to have a monopoly. Even Fanboys are geting grinded. (btw this would also be true for an AMD Monopoly - I tend to be an AMD fan but I'm also a realist)

[bridgman]

so according to my dmesg log, this has a ReBAR right? because it allows to set a BAR bigger than 256MB

[ 1.413260] [drm] Detected VRAM RAM=6128M, BAR=8192M

so this is not really a BAR but a ReBAR... the mesa driver has had this feature for a long time, and you don't even need a NAVI 6000 for this nor a bios option to enable it.
it's just that just a few months ago the amdgpu driver started doing optimizations for taking advantage of the ReBAR, which is what nvidia is doing now a little later than the mesa driver.

Correct, although note that resizeable BAR is a kernel driver feature (which has been in place for a year or more) while "taking advantage of the larger BAR" mostly happens in the userspace drivers (radeonsi, possibly radv). That's one of the reasons we keep SAM and resizeable BAR separate when talking about them.

[obri]

nice performance uplifts indeed. Just let us all thank AMD to push this idea like low level apis before we hail nvidia.

I think it is good, that we have them both available for use on Linux.
The different approach in the driver strategy is no problem, as long as everyone can choose what fits his needs.

[msjyoshi]

what you are saying is that my B350 motherboard supports ReBAR? because according to AMD you need a b450 motherboard

It seems so. I have an asrock ab350M, which has it.
There is a post by an X.org dev instructing how to check for it in the forums.

[andre30correia]

If there is any performance difference, Nvidia Linux is slower than Nvidia Windows in games.

spykes Nvidia does not provide vbios updates with rBAR for anything older than Ampere.

not true in all games but ok

[CochainComplex]

nice performance uplifts indeed. Just let us all thank AMD to push this idea like low level apis before we hail nvidia.

[Termy]

Interesting thing is that on Windows, ReBAR didn'T improve Nvidias performance nearly as much from what benchmarks i've seen - i wonder what the cause is?

[khnazile]

what you are saying is that my B350 motherboard supports ReBAR? because according to AMD you need a b450 motherboard

The ReBAR feature works for me with gigabyte ab350 motherboard, ryzen 1700 (week 4 2017), rx550. Only on linux ofc, and I'm not sure if mesa actually uses it, but at least kernel reports 4GiB BAR.

[Mel Spektor]

thanks bridgman

so according to my dmesg log, this has a ReBAR right? because it allows to set a BAR bigger than 256MB

[ 1.412949] amdgpu 0000:08:00.0: amdgpu: Fetched VBIOS from ROM BAR
[ 1.413062] amdgpu 0000:08:00.0: BAR 2: releasing [mem 0xf0000000-0xf01fffff 64bit pref]
[ 1.413065] amdgpu 0000:08:00.0: BAR 0: releasing [mem 0xe0000000-0xefffffff 64bit pref]
[ 1.413094] pcieport 0000:07:00.0: BAR 15: releasing [mem 0xe0000000-0xf01fffff 64bit pref]
[ 1.413097] pcieport 0000:06:00.0: BAR 15: releasing [mem 0xe0000000-0xf01fffff 64bit pref]
[ 1.413099] pcieport 0000:03:04.0: BAR 15: releasing [mem 0xe0000000-0xf01fffff 64bit pref]
[ 1.413101] pcieport 0000:02:00.2: BAR 15: releasing [mem 0xe0000000-0xf01fffff 64bit pref]
[ 1.413104] pcieport 0000:00:01.3: BAR 15: releasing [mem 0xe0000000-0xf01fffff 64bit pref]
[ 1.413116] pcieport 0000:00:01.3: BAR 15: assigned [mem 0x500000000-0x7ffffffff 64bit pref]
[ 1.413119] pcieport 0000:02:00.2: BAR 15: assigned [mem 0x500000000-0x7ffffffff 64bit pref]
[ 1.413122] pcieport 0000:03:04.0: BAR 15: assigned [mem 0x500000000-0x7ffffffff 64bit pref]
[ 1.413125] pcieport 0000:06:00.0: BAR 15: assigned [mem 0x500000000-0x7ffffffff 64bit pref]
[ 1.413127] pcieport 0000:07:00.0: BAR 15: assigned [mem 0x500000000-0x7ffffffff 64bit pref]
[ 1.413130] amdgpu 0000:08:00.0: BAR 0: assigned [mem 0x600000000-0x7ffffffff 64bit pref]
[ 1.413143] amdgpu 0000:08:00.0: BAR 2: assigned [mem 0x500000000-0x5001fffff 64bit pref]
[ 1.413260] [drm] Detected VRAM RAM=6128M, BAR=8192M

so this is not really a BAR but a ReBAR... the mesa driver has had this feature for a long time, and you don't even need a NAVI 6000 for this nor a bios option to enable it.
it's just that just a few months ago the amdgpu driver started doing optimizations for taking advantage of the ReBAR, which is what nvidia is doing now a little later than the mesa driver.

[smitty3268]

I don't understand is BAR and ReBAR the same thing?? because lots of hardware review sites seem to be confusing the terms... Also if I understand correctly AMD SAM it's a different thing.

Bridgman is correct in defining what the terms actually mean, but I think it's worthwhile to add that if you ever see any of these terms in any hardware review site articles they are certainly all referring to the same thing. Just mixing up their terminology a bit.