Fabric Manager Users Guide, Version 6.1, Revision A - QLogic

2–Advanced Fabric Manager Capabilities
Fabric Sweeping
• Constant
• Dynamic
NOTE:
Some applications, especially MPI middleware, have independent
configuration of QP time-outs and ignore the values provided by the SM. For
such applications configuration of SM Packet LifeTime will have no effect.
The constant approach causes the SM to return the same value for Packet
LifeTime for all queries.
The dynamic approach causes the SM to return a different value for the Packet
LifeTime depending on the number of hops through the fabric in the given path.
This allows the SM to account for the fact that longer routes have more
opportunities for congestion, queuing delays, and so on and should have larger
time-outs. When using the dynamic approach, a unique Packet LifeTime can be
configured for one hop to nine hop paths. Paths greater than nine hops will all use
the nine hop value.
Fabric Sweeping
The SM periodically sweeps the fabric. During a sweep the fabric is analyzed,
routes are computed and switches and Channel Adapters are configured. The SM
sweep algorithms attempt to carefully balance:
• Responsiveness to fabric changes
• Limit performance overhead of SM
• Efficiency of fabric analysis and configuration
• Potential hardware limitations
• Handle possibility that fabric is changing while being analyzed/configured
The SM performs sweeps at fixed intervals. The SM immediately performs a
sweep when a switch reports a port state change trap. Such traps indicate a link
has come up or down. Generally traps will trigger rapid sweeps to respond to
fabric changes. The fixed sweeps are a safety net in case traps are lost or there
are switches whose attempts to send traps are failing. To limit overhead, the SM
fixed sweep is very slow (5 minutes).
During a sweep, the SM must query and configure many devices in the fabric.
These operations occur using the SM protocol over VL15. VL15 is a
non-flow-controlled VL. This combined with the fact that the fabric could be
changing while the SM is sweeping, means that packets may be lost during the
sweep.
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