It's possible to set the budget of a partition to zero as long as the
SCHED_APS_SEC_NONZERO_BUDGETS
security flag isn't set—see the
SCHED_APS_ADD_SECURITY
command for SchedCtl().
Threads in a zero-budget partition run only in these cases:
- You're using the default scheduling policy (SCHED_APS_SCHEDPOL_DEFAULT),
and the highest-priority thread in the system belongs to the zero-budget partition.
- You're using SCHED_APS_SCHEDPOL_FREETIME_BY_RATIO, and
all other nonzero-budget partitions are idle.
- The zero-budget partition has a nonzero critical budget, in which case its critical threads run.
- A thread receives a message from a partition with a nonzero budget, in which case the receiving thread runs temporarily in the sender's partition.
When is it useful to set the budget of a partition to zero?
It useful to set the budget of a partition to zero when:
- A partition is permanently empty of running threads; you can set
its budget to zero to effectively turn it off.
When a zero-budget partition is idle, it isn't considered to produce free time (see
“Summary of scheduling behavior”
in the Using the Thread Scheduler chapter of this guide).
A partition with a nonzero budget that never runs threads puts the
thread scheduler permanently in free-time mode, which may not be the desired
behavior.
- You want noncritical code to run only when some other partition
is idle.
- The partition is populated by resource managers, or other
software, that runs only in response to receiving messages.
Putting them in a zero-budget partition means you don't have to
separately engineer a partition budget for them.
(Those resource managers automatically bill their time to the partitions
of their clients.)
Note:
Typically, setting a partition's budget to zero is not recommended.
(This is why the
SCHED_APS_SEC_RECOMMENDED security setting
doesn't permit partition budgets to be zero.)
The main risk in placing code into a zero-budget partition is that it may run
in response to a pulse or event (i.e., not a message), and therefore, not run
in the sender's partition. As a result, when the system is loaded (i.e., there's no free time), those threads may
simply not run; they might hang, or things might occur in the wrong order.
For example, it's hazardous to set the System partition's budget to zero.
On a loaded machine with a System partition of zero, requests to
procnto to create processes and threads may hang, for example, when
MAP_LAZY is used.
In addition, if your system uses zero-budget partitions, you should carefully test it with
all other partitions fully loaded with while(1) loops.