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OpenSolaris 2009.06 - man page for fss (opensolaris section 7)

FSS(7)				  Device and Network Interfaces 			   FSS(7)

NAME
       FSS - Fair share scheduler

DESCRIPTION
       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  avail-
       able  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  entitle-
       ment  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  priori-
       ties.  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  periodi-
       cally  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 rela-
       tive CPU usage converge with its entitlement.

       While FSS is designed to fairly allocate cycles over a long-term time period, it is possi-
       ble  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 unreli-
       able.

       Note that share is not the same as utilization. A project may be allocated 50% of the sys-
       tem, 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. Allo-
       cating a small share to a busy project slows it down but does not prevent it from complet-
       ing 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. See
       resource_controls(5) for a description of the resource controls supported in  the  current
       release of the Solaris operating system.

       By  default, project system (project ID 0) includes all system daemons started by initial-
       ization 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

CONFIGURING SCHEDULER WITH DISPADMIN
       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 recip-
       rocal 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  QUAN-
       TUM 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 infor-
       mation.

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

       +-----------------------------------------------------------+
       |ATTRIBUTE TYPE		      ATTRIBUTE VALUE		   |
       |Architecture		      SUNWcsu			   |
       +-----------------------------------------------------------+

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

       System Administration Guide:  Virtualization Using the Solaris Operating System

SunOS 5.11				    1 Oct 2004					   FSS(7)


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