Enabling 64-bit Huge DMA

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On Power® 8 systems, you can enable extra I/O space to support 64-bit Huge DMA. 64-bit Huge DMA is a PCIe slot capability on IBM® Power Systems servers that enables a DMA window to be wider. This action allows all the partition memory to be mapped for DMA. This feature avoids increased system usage when DMA mappings are requested by the driver, because all the system memory that is assigned to the partition is already mapped. Therefore, this feature enables the data transfer between the I/O card that is placed in this slot and the system memory to be more efficient and with lower latency. The performance benefit varies depending on the adapter that is used.

Note: This capability is also known as Huge Dynamic DMA Window.

Not all PCIe slots or PCIe adapters support 64-bit DMA Huge DMA. If the card or the device driver does not support the 64-bit Huge DMA feature space, the PCIe slot works in a standard way and is not differentiated from the other slots.

You can enable addition I/O space through the ASMI interface:

  1. Power your system off.
  2. From the ASMI menu, select System Configuration > I/O Adapter Enlarged Capacity.
  3. Select to Enable I/O Adapter Enlarged Capacity.
  4. For each Node, select a value from the menu.
  5. Select Save Settings.
  6. Restart your system.

To determine the I/O Adapter Enlarged Capacity assignment order, see Table 3. PCIe slot locations and descriptions for your server model: