User guide

IB0054606-02 A F-1
F Write Combining
Introduction
Write Combining improves write bandwidth to the QLogic driver by writing multiple
words in a single bus transaction (typically 64 bytes). Write combining applies only
to x86_64 systems.
The x86 Page Attribute Table (PAT) mechanism allocates Write Combining (WC)
mappings for the PIO buffers, and is the default mechanism for WC.
If PAT is unavailable or PAT initialization fails, the software will generate a
message in the log and fall back to the Memory Type Range Registers (MTRR)
mechanism. If write combining is not working properly, lower than expected
bandwidth may occur.
The following sections provide instructions for enabling and disabling WC using
PAT and MTRR, and for verifying that write combining is working.
PAT and Write Combining
The wc_pat parameter is set in /etc/modprobe.conf (on Red Hat systems) or
/etc/modprobe.conf.local (on SLES systems) to:
 0) Disable PAT and use MTRR.
 1) Configure WC by programming the PAT at the memory page level instead
of the physical memory ranges.
 2) Configure WC by programming the PAT at the memory page level and
overwriting the operating system PAT configuration to enable WC uniformly
across CPUs that have it disabled. This is the default setting.
The default wc_pat parameter is:
option ib_qib wc_pat=2
If PAT is unavailable or PAT initialization fails, the code generates a message in
the log and falls back to the MTRR mechanism. To use MTRR, disable PAT by
setting the wc_pat parameter to 0 (as a root user):
option ib_qib wc_pat=0