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- May 2, 2020
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Since OC 0.7.5 final release should be arriving later today a short guide how to have resizable BAR enabled with macOS.
Shortly about resizable BAR. Each PCIe device has a "window" which the CPU can access the devices memory at once. The default is 256 MB, since VRAM has grown from Megabytes to Gigabytes since PCIe has been introduced, this Window has gotten small and makes it inefficient for todays GPUs. This is where resizable BAR comes in, which has been a feature in PCIe for years, but was largely ignored until AMD announced Smart Access Memory. With resizableBAR, the OS can negotiate a bigger window to access the full VRAM at once. But since macOS does not support that, it could not be enabled when booting macOS.
OC 0.7.5 now has a quirk to fix the BAR for macOS only ( For MacOS on the other hand we can set
For macOS it's recommended to set
TLDR:
Shortly about resizable BAR. Each PCIe device has a "window" which the CPU can access the devices memory at once. The default is 256 MB, since VRAM has grown from Megabytes to Gigabytes since PCIe has been introduced, this Window has gotten small and makes it inefficient for todays GPUs. This is where resizable BAR comes in, which has been a feature in PCIe for years, but was largely ignored until AMD announced Smart Access Memory. With resizableBAR, the OS can negotiate a bigger window to access the full VRAM at once. But since macOS does not support that, it could not be enabled when booting macOS.
OC 0.7.5 now has a quirk to fix the BAR for macOS only (
ResizeAppleGpuBars
) and for other OS (ResizeGpuBars
). To leave it to the OS -1 can be set on both, which is what we do for ResizeGpuBars
since Windows/Linux can handle it.ResizeAppleGpuBars
to 8 (2 power 8 = 256MB) which is what macOS works best with (when rBAR is disabled). macOS can actually supports a BAR up to 1 GB (ResizeAppleGpuBars
= 10), but for many this breaks sleep.For macOS it's recommended to set
ResizeAppleGpuBars
= 0, 8 would usually work but apparently can cause high GPU memory usage, 9 or 10 can break sleep without any real performance gain.TLDR:
- Update to OC 0.7.5
- Enable resizable BAR in BIOS
- Set
ResizeAppleGpuBars
= 0 - Set
ResizeGpuBars
= -1 - Enjoy SAM on Windows
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