The fair share scheduler (FSS) guarantees application performance by explicitly allocating shares of CPU resources to projects. A share indicates a project's entitlement to available CPU resources. Because shares are meaningful only in comparison with other project's shares, the absolute quantity of shares is not important. Any number that is in proportion with the desired CPU entitlement can be used.

The goals of the FSS scheduler differ from the traditional time-sharing scheduling class (TS). In addition to scheduling individual LWPs, the FSS scheduler schedules projects against each other, making it impossible for any project to acquire more CPU cycles simply by running more processes concurrently.

A project's entitlement is individually calculated by FSS independently for each processor set if the project contains processes bound to them. If a project is running on more than one processor set, it can have different entitlements on every set. A project's entitlement is defined as a ratio between the number of shares given to a project and the sum of shares of all active projects running on the same processor set. An active project is one that has at least one running or runnable process. Entitlements are recomputed whenever any project becomes active or inactive, or whenever the number of shares is changed.

Processor sets represent virtual machines in the FSS scheduling class and processes are scheduled independently in each processor set. That is, processes compete with each other only if they are running on the same processor set. When a processor set is destroyed, all processes that were bound to it are moved to the default processor set, which always exists. Empty processor sets (that is, sets without processors in them) have no impact on the FSS scheduler behavior.

If a processor set contains a mix of TS/IA and FSS processes, the fairness of the FSS scheduling class can be compromised because these classes use the same range of priorities. Fairness is most significantly affected if processes running in the TS scheduling class are CPU-intensive and are bound to processors within the processor set. As a result, you should avoid having processes from TS/IA and FSS classes share the same processor set. RT and FSS processes use disjoint priority ranges and therefore can share processor sets.

As projects execute, their CPU usage is accumulated over time. The FSS scheduler periodically decays CPU usages of every project by multiplying it with a decay factor, ensuring that more recent CPU usage has greater weight when taken into account for scheduling. The FSS scheduler continually adjusts priorities of all processes to make each project's relative CPU usage converge with its entitlement.

While FSS is designed to fairly allocate cycles over a long-term time period, it is possible that projects will not receive their allocated shares worth of CPU cycles due to uneven demand. This makes one-shot, instantaneous analysis of FSS performance data unreliable.

Note that share is not the same as utilization. A project may be allocated 50% of the system, although on the average, it uses just 20%. Shares serve to cap a project's CPU usage only when there is competition from other projects running on the same processor set. When there is no competition, utilization may be larger than entitlement based on shares. Allocating a small share to a busy project slows it down but does not prevent it from completing its work if the system is not saturated.

The configuration of CPU shares is managed by the name server as a property of the project(4) database. In the following example, an entry in the /etc/project file sets the number of shares for project "x-files" to 10:

x-files:100::::project.cpu-shares=(privileged,10,none)

Projects with undefined number of shares are given one share each. This means that such projects are treated with equal importance. Projects with 0 shares only run when there are no projects with non-zero shares competing for the same processor set. The maximum number of shares that can be assigned to one project is 65535.

You can use the prctl(1) command to determine the current share assignment for a given project:

$ prctl -n project.cpu-shares -i project x-files

or to change the amount of shares if you have root privileges:

# prctl -r -n project.cpu-shares -v 5 -i project x-files

See the prctl(1) man page for additional information on how to modify and examine resource controls associated with active processes, tasks, or projects on the system.

By default, project "system" (project ID 0) includes all system daemons started by initialization scripts and has an "unlimited" amount of shares. That is, it is always scheduled first no matter how many shares are given to other projects.

The following command sets FSS as the default scheduler for the system:

# dispadmin -d FSS

This change will take effect on the next reboot. Alternatively, you can move processes from the time-share scheduling class (as well as the special case of init) into the FSS class without changing your default scheduling class and rebooting by becoming root, and then using the priocntl(1) command, as shown in the following example:

# priocntl -s -c FSS -i class TS # priocntl -s -c FSS -i pid 1

You can use the dispadmin(1M) command to examine and "tune" the FSS scheduler's time quantum value. Time quantum is the amount of time that a thread is allowed to run before it must relinquish the processor. The following example dumps the current time quantum for the fair share scheduler:

$ dispadmin -g -c FSS
	#
	# Fair Share Scheduler Configuration
	#
	RES=1000
	#
	# Time Quantum
	#
	QUANTUM=110

The value of the QUANTUM represents some fraction of a second with the fractional value determied by the reciprocal value of RES. With the default value of RES = 1000, the reciprocal of 1000 is .001, or milliseconds. Thus, by default, the QUANTUM value represents the time quantum in milliseconds.

If you change the RES value using dispadmin with the -r option, you also change the QUANTUM value. For example, instead of quantum of 110 with RES of 1000, a quantum of 11 with a RES of 100 results. The fractional unit is different while the amount of time is the same.

You can use the -s option to change the time quantum value. Note that such changes are not preserved across reboot. Please refer to the dispadmin(1M) man page for additional information.

See attributes(5) for descriptions of the following attributes:

prctl(1), priocntl(1), dispadmin(1M), psrset(1M), priocntl(2), project(4), attributes(5)

System Administration Guide