Q-Logic IB6054601-00 D manual Mtrr Mapping and Write Combining, Incorrect Mtrr Mapping

Q

C – Troubleshooting BIOS Settings

C.2.1

MTRR Mapping and Write Combining

MTRR (Memory Type Range Registers) is used by the InfiniPath driver to enable write combining to the InfiniPath on-chip transmit buffers. This improves write bandwidth to the InfiniPath chip by writing multiple words in a single bus transaction (typically 64). This applies only to x86_64 systems. To see if is working correctly and to check your bandwidth use this command:

$ ipath_pkt_test -B

When configured correctly, PCIe InfiniPath will normally report in the range of 1150-1500 MB/s, while HTX InfiniPath cards will normally report in the range of 2300-2650 MB/s.

However, some BIOSes don’t have the MTRR mapping option. It may be referred to in a different way, dependent upon chipset, vendor, BIOS, or other factors. For example, it is sometimes referred to as "32 bit memory hole", which should be enabled.

If there is no setting for MTRR mapping or 32 bit memory hole, please contact your system or motherboard vendor and inquire as to how write combining may be enabled.

C.2.2

Incorrect MTRR Mapping

In some cases, the InfiniPath driver may be unable to configure the CPU Write Combining attributes for the QLogic InfiniPath IBA6110. This would normally be seen for a new system, or after the system’s BIOS has been upgraded or reconfigured.

If this error occurs, the InfiniPath interconnect will operate, but in a degraded performance mode. Typically the latency will increase to several microseconds, and the bandwidth may decrease to as little as 200 MBytes/sec.

A message similar to this will be printed on the console, and normally to the system log (typically in /var/log/messages):

infinipath: mtrr_add(feb00000,0x100000,WC,0) failed (-22)

infinipath: probe of 0000:04:01.0 failed with error -22

If you see this error message, you should edit the BIOS setting for MTRR Mapping. The setting should look like this:

MTRR Mapping [Discrete]

You can check and adjust the BIOS settings using the BIOS Setup Utility. Check the hardware documentation that came with your system for more information on how to do this. Section C.2.3, below, documents a related issue.

IB6054601-00 D C-3