KERN_YIELD(9) FreeBSD Kernel Developer's Manual KERN_YIELD(9)
NAME
kern_yield, maybe_yield, should_yield - functions for yielding execution
of the current thread
SYNOPSIS
void
kern_yield(int prio);
void
maybe_yield();
bool
should_yield();
DESCRIPTION
The kern_yield() function causes the currently running thread to
voluntarily, but unconditionally, surrender its execution to the
scheduler. The prio argument specifies the scheduling priority to be
assigned before the context switch, which has an influence on when
execution will resume. Note that the requested priority will take effect
until the thread returns to usermode, after which its base user priority
will be restored. Valid values for prio are any of the PRI_* values
defined in <sys/priority.h>, as well as the following special values:
PRI_USER Schedule the thread with its base user priority; the value
corresponding to setpriority(2) / nice(3).
PRI_UNCHANGED Yield the thread without changing its priority.
The should_yield() function checks if sufficient time has passed since
the thread's last voluntary context switch that yielding would be a
useful service to other threads. It returns true when this is the case.
See USAGE NOTES for an elaboration of what this means.
The maybe_yield() function is a helper function for the common task of
optionally yielding the processor. Internally, kern_yield(PRI_USER) will
be called if should_yield() returns true.
USAGE NOTES
Although the kernel supports preemption, this is usually reserved for
high-priority realtime or interrupt threads. Kernel worker threads and
timesharing threads are not guaranteed to preempt each another. Thus,
threads executing in the kernel are expected to behave cooperatively with
respect to other threads in the system. The yield functions are mostly
intended to be used by threads which perform a lot of work inside the
kernel. For example: maybe_yield() is called by the vlnru process each
time it reclaims a vnode.
The scheduler aims to identify threads which monopolize the CPU, and will
schedule them with decreased priority. Threads which regularly yield the
processor will be given the chance to run more often. The possibly
surprising effect of this is that, depending on the disposition of other
threads on the CPU's runqueue, a call to kern_yield() does not guarantee
that the yielding thread will be taken off the CPU.
With the above considerations in mind, it is advised that code written
using kern_yield() be measured to confirm that its use has a positive
effect on relevant performance or responsiveness metrics. Switching
thread contexts has a non-zero cost, and thus yielding the processor too
eagerly could have a negative impact on performance.
Additionally, since yielding is a cooperative action, it is advised that
the yielding thread release any locks that it may be holding, when
possible. Otherwise, threads which have been given the chance to run
could end up waiting on the yielding thread to release the lock, largely
defeating the purpose of the yield.
SEE ALSO
setpriority(2), nice(3), mi_switch(9)
AUTHORS
This manual page was written by Mitchell Horne <
[email protected]>.
FreeBSD 14.1-RELEASE-p8 January 30, 2023 FreeBSD 14.1-RELEASE-p8