Turbocharge legacy Intel systems with Resizable BAR and ReBarUEFI

Technology Resizable BAR (Base Address Register) allows a graphics card to directly and more efficiently access system memory. This can lead to performance improvements in games and other applications. AMD made it famous with the introduction of its Ryzen 5000 series CPUs and RX 6000 GPUs, in 2020.

The GPU modern benefit greatly from enabling the Resizable BAR, at least in some scenarios. It’s not uncommon for Radeon GPUs to gain between 5% and 24% in performance. Just think that in the field gaming both AMD and NVidia use specific profiles to manage Resizable BARenabling them only when this is appropriate and can lead to real gains in terms of performance.

Resizable BAR it has actually existed since the interface was presented PCIe Gen 2.0 (2007). However, it never caught the attention of the producers mainstream until AMD realized its potential years later and presented its solution Smart Access Memory.

ReBarUEFI: Performance optimization via Resizable BAR on unofficially supported systems

An independent developer has done extensive research on the behavior of Resizable BAR explaining that this tool not only provides performance benefits general but is even necessary to ensure optimal functioning of the GPU Intel Arc.

The change published on GitHub, which takes the form of a patch for UEFI (Unified Extensible Firmware Interface), allows you to enable Resizable BAR on systems prior to 2020, even going as far as CPU Intel Sandy Bridge of the now distant 2011.

ReBar UEFI is a DXE driver (Driver Execution Environment) for UEFI designed for activating Resizable BAR on all those systems that do not officially support this function. Basically, support focuses on systems equipped with Intel Arc GPUs; If you are using an NVidia Turing GPU, the project developer suggests using NvStrapsReBar to enable Resizable BAR.

Resizable BAR Intel GPU

For possible installation and use of ReBar UEFI, it is recommended to carefully follow the guide on the wiki page. After the firmware has been changed, it is important to check that 4G Decoding is enabled and CSM is disabled. The module ReBarStateto be executed later, allows you to set the size of Resizable BAR. The recommended size is 32 (unlimited), but you may need to use a smaller size if you have problems.

What are 4G Decoding and CSM

When the option 4G Decoding is enabled, the operating system and UEFI/BIOS firmware can use more than 4 GB of physical address space to handle I/O devices. In particular, this option is often necessary when using expansion cards that require a significant amount of address spacesuch as advanced graphics cards, storage controllers or other high-performance PCIe peripherals.

CSMacronym for Compatibility Support Moduleis a feature of the UEFI environment designed to ensure compatibility with operating systems and applications designed for the traditional BIOS, known as Legacy BIOS. Using CSM may slow down some advanced features and performance offered by the UEFI environment. Furthermore, deactivating it helps free up resources and optimize the overall efficiency of the system. In another article we saw what CMS is in the UEFI BIOS and when it is useful.

How an increase in the Resizable BAR value can improve performance

The Resizable BAR modifies the “opening” of the bus PCIe to allow the transfer of large volumes of data within a single “package”, over 256 MB.

Without resorting to Resizable BAR, packet sizes are limited; by enabling the option, the value can grow substantially by improving the performance overall.

Today both AMD and NVidia, as previously mentioned, actively intervene to define the specifications of Resizable BARfrom time to time, in such a way as to obtain the best possible performance.

With the unofficial “mod” made and published on GitHub, owners of older systems can have the opportunity to get improved performance. This is especially useful for all those who are oriented towards upgrading their machines with newer GPUs, which are themselves compatible with Resizable BAR. Some GPUs, like those from serie Arc Athey are so sensitive to the presence of Resizable BAR that Intel requires it to be enabled in order to ensure good performance.


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