1FSS(7) Device and Network Interfaces FSS(7)
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6 FSS - Fair share scheduler
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9 The fair share scheduler (FSS) guarantees application performance by
10 explicitly allocating shares of CPU resources to projects. A share
11 indicates a project's entitlement to available CPU resources. Because
12 shares are meaningful only in comparison with other project's shares,
13 the absolute quantity of shares is not important. Any number that is in
14 proportion with the desired CPU entitlement can be used.
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17 The goals of the FSS scheduler differ from the traditional time-sharing
18 scheduling class (TS). In addition to scheduling individual LWPs, the
19 FSS scheduler schedules projects against each other, making it impossi‐
20 ble for any project to acquire more CPU cycles simply by running more
21 processes concurrently.
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24 A project's entitlement is individually calculated by FSS independently
25 for each processor set if the project contains processes bound to them.
26 If a project is running on more than one processor set, it can have
27 different entitlements on every set. A project's entitlement is defined
28 as a ratio between the number of shares given to a project and the sum
29 of shares of all active projects running on the same processor set. An
30 active project is one that has at least one running or runnable
31 process. Entitlements are recomputed whenever any project becomes
32 active or inactive, or whenever the number of shares is changed.
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35 Processor sets represent virtual machines in the FSS scheduling class
36 and processes are scheduled independently in each processor set. That
37 is, processes compete with each other only if they are running on the
38 same processor set. When a processor set is destroyed, all processes
39 that were bound to it are moved to the default processor set, which
40 always exists. Empty processor sets (that is, sets without processors
41 in them) have no impact on the FSS scheduler behavior.
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44 If a processor set contains a mix of TS/IA and FSS processes, the fair‐
45 ness of the FSS scheduling class can be compromised because these
46 classes use the same range of priorities. Fairness is most signifi‐
47 cantly affected if processes running in the TS scheduling class are
48 CPU-intensive and are bound to processors within the processor set. As
49 a result, you should avoid having processes from TS/IA and FSS classes
50 share the same processor set. RT and FSS processes use disjoint prior‐
51 ity ranges and therefore can share processor sets.
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54 As projects execute, their CPU usage is accumulated over time. The FSS
55 scheduler periodically decays CPU usages of every project by multiply‐
56 ing it with a decay factor, ensuring that more recent CPU usage has
57 greater weight when taken into account for scheduling. The FSS sched‐
58 uler continually adjusts priorities of all processes to make each
59 project's relative CPU usage converge with its entitlement.
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62 While FSS is designed to fairly allocate cycles over a long-term time
63 period, it is possible that projects will not receive their allocated
64 shares worth of CPU cycles due to uneven demand. This makes one-shot,
65 instantaneous analysis of FSS performance data unreliable.
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68 Note that share is not the same as utilization. A project may be allo‐
69 cated 50% of the system, although on the average, it uses just 20%.
70 Shares serve to cap a project's CPU usage only when there is competi‐
71 tion from other projects running on the same processor set. When there
72 is no competition, utilization may be larger than entitlement based on
73 shares. Allocating a small share to a busy project slows it down but
74 does not prevent it from completing its work if the system is not satu‐
75 rated.
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78 The configuration of CPU shares is managed by the name server as a
79 property of the project(4) database. In the following example, an entry
80 in the /etc/project file sets the number of shares for project x-files
81 to 10:
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83 x-files:100::::project.cpu-shares=(privileged,10,none)
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87 Projects with undefined number of shares are given one share each. This
88 means that such projects are treated with equal importance. Projects
89 with 0 shares only run when there are no projects with non-zero shares
90 competing for the same processor set. The maximum number of shares that
91 can be assigned to one project is 65535.
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94 You can use the prctl(1) command to determine the current share assign‐
95 ment for a given project:
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97 $ prctl -n project.cpu-shares -i project x-files
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101 or to change the amount of shares if you have root privileges:
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103 # prctl -r -n project.cpu-shares -v 5 -i project x-files
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107 See the prctl(1) man page for additional information on how to modify
108 and examine resource controls associated with active processes, tasks,
109 or projects on the system. See resource_controls(5) for a description
110 of the resource controls supported in the current release of the
111 Solaris operating system.
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114 By default, project system (project ID 0) includes all system daemons
115 started by initialization scripts and has an "unlimited" amount of
116 shares. That is, it is always scheduled first no matter how many shares
117 are given to other projects.
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120 The following command sets FSS as the default scheduler for the system:
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122 # dispadmin -d FSS
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126 This change will take effect on the next reboot. Alternatively, you can
127 move processes from the time-share scheduling class (as well as the
128 special case of init) into the FSS class without changing your default
129 scheduling class and rebooting by becoming root, and then using the
130 priocntl(1) command, as shown in the following example:
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132 # priocntl -s -c FSS -i class TS
133 # priocntl -s -c FSS -i pid 1
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137 You can use the dispadmin(1M) command to examine and tune the FSS
138 scheduler's time quantum value. Time quantum is the amount of time that
139 a thread is allowed to run before it must relinquish the processor. The
140 following example dumps the current time quantum for the fair share
141 scheduler:
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143 $ dispadmin -g -c FSS
144 #
145 # Fair Share Scheduler Configuration
146 #
147 RES=1000
148 #
149 # Time Quantum
150 #
151 QUANTUM=110
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155 The value of the QUANTUM represents some fraction of a second with the
156 fractional value determied by the reciprocal value of RES. With the
157 default value of RES = 1000, the reciprocal of 1000 is .001, or mil‐
158 liseconds. Thus, by default, the QUANTUM value represents the time
159 quantum in milliseconds.
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162 If you change the RES value using dispadmin with the -r option, you
163 also change the QUANTUM value. For example, instead of quantum of 110
164 with RES of 1000, a quantum of 11 with a RES of 100 results. The frac‐
165 tional unit is different while the amount of time is the same.
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168 You can use the -s option to change the time quantum value. Note that
169 such changes are not preserved across reboot. Please refer to the dis‐
170 padmin(1M) man page for additional information.
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173 See attributes(5) for descriptions of the following attributes:
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178 ┌───────────────────────────────────────────────────────────┐
179 │ATTRIBUTE TYPE ATTRIBUTE VALUE │
180 │Architecture SUNWcsu │
181 └───────────────────────────────────────────────────────────┘
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184 prctl(1), priocntl(1), dispadmin(1M), psrset(1M), priocntl(2),
185 project(4), attributes(5), resource_controls(5)
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188 System Administration Guide: Virtualization Using the Solaris Operat‐
189 ing System
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193SunOS 5.11 1 Oct 2004 FSS(7)