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
Summary of Contents for OFED+ Host
Page 1: ...IB0054606 02 A OFED Host Software Release 1 5 4 User Guide...
Page 14: ...xiv IB0054606 02 A OFED Host Software Release 1 5 4 User Guide...
Page 22: ...1 Introduction Interoperability 1 4 IB0054606 02 A...
Page 96: ...4 Running MPI on QLogic Adapters Debugging MPI Programs 4 24 IB0054606 02 A...
Page 140: ...6 SHMEM Description and Configuration SHMEM Benchmark Programs 6 32 IB0054606 02 A...
Page 148: ...8 Dispersive Routing 8 4 IB0054606 02 A...
Page 164: ...9 gPXE HTTP Boot Setup 9 16 IB0054606 02 A...
Page 176: ...A Benchmark Programs Benchmark 3 Messaging Rate Microbenchmarks A 12 IB0054606 02 A...
Page 202: ...B SRP Configuration OFED SRP Configuration B 26 IB0054606 02 A Notes...
Page 206: ...C Integration with a Batch Queuing System Clean up PSM Shared Memory Files C 4 IB0054606 02 A...
Page 238: ...E ULP Troubleshooting Troubleshooting SRP Issues E 20 IB0054606 02 A...
Page 242: ...F Write Combining Verify Write Combining is Working F 4 IB0054606 02 A Notes...
Page 280: ...G Commands and Files Summary of Configuration Files G 38 IB0054606 02 A...
Page 283: ......
