1

Traditional Unix SchedulingTraditional Unix Scheduling• “Traditional” means

– No consideration on realtime processes.– Typically 4.3BSD and SVR3

• MFQ-based– Preemptive (time quantum based)– Round robin in the same priority queue– Same time quantum to different priority queues– Dynamic priority assignment

• Priority – Integer value between 0 and 127

• Numerically lower values correspond to higher priorities.• Priorities between 0 and 49 are reserved for the kernel• Priorities between 50 and 127 are for user-level programs

2

• PCB: the most important data structure in an OS – Process identification

• PID

– CPU state information• The contents of CPU registers• Program counter

– Indicates the address of the next instruction to be executed.

– Process control information• Event that a process is waiting on. • I/O status (e.g., open files)• Scheduling info

Process Control

Block

User Program

User Data/Stack

Kernel Stack

Process Image

3

• PCB contains, as process scheduling info,– Current scheduling priority (pri) – User mode priority (usrpri)– Measure of recent CPU

utilization (cpu)– Nice value (user-controllable

adjustment factor; nice)

• Current scheduling priority (pri)– Used to decide which process

to schedule. – Same as user mode priority

(usrpri), when a process is in user mode.

– Goes down to place higher priority to kernel mode processing upon I/O completion.

• Down to a predefined priority (0 =< sleep priority =< 49).

– 10 for file access– 20 for disk I/O

• This enables a prompt I/O completion (a prompt completion of system call).

– Replaced with the current user mode priority (usrpri) when it is back in use mode.

4

7-State Process Behavior Model7-State Process Behavior Model

• Ready: The process is in main memory and available for execution.

• Blocked: The process is in main memory and blocked on an event.

• Blocked, suspended: The process is in virtual memory and blocked on an event.

• Ready, suspended: The process is in virtual memory but is available for execution as soon as it is loaded to main memory.

Ready,Suspended

Ready Running Terminated

Blocked

activatedDispatched to a CPU

interrupted

I/O or event waitEvent completion

exit

Blocked,Blocked,SuspendedSuspended

Suspended

Suspended

New

admitted

activated

5

• User mode priority (usrpri)– Depends two factors:

• Recent CPU utilization (cpu)

• Nice value (nice)

• CPU utilization– Initialized to 0 when a

process is created. – Incremented (for the

currently-running process) at every clock interrupt.

• 10 msec

• Nice value– An integer value between

0 and 39• The default value is 20.

– Increasing this value decreases the priority.

• Only OS administrators can decrease it.

– The nice command is used to change the nice value for each process.

• Accepts -20 to 19 as an increment to the current value.

6

• cpuj(i)– CPU utilization by process j through interval i. – Unit time for i: 10 msec.

• DR: decay rate– cpuj is reduced by a decay rate.– 1/2 in SVR3.

• PUSER (baseline priority): 50– used to divide processes into fixed bands of priority levels.

€

cpu j (i) = DR*cpu j (i −1)

usrpri j (i) = PUSER +cpu j (i)

2+ nice j

• Every second, user mode priority (usrpri) is updated as follows:

7

An ExampleAn Example

€

cpu j (i) =cpu j (i −1)

2

usrpri j (i) = 50 +cpu j (i)

2+ 0

DR: 1/2PUSER: 50nice: 0

3 processes in thesame priority band

Time quantum/process:1 sec

Process 1usrpri cpu

Process 2usrpri cpu

Process 3usrpri cpuTime

0

1 sec

2 sec

3 sec

4 sec

50 01

2

100

50 0

50 0

75 50

50 01

2

100

50 0

62 25

75 50

50 01

2

100

75 50

56 1213

112

62 25

78 56 62 25

56 1213

112

10 ms20 ms

8

4.3BSD Scheduler4.3BSD Scheduler

• 32 run (ready) queue– A queue for 4 adjacent priorities

• RQ0 for priorities 0 to 3• RQ1 for priorities 4 to 7

• RR in each RQ– RR time quantum: 100 msec– The highest-priority process is

always scheduled first in each RQ.

• MFQ across RQs– If all other ready processes are

on lower RQs, the current process continues to run even if its quantum has expired.

• Priority update– Once every second– Process promotion and

demotion across RQs

• Timing of context switches– The current process blocks on

a resource or exits.• Voluntary context switch

– RR-based preemption– Priority-based preemption– I/O (event) completion

CPU

Run Queue 0admitteddispatched

Run Queue 1

Run Queue 31

9

HWHW• The decay factor used in 4.3BSD is:

• LoadAverage– The average number of ready processes over the

last second.

• Discuss how this decay factor impacts process scheduling.

• Compare the scheduling algorithms in SVR3 and 4.3BSD.

€

DR =2∗LoadAverage2∗LoadAverage +1

10

Considering Process GroupsConsidering Process Groups

• In traditional Unix scheduling,– A priority is assigned per process. – Processes compete for CPU time.

• What about a group of processes? – Processes created by each application– Processes created by each user– Processes in the same session (or job)

• ps -j• ps -Aj

11

Fair Share Scheduling (FSS)Fair Share Scheduling (FSS)• Group-aware priority

assignment– Group: application, user or

session

• Assigns the same/similar priority (share of the CPU) to the processes in the same group – Give fewer CPU time to

processes that have had more than their fair share

– Give more CPU time to processes that have had less than their fair share.

• Considers the CPU utilization history of a group of processes– along with the CPU utilization

history of individual processes.• e.g., 25% share of the total

CPU time, when there are 4 groups.

• Can assign different priorities to different groups– Foreground and background

processes– OS administrators, normal

users and guests

• An extension to the traditional Unix scheduler.

12

• cpuj(i)– CPU utilization by process j through interval i. – Unit time for i: 10 msec.

• gcpuj(i)– CPU utilization by group k through interval i.

• DRp DRg: decay rate– cpuj and cpuk are reduced by DRp and DRg, respectively.

• PUSER (baseline priority):– The default value: 50

• Wk: weight assigned to group k.– 0 < Wk < 1 and

€

cpu j (i) = DRp *cpu j (i −1)

gcpuk = DRg * gcpuk (i −1)

usrpri j (i) = PUSER +cpu j (i)

2+gcpuk (i)

4 *Wk

• Every second, user mode priority (usrpri) is updated as follows:

€

Wk

k

∑ =1

13

An ExampleAn Example

€

cpu j (i) =cpu j (i −1)

2

gcpuk (i) =gcpuk (i −1)

2usrpri j (i) =

50 +cpu j (i)

2+gcpuk (i)

2

DRp, DRg: 1/2PUSER: 50W1 and W2: 1/2

3 processes in thesame priority band

P1 is in Group 1P2 and P3 in Group 2

Time quantum/process:1 sec

Time0

1 sec

2 sec

3 sec

4 sec

10 ms20 ms

5 sec

Process 1usrpri cpu gcpu

50 0 0 1 1 2 2

100 100

50 0 0 1 1 2 2

100 100

106 50 62

Process 2usrpri cpu gcpu

Process 3usrpri cpu gcpu

50 0 0

50 0 0

100 50 50

Group 1 Group 2

50 0 0 1 2

100

74 25 25

125 125

100 50 50

75 0 50

112 62 62

74 25 25

125

62 0 25

100 125

87 12 62

80 31 31

131 131

14

Process 1Usrpri cpu

Process 2usrpri cpu

Process 3usrpri cpuTime

0

1 sec

2 sec

3 sec

4 sec

50 01

2

100

50 0

50 0

75 50

50 0 1

100

50 0

62 25

75 50

50 01

100

75 50

56 1213

112

62 25

78 56 62 25

56 1213

112

10 ms20 ms

Process 1usrpri cpu gcpu

50 0 0 1 1 2 2

100 100

50 0 0 1 1 2 2

100 100

106 50 62

Process 2usrpri cpu gcpu

Process 3usrpri cpu gcpu

50 0 0

50 0 0

100 50 50

Group 1 Group 2

50 0 0 1 2

100

74 25 25

125 125

100 50 50

75 0 50

112 62 62

74 25 25

125

62 0 25

100 125

87 12 62

80 31 31

131 131

Traditional Unix Scheduling Fair Share Scheduling