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   6 .TH FSS 7 "May 13, 2017"
   7 .SH NAME
   8 FSS \- Fair share scheduler
  10 .LP
  11 The fair share scheduler (FSS) guarantees application performance by explicitly
  12 allocating shares of CPU resources to projects. A share indicates a project's
  13 entitlement to available CPU resources. Because shares are meaningful only in
  14 comparison with other project's shares, the absolute quantity of shares is not
  15 important. Any number that is in proportion with the desired CPU entitlement
  16 can be used.
  17 .sp
  18 .LP
  19 The goals of the FSS scheduler differ from the traditional time-sharing
  20 scheduling class (TS). In addition to scheduling individual LWPs, the FSS
  21 scheduler schedules projects against each other, making it impossible for any
  22 project to acquire more CPU cycles simply by running more processes
  23 concurrently.
  24 .sp
  25 .LP
  26 A project's entitlement is individually calculated by FSS independently for
  27 each processor set if the project contains processes bound to them. If a
  28 project is running on more than one processor set, it can have different
  29 entitlements on every set. A project's entitlement is defined as a ratio
  30 between the number of shares given to a project and the sum of shares of all
  31 active projects running on the same processor set. An active project is one
  32 that has at least one running or runnable process. Entitlements are recomputed
  33 whenever any project becomes active or inactive, or whenever the number of
  34 shares is changed.
  35 .sp
  36 .LP
  37 Processor sets represent virtual machines in the FSS scheduling class and
  38 processes are scheduled independently in each processor set. That is, processes
  39 compete with each other only if they are running on the same processor set.
  40 When a processor set is destroyed, all processes that were bound to it are
  41 moved to the default processor set, which always exists. Empty processor sets
  42 (that is, sets without processors in them) have no impact on the FSS scheduler
  43 behavior.
  44 .sp
  45 .LP
  46 If a processor set contains a mix of TS/IA and FSS processes, the fairness of
  47 the FSS scheduling class can be compromised because these classes use the same
  48 range of priorities. Fairness is most significantly affected if processes
  49 running in the TS scheduling class are CPU-intensive and are bound to
  50 processors within the processor set. As a result, you should avoid having
  51 processes from TS/IA and FSS classes share the same processor set. RT and FSS
  52 processes use disjoint priority ranges and therefore can share processor sets.
  53 .sp
  54 .LP
  55 As projects execute, their CPU usage is accumulated over time. The FSS
  56 scheduler periodically decays CPU usages of every project by multiplying it
  57 with a decay factor, ensuring that more recent CPU usage has greater weight
  58 when taken into account for scheduling. The FSS scheduler continually adjusts
  59 priorities of all processes to make each project's relative CPU usage converge
  60 with its entitlement.
  61 .sp
  62 .LP
  63 While FSS is designed to fairly allocate cycles over a long-term time period,
  64 it is possible that projects will not receive their allocated shares worth of
  65 CPU cycles due to uneven demand. This makes one-shot, instantaneous analysis of
  66 FSS performance data unreliable.
  67 .sp
  68 .LP
  69 Note that share is not the same as utilization. A project may be allocated 50%
  70 of the system, although on the average, it uses just 20%. Shares serve to cap a
  71 project's CPU usage only when there is competition from other projects running
  72 on the same processor set. When there is no competition, utilization may be
  73 larger than entitlement based on shares. Allocating a small share to a busy
  74 project slows it down but does not prevent it from completing its work if the
  75 system is not saturated.
  76 .sp
  77 .LP
  78 The configuration of CPU shares is managed by the name server as a property of
  79 the \fBproject\fR(4) database. In the following example, an entry in the
  80 \fB/etc/project\fR file sets the number of shares for project \fBx-files\fR to
  81 10:
  82 .sp
  83 .in +2
  84 .nf
  85 x-files:100::::project.cpu-shares=(privileged,10,none)
  86 .fi
  87 .in -2
  89 .sp
  90 .LP
  91 Projects with undefined number of shares are given one share each. This means
  92 that such projects are treated with equal importance. Projects with 0 shares
  93 only run when there are no projects with non-zero shares competing for the same
  94 processor set. The maximum number of shares that can be assigned to one project
  95 is 65535.
  96 .sp
  97 .LP
  98 You can use the \fBprctl\fR(1) command to determine the current share
  99 assignment for a given project:
 100 .sp
 101 .in +2
 102 .nf
 103 $ prctl -n project.cpu-shares -i project x-files
 104 .fi
 105 .in -2
 107 .sp
 108 .LP
 109 or to change the amount of shares if you have root privileges:
 110 .sp
 111 .in +2
 112 .nf
 113 # prctl -r -n project.cpu-shares -v 5 -i project x-files
 114 .fi
 115 .in -2
 117 .sp
 118 .LP
 119 See the \fBprctl\fR(1) man page for additional information on how to modify and
 120 examine resource controls associated with active processes, tasks, or projects
 121 on the system. See \fBresource_controls\fR(5) for a description of the resource
 122 controls supported in the current release of the Solaris operating system.
 123 .sp
 124 .LP
 125 By default, project \fBsystem\fR (project ID 0) includes all system daemons
 126 started by initialization scripts and has an "unlimited" amount of shares. That
 127 is, it is always scheduled first no matter how many shares are given to other
 128 projects.
 129 .sp
 130 .LP
 131 The following command sets FSS as the default scheduler for the system:
 132 .sp
 133 .in +2
 134 .nf
 135 # dispadmin -d FSS
 136 .fi
 137 .in -2
 139 .sp
 140 .LP
 141 This change will take effect on the next reboot. Alternatively, you can move
 142 processes from the time-share scheduling class (as well as the special case of
 143 init) into the FSS class without changing your default scheduling class and
 144 rebooting by becoming \fBroot\fR, and then using the \fBpriocntl\fR(1) command,
 145 as shown in the following example:
 146 .sp
 147 .in +2
 148 .nf
 149 # priocntl -s -c FSS -i class TS
 150 # priocntl -s -c FSS -i pid 1
 151 .fi
 152 .in -2
 155 .LP
 156 You can use the \fBdispadmin\fR(1M) command to examine and tune the FSS
 157 scheduler's time quantum value. Time quantum is the amount of time that a
 158 thread is allowed to run before it must relinquish the processor. The following
 159 example dumps the current time quantum for the fair share scheduler:
 160 .sp
 161 .in +2
 162 .nf
 163 $ dispadmin -g -c FSS
 164         #
 165         # Fair Share Scheduler Configuration
 166         #
 167         RES=1000
 168         #
 169         # Time Quantum
 170         #
 171         QUANTUM=110
 172 .fi
 173 .in -2
 175 .sp
 176 .LP
 177 The value of the QUANTUM represents some fraction of a second with the
 178 fractional value determied by the reciprocal value of RES. With the default
 179 value of RES = 1000, the reciprocal of 1000 is .001, or milliseconds. Thus, by
 180 default, the QUANTUM value represents the time quantum in milliseconds.
 181 .sp
 182 .LP
 183 If you change the RES value using \fBdispadmin\fR with the \fB-r\fR option, you
 184 also change the QUANTUM value. For example, instead of quantum of 110 with RES
 185 of 1000, a quantum of 11 with a RES of 100 results. The fractional unit is
 186 different while the amount of time is the same.
 187 .sp
 188 .LP
 189 You can use the \fB-s\fR option to change the time quantum value. Note that
 190 such changes are not preserved across reboot. Please refer to the
 191 \fBdispadmin\fR(1M) man page for additional information.
 194 .LP
 195 \fBprctl\fR(1), \fBpriocntl\fR(1), \fBdispadmin\fR(1M), \fBpsrset\fR(1M),
 196 \fBpriocntl\fR(2), \fBproject\fR(4), \fBresource_controls\fR(5)
 197 .sp
 198 .LP
 199 \fISystem Administration Guide:  Virtualization Using the Solaris Operating
 200 System\fR