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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  in-
       dicates	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 impossi-
       ble  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 ac-
       tive 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 al-
       ways  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 fair-
       ness  of	 the  FSS  scheduling  class  can be compromised because these
       classes use the same range of priorities.  Fairness  is	most  signifi-
       cantly  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	prior-
       ity 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  multiply-
       ing  it	with  a	 decay factor, ensuring	that more recent CPU usage has
       greater weight when taken into account for scheduling. The  FSS	sched-
       uler  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 allo-
       cated 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 competi-
       tion 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 satu-
       rated.

       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 assign-
       ment 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 So-
       laris operating 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

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 de-
       fault value of RES = 1000, the reciprocal of 1000 is .001, or millisec-
       onds. 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 frac-
       tional 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 dis-
       padmin(1M) man page for additional information.

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: N1 Grid Containers, Resource Management,
       and Solaris Zones

SunOS 5.10			  1 Oct	2004				FSS(7)

NAME | DESCRIPTION | CONFIGURING SCHEDULER WITH DISPADMIN | ATTRIBUTES | SEE ALSO

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