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AXP1406/AXP1600 Subsystem IPMI Programmer’s Reference (6806800B66C)
System Behavior in Response to Sensor Events Adaptive Adjustment of the Minimum Fan Level
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When at least one nonrecoverable temperature threshold has been exceeded on a FRU, the
Shelf Manager powers down the FRU (sending them the SetPowerLevel(0) command) and
refuses to power the FRU back up for COOLING_POLL_TIMEOUT* seconds. In that state, the
Shelf Manager also immediately raises the level of all fans to the maximum level and keeps it
at maximum.
A.4 Adaptive Adjustment of the Minimum Fan Level
The Shelf Manager changes the minimum fan level over time to prevent oscillations of the shelf
between normal and abnormal cooling mode. When the shelf transitions from normal to
abnormal cooling mode, this means that the current fan level is insufficient for effective cooling
of the shelf. In that case, the Shelf Manager dynamically changes the minimum fan level to the
current level + 1, so that next time in normal mode the Shelf Manager will not try to decrease
the fan level so low. (In the case of multiple fans and different fan levels, the Shelf Manager
chooses the maximum current fan level.) This makes the algorithm to converge to a certain fan
level after several oscillations; at that level, the shelf operates in normal cooling mode and this
is the minimum possible fan level at which the shelf can operate in normal cooling mode.
To accommodate for possible changes in the thermal load in the shelf in a long run, an
additional enhancement is implemented. After the shelf stays in normal cooling mode at a
stable fan level for a substantial period of time (specified as a configuration parameter
NORMAL_STABLE_TIME*, with a default of 1 hour), the minimum fan level is decreased by
one and the current fan level is allowed to drop to the new minimum. If the thermal load in the
shelf has decreased, the shelf will continue to operate at the reduced fan level. Otherwise, the
shelf will transition to the abnormal cooling state and the fan level will converge to a new stable
value after several oscillations.