User guide

F–Write Combining
MTRR Mapping and Write Combining
F-2 IB0054606-02 A
Revert to using MTRR-only behavior by following one of the two suggestions in
MTRR Mapping and Write Combining.
The driver must be restarted after the changes have been made.
.
MTRR Mapping and Write Combining
Two suggestions for properly enabling MTRR mapping for write combining are
described in the following sections.
See “Performance Issues” on page D-9 for more details on a related performance
issue.
Edit BIOS Settings to Fix MTRR Issues
You can edit the BIOS setting for MTRR mapping. The BIOS setting looks similar
to:
MTRR Mapping [Discrete]
For systems with very large amounts of memory (32GB or more), it may also be
necessary to adjust the BIOS setting for the PCI hole granularity to 2GB. This
setting allows the memory to be mapped with fewer MTRRs, so that there will be
one or more unused MTRRs for the InfiniPath driver.
Some BIOS’ do not have the MTRR mapping option. It may have a different
name, depending on the chipset, vendor, BIOS, or other factors. For example, it is
sometimes referred to as 32 bit memory hole. This setting must be enabled.
If there is no setting for MTRR mapping or 32 bit memory hole, and you have
problems with degraded performance, contact your system or motherboard
vendor and ask how to enable write combining.
Use the ipath_mtrr Script to Fix MTRR Issues
QLogic also provides a script, ipath_mtrr, which sets the MTRR registers,
enabling maximum performance from the InfiniPath driver. This Python script is
available as a part of the InfiniPath software download, and is contained in the
infinipath* RPM. It is installed in /bin.
To diagnose the machine, run it with no arguments (as a root user):
# ipath_mtrr
NOTE
There will not be a WC entry in /proc/mtrr when using PAT.