Product specifications

E–Write Combining
PAT and Write Combining
E-2 IB6054601-00 H
S
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If you do not see any of these messages on your console, but suspect this
problem, check the /var/log/messages file. Some systems suppress driver
load messages but still output them to the log file.
Methods for enabling and disabling the two write combining mechanisms are
described in the following sections. There are no conflicts between the two
methods.
PAT and Write Combining
This is the default mechanism for allocating Write Combining (WC) mappings for
the PIO buffers. It is set as a parameter in /etc/modprobe.conf (on Red Hat
systems) or /etc/modprobe.conf.local (on SLES systems). The default is:
option ib_ipath wc_pat=1
If PAT is unavailable or PAT initialization fails, the code generates a message in
the log and falls back to the Memory Type Range Registers (MTRR) mechanism.
To use MTRR, disable PAT by setting this module parameter to 0 (as a root user):
option ib_ipath wc_pat=0
Then, revert to using MTRR-only behavior by following one of the two suggestions
in “MTRR Mapping and Write Combining” on page E-2.
The driver must be restarted after the changes have been made.
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MTRR Mapping and Write Combining
Two suggestions for properly enabling MTRR mapping for write combining are
described in the following sections.
See “Unexpected Low Bandwidth or Poor Latency” 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]
NOTE:
There will be no WC entry in /proc/mtrr when using PAT.