Basic Concepts Preemptive Vs. Non-preemptive CPU...
17
1 CPU Scheduling Outline What is scheduling in the OS ? What are common scheduling criteria ? How to evaluate scheduling algorithms ? What are common scheduling algorithms ? How is thread scheduling different from process scheduling ? What are the issues in multiple-processor scheduling ? Operating systems case studies. 2 Basic Concepts Multiprogramming most processes alternate between CPU bursts and I/O bursts CPU free and idle during I/O burst schedule another process on the CPU maximizes CPU utilization CPU bound process spends most of its time in the CPU at least a few long CPU bursts I/O bound process spends most its time performing I/O several short CPU bursts 3 CPU Scheduler Responsible for the selection of the next running process part of the OS dispatcher selects from among the processes in memory that are ready to execute based on a particular strategy When does CPU scheduling happen ? process switches from running to waiting state process switches from running to ready state process switches from waiting to ready state process terminates Scheduling under 1 and 4 is nonpreemptive All other scheduling is preemptive 4 Preemptive Vs. Non-preemptive CPU Scheduling Non-preemptive scheduling process voluntarily releases the CPU (conditions 1 and 4) easy, requires no special hardware poor response time for interactive and real-time systems Preemptive scheduling OS can force a running process involuntarily relinquish the CPU arrival of a higher priority process running process exceeds its time-slot may require special hardware, eg., timer may require synchronization mechanisms to maintain data consistency complicates design of the kernel favored by most OSes
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Transcript of Basic Concepts Preemptive Vs. Non-preemptive CPU...
1
CP
U S
ch
ed
ulin
g
Ou
tlin
e
Wha
t is
sched
ulin
g in the
OS
?
Wha
t are
co
mm
on s
ched
ulin
g c
rite
ria
?
How
to e
valu
ate
sched
ulin
g a
lgori
thm
s ?
Wh
at
are
co
mm
on
sch
ed
ulin
g a
lgo
rith
ms ?
How
is thre
ad s
ched
ulin
g d
iffe
rent fr
om
pro
ce
ss s
ch
edu
ling ?
Wha
t are
the
issu
es in
multip
le-p
roce
ssor
sched
ulin
g ?
Opera
tin
g s
yste
ms c
ase
stu
die
s.
2
Basic
Co
ncep
ts
Multip
rogra
mm
ing
mo
st
pro
ce
sses a
lte
rna
te b
etw
een
CP
U b
urs
tsand I
/O b
urs
ts
CP
U f
ree a
nd
idle
during
I/O
bu
rst
sch
ed
ule
ano
the
r p
roce
ss o
n t
he C
PU
ma
xim
ize
s C
PU
utiliz
ation
CP
U b
ou
nd p
roce
ss
sp
en
ds m
ost
of its t
ime in
th
e C
PU
at
lea
st
a f
ew
lon
gC
PU
burs
ts
I/O
bou
nd p
roce
ss
sp
en
ds m
ost
its t
ime p
erf
orm
ing
I/O
se
vera
l sh
ort
CP
U b
urs
ts
3
CP
U S
ch
ed
ule
r
Respon
sib
le for
the
se
lectio
n o
f th
e n
ext
run
nin
g p
roce
ss
part
of
the O
S d
isp
atc
her
se
lects
fro
m a
mon
g t
he p
roce
sses in m
em
ory
th
at
are
re
ad
y t
o e
xe
cute
base
d o
n a
part
icu
lar
str
ate
gy
Whe
n d
oes C
PU
sched
ulin
g h
ap
pen
?
pro
ce
ss s
witch
es f
rom
ru
nn
ing t
o w
aitin
g s
tate
pro
ce
ss s
witch
es f
rom
ru
nn
ing t
o r
ead
y s
tate
pro
ce
ss s
witch
es f
rom
wa
itin
g t
o r
ead
y s
tate
pro
ce
ss t
erm
ina
tes
Sch
edu
ling u
nde
r 1 a
nd 4
is n
on
pre
em
pti
ve
All
oth
er
sched
ulin
g is p
reem
pti
ve
4
Pre
em
pti
ve V
s. N
on
-pre
em
pti
ve
CP
U S
ch
ed
ulin
g
Non
-pre
em
ptive s
ched
ulin
g
pro
ce
ss v
olu
nta
rily
re
lease
s t
he C
PU
(co
nd
itio
ns 1
an
d 4
)
easy,
requires n
o s
pecia
l hard
ware
poo
r re
spo
nse
tim
e f
or
inte
ractive
and
rea
l-tim
e s
yste
ms
Pre
em
ptive s
ched
ulin
g
OS
ca
n f
orc
e a
ru
nn
ing p
roce
ss invo
lunta
rily
re
linq
uis
h t
he C
PU
arr
ival of
a h
igher
priority
pro
cess
runn
ing p
roce
ss e
xce
ed
s its
tim
e-s
lot
ma
y r
equ
ire
sp
ecia
l h
ard
wa
re,
eg.,
tim
er
ma
y r
equ
ire
syn
chro
niz
atio
n m
echa
nis
ms t
o m
ain
tain
data
co
nsis
tency
co
mplic
ate
s d
esig
n o
f th
e k
ern
el
favore
d b
y m
ost O
Se
s
5
Dis
patc
her
Sch
edu
ler
is a
part
of
the
dis
patc
her
module
in
the
OS
Fun
ctio
ns o
f th
e d
isp
atc
her
get
the n
ew
pro
ce
ss f
rom
th
e s
ch
ed
ule
r
sw
itch
out
the c
onte
xt
of
the c
urr
ent
pro
ce
ss
giv
e C
PU
co
ntr
ol to
th
e n
ew
pro
ce
ss
jum
p t
o th
e p
rop
er
loca
tion
in t
he n
ew
pro
gra
m t
o r
esta
rt t
hat
pro
gra
m
Tim
e t
aken b
y the d
isp
atc
her
to s
top o
ne
pro
ce
ss a
nd s
tart
ano
the
r ru
nnin
g is c
alle
d the d
isp
atc
h late
ncy
6
Sch
ed
ulin
g Q
ueu
es
Jo
b q
ueu
e: consis
ts o
f a
ll pro
ce
sses
all
job
s (
pro
cesse
s),
once
su
bm
itte
d,
are
in
th
e jo
b q
ue
ue
sch
ed
ule
d b
y t
he lo
ng-t
erm
sch
ed
ule
r
Read
y q
ue
ue:
co
nsis
ts o
f p
rocesse
s in
mem
ory
pro
ce
sses r
ead
y a
nd
wa
itin
g f
or
exe
cutio
n
sch
ed
ule
d b
y t
he s
hort
-te
rm o
r C
PU
sche
du
ler
Devic
e q
ueue
: pro
ce
sses w
aitin
g fo
r a d
evic
e
mu
ltip
le p
roce
sses c
an b
e b
locke
d f
or
the s
am
e d
evic
e
I/O
com
ple
tion
mo
ves p
roce
ss b
ack t
o r
ead
y q
ue
ue
7
Sch
ed
ulin
g Q
ueu
es (
2)
8
Perf
orm
an
ce M
etr
ics f
or
CP
U S
ch
ed
ulin
g
CP
U u
tiliz
atio
n:
perc
enta
ge o
f tim
e t
hat
the
CP
U is b
usy
Thro
ugh
put:
num
be
r of p
rocesse
s t
ha
t com
ple
te t
he
ir
execu
tio
n p
er
tim
e u
nit
Turn
aro
und
tim
e:
am
oun
t of tim
e t
o e
xecute
a p
art
icu
lar
pro
ce
ss (
subm
issio
n t
ime t
o c
om
ple
tio
n t
ime)
Waitin
g tim
e:
am
oun
t of tim
e a
pro
cess h
as b
ee
n w
aitin
g in the
re
ady q
ue
ue
Respon
se
tim
e:
am
oun
t of tim
e it
takes fro
m w
hen
a r
eque
st
wa
s s
ub
mitte
d u
ntil th
e f
irst
resp
on
se
is p
rod
uce
d
Sch
edu
ling
goa
ls
ma
xim
ize
CP
U u
tiliz
ation
and
th
rou
gh
pu
t
min
imiz
e t
urn
aro
und
tim
e,
wa
itin
g t
ime
an
d r
espo
nse
tim
e
be f
air t
o a
ll pro
cesses a
nd a
ll users
9
Meth
od
fo
r E
valu
ati
ng
C
PU
Sch
ed
ulin
g A
lgo
rith
ms
Evalu
atio
n c
rite
ria
defin
e r
ela
tive im
po
rtan
ce o
f th
e p
erf
orm
ance
me
tric
s
inclu
de o
ther
syste
m-s
pecific
measure
s
Dete
rmin
istic m
odelin
g
takes a
part
icu
lar
pre
dete
rmin
ed w
ork
load
and
defin
es t
he p
erf
orm
ance
o
f e
ach
alg
orith
m f
or
that
wo
rklo
ad
sim
ple
and
fa
st,
giv
es e
xa
ct
num
bers
difficu
lt t
o g
en
era
lize r
esults
ca
n r
ecog
niz
e a
lgorith
m p
erf
orm
ance
tre
nd
s o
ve
r seve
ral in
puts
used
fo
r e
xp
lain
ing s
ch
ed
ulin
g a
lgorith
ms
used
in t
he r
est
of th
is c
ha
pte
r !
10
Wo
rklo
ad
Mo
dels
an
d G
an
tt C
hart
s
Work
load m
odel:
Gantt c
hart
s:
bar
ch
art
to
illu
str
ate
a p
art
icula
r schedule
fig
ure
sh
ow
s b
atc
h s
ch
ed
ule
Process
Arriv
al Tim
eBurst T
ime
P1
08
P2
14
P3
110
P4
62
P1
P2
P3
P4
08
012
22
24
11
Dete
rmin
isti
c M
od
elin
g E
xam
ple
Supp
ose w
e h
ave p
roce
sses A
, B
, a
nd C
, subm
itte
d a
t tim
e 0
.
We w
an
t to
know
the
respon
se
tim
e,
waitin
g tim
e,
and
tu
rnaro
und
tim
e o
f p
rocess A
AB
CA
BC
AC
AC
Tim
e
respo
nse
tim
e =
0+
+w
ait t
ime
turn
aro
und
tim
e
12
Dete
rmin
isti
c M
od
elin
g E
xam
ple
Supp
ose w
e h
ave p
roce
sses A
, B
, a
nd C
, subm
itte
d a
t tim
e 0
.
We w
an
t to
know
the
respon
se
tim
e,
waitin
g tim
e,
and
tu
rnaro
und
tim
e o
f p
rocess B
.
AB
CA
BC
AC
AC
Tim
e
respo
nse
tim
e+
wa
it t
ime
turn
aro
und
tim
e
13
Dete
rmin
isti
c M
od
elin
g E
xam
ple
Supp
ose w
e h
ave p
roce
sses A
, B
, a
nd C
, subm
itte
d a
t tim
e 0
.
We w
an
t to
know
the
respon
se
tim
e,
waitin
g tim
e,
and
tu
rnaro
und
tim
e o
f p
rocess C
AB
CA
BC
AC
AC
Tim
e
respo
nse
tim
e+
++
wa
it t
ime
turn
aro
und t
ime
14
Meth
od
fo
r E
valu
ati
ng
C
PU
Sch
ed
ulin
g A
lgo
rith
ms (
2)
Queue
ing m
odels
ana
lytica
lly m
ode
l th
e q
ue
ue
beh
avio
r (u
nd
er
so
me a
ssum
ption
s)
involv
es a
lot
of
com
plic
ate
d m
ath
ca
n o
nly
han
dle
a lim
ite
d n
um
ber
of d
istr
ibutio
ns a
nd
alg
orith
ms
ma
y n
ot
be v
ery
accura
te b
eca
use
of
unre
alis
tic a
ssum
ption
s
Sim
ula
tio
ns
get
a w
ork
load info
rmation f
rom
a s
yste
m
sim
ula
te t
he s
ch
ed
ulin
g a
lgorith
m
co
mpu
te t
he p
erf
orm
ance
me
tric
s
tim
e a
nd s
pace
inte
nsiv
e
is p
ractica
lly t
he b
est
evalu
ation
me
tho
d
15
Sim
ula
tio
n Illu
str
ati
on
16
Sch
ed
ulin
g A
lgo
rith
ms
Fir
st C
om
e,
Fir
st S
erv
ed (
FC
FS
)
Short
est Job F
irst
(SJF
)
Prio
rity
Based
Ro
un
d R
ob
in (
RR
)
Multile
vel Q
ueue
Sch
edu
ling
Multile
vel F
ee
dba
ck Q
ueue
Sch
edu
ling
17
Fir
st
Co
me, F
irst
Serv
ed
Sch
ed
ulin
g
Assig
ns the
CP
U b
ased o
n t
he
ord
er
of th
e r
eq
uests
.
Imple
mente
d u
sin
g a
FIF
O q
ue
ue.
No p
reem
ption
Ad
va
nta
ge
s
str
aig
htf
orw
ard
, sim
ple
to
write
and
und
ers
tand
Dis
advanta
ge
s
avera
ge
wa
itin
g t
ime
ma
y b
e t
oo lo
ng
hug
e v
aria
tion
base
d o
n w
hen
pro
ce
sses a
rriv
e
ca
nn
ot
bala
nce C
PU
-bou
nd
and
I/O
-bou
nd
pro
cesse
s
co
nvo
y e
ffe
ct,
sh
ort
pro
ce
ss b
ehin
d lon
g p
roce
ss
ca
nn
ot
be u
sed
fo
r tim
e-s
haring
syste
ms
18
FC
FS
Sch
ed
ulin
g E
xam
ple
Supp
ose th
at th
e p
roce
sses a
rriv
e in
the
ord
er:
P1
, P
2,
P3
Waitin
g tim
e f
or
P1
= 0
; P
2=
24
; P
3 =
27
Avera
ge
waitin
g tim
e:
(0 +
24
+ 2
7)/
3 =
17
P1
P2
P3
24
27
30
0
Process
Arriv
al Tim
eBurst T
ime
P1
024
P2
03
P3
03
19
FC
FS
Sch
ed
ulin
g (
2)
Supp
ose th
at th
e p
roce
sses a
rriv
e in
the
ord
er
P2
, P
3,
P1
The
Gantt c
hart
for
the
sch
edu
le is:
Wa
itin
g t
ime
fo
r P
1 =
6;P
2=
0; P
3 =
3
Avera
ge
waitin
g tim
e:
(6
+ 0
+ 3
)/3 =
3
Much
bett
er
tha
n p
revio
us c
ase
but
larg
e v
aria
tion
P1
P3
P2
63
30
0
20
Sh
ort
est
Jo
b F
irst
(SJF
) S
ch
ed
ulin
g
Ord
er
each p
rocess b
ased o
n t
he
le
ngth
of its n
ext
CP
U b
urs
t
Allo
cate
CP
U t
o t
he p
roce
ss fro
m t
he
fro
nt
of th
e lis
t
sh
ort
est
next
CP
U b
urs
t
Ad
va
nta
ge
s
SJF
is o
ptim
al
achie
ve
s m
inim
um
avera
ge
wa
itin
g t
ime
fo
r a
giv
en s
et o
f p
roce
sses
Dis
advanta
ge
s
difficu
lt t
o k
now
th
e le
ng
th o
f th
e n
ext
CP
U r
equ
est
ca
n a
sk t
he u
ser,
wh
o m
ay n
ot
kn
ow
any b
ett
er
!
mo
de
l-b
ase
d p
red
ictio
n
ca
n le
ad
to
pro
ce
ss s
tarv
atio
n
21
Exam
ple
of
SJF
SJF
sch
edu
ling c
ha
rt
Avera
ge
waitin
g tim
e =
(3
+ 1
6 +
9 +
0)
/ 4 =
7
P4
P3
P1
316
09
P2
24
Process
Arriv
al Tim
eBurst T
ime
P1
06
P2
08
P3
07
P4
03
22
Esti
mate
Len
gth
of
Next
CP
U B
urs
t
Can o
nly
estim
ate
the
le
ngth
next
CP
U b
urs
t sim
ilar
to p
revio
us C
PU
burs
ts ?
how
re
levan
t is
th
e h
isto
ry o
f p
revio
us C
PU
burs
ts ?
Calc
ula
ted a
s a
n e
xpone
ntial avera
ge o
f th
e p
revio
us C
PU
burs
ts f
or
the
pro
ce
ss
Form
ula
:
10 ,
3
.
burs
t
CP
Un
ex
t
for
the
v
alu
ep
redic
ted
2.
burs
t
CP
U
o
fle
ngth
actu
al
1
.
1n
th
nn
t
nn
nt
1
1
23
Esti
mate
Len
gth
of
the N
ext
CP
U B
urs
t (2
)
=0 n
+1
=
n
Rece
nt
his
tory
doe
s n
ot
co
un
t
=1
n+
1=
t n
On
ly t
he a
ctu
al la
st
CP
U b
urs
t co
un
ts
If w
e e
xpand
the
form
ula
, w
e g
et:
n+
1=
t n
+(1
-)
t n-1
+(1
-)j
t n-j
+(1
-)n
+1
0
Sin
ce
both
and
(1 -
) are
le
ss tha
n o
r equ
al to
1, e
ach
su
ccessiv
e term
has le
ss w
eig
ht th
an
its
pre
decesso
r
24
Esti
mate
Len
gth
of
the N
ext
CP
U B
urs
t (3
)
25
Pre
em
pti
ve S
JF
Sch
ed
ulin
g
New
sh
ort
er
pro
ce
ss c
an p
ree
mpt
longe
r cu
rren
t ru
nnin
g
pro
ce
ss
Sh
ort
est
Re
ma
inin
g T
ime
First
sch
ed
ulin
g c
ha
rt:
Ave
rag
e w
aitin
g t
ime
= 6
.5
Process
Arriv
al Tim
eBurst T
ime
P1
08
P2
14
P3
29
P4
35
P 1
P2
P4
P1
P3
01
510
17
26
26
Pri
ori
ty S
ch
ed
ulin
g
A p
riori
ty n
um
be
r asso
cia
ted w
ith
ea
ch
pro
ce
ss
CP
U a
llocate
d to
the
pro
ce
ss w
ith
the
hig
he
st
pri
ori
ty
equ
al p
rio
rity
pro
ce
sses s
ch
ed
ule
d in F
CF
S o
rde
r
Inte
rna
lly d
ete
rmin
ed
prio
ritie
s
tim
e lim
it,
me
mory
re
qu
ire
men
ts,
etc
SJF
use
s n
ext
CP
U b
urs
t fo
r its p
rio
rity
(h
ow
?)
Exte
rnally
sp
ecifie
d p
rio
ritie
s
pro
ce
ss im
po
rtan
ce,
user-
leve
l, e
tc
Can b
e p
reem
ptive o
r non
-pre
em
ptive
Text
uses lo
w n
um
be
rs f
or
hig
h p
riori
tie
s
27
Pri
ori
ty S
ch
ed
ulin
g (
2)
Advanta
ges
prio
ritie
s c
an b
e m
ade
as g
ene
ral a
s n
eed
ed
Dis
advanta
ge
low
prio
rity
pro
ce
ss m
ay n
eve
r e
xe
cute
(in
defin
ite
blo
ckin
g o
r sta
rvatio
n)
Agin
g
techn
ique
to
pre
ve
nt
sta
rvatio
n
incre
ase
prio
rity
of
pro
ce
sses w
ith
tim
e
28
Ro
un
d R
ob
in S
ch
ed
ulin
g (
RR
)
Roun
d r
ob
in a
lgori
thm
arr
an
ge
job
s in F
CF
S o
rde
r
allo
ca
te C
PU
to
th
e f
irst
job in
th
e q
ue
ue
fo
r o
ne
tim
e-s
lice
pre
em
pt
job
, a
dd
it to
th
e e
nd
of th
e q
ue
ue
allo
ca
te C
PU
to
th
e n
ext
job
and
co
ntin
ue
...
One t
ime s
lice is c
alle
d a
tim
e q
ua
ntu
m
Is b
y d
efin
itio
n p
reem
ptive
ca
n b
e c
onsid
ere
d a
s F
CF
S w
ith
pre
em
ptio
n
Advanta
ges
sim
ple
, a
void
s s
tarv
atio
n
Dis
advanta
ge
s
ma
y invo
lve
a larg
e c
onte
xt
sw
itch
overh
ea
d
hig
her
avera
ge w
aitin
g t
ime t
han S
JF
an I
/O b
ou
nd
pro
ce
ss o
n a
hea
vily
loa
de
d s
yste
m w
ill r
un s
low
er
29
Exam
ple
of
RR
wit
h T
ime Q
uan
tum
= 4
The
Gantt s
ched
ulin
g c
hart
is:
Avera
ge
waitin
g tim
e =
5.6
6
P1
P2
P3
P1
P1
P1
P1
P1
04
710
14
18
22
26
30
Process
Burst T
imes
P1
24
P2
3
P3
3
30
Ro
un
d R
ob
in S
ch
ed
ulin
g (
2)
Perf
orm
ance
dep
en
ds o
n t
he le
ng
th o
f th
e t
ime q
ua
ntu
m
Genera
lly,
tim
e q
uan
ta r
ang
e f
rom
10
100
mill
ise
con
ds
co
nte
xt
sw
itch
tim
e is less t
han
10
mic
rose
con
ds
RR
has la
rge
r w
aitin
g tim
e,
but
pro
vid
es b
ett
er
respon
se
tim
e f
or
inte
ractive
syste
ms
Turn
aro
und
tim
e d
ep
end
s o
n t
he
siz
e o
f th
e t
ime q
ua
ntu
m
31
Tu
rnaro
un
d T
ime V
ari
es W
ith
Th
e T
ime Q
uan
tum
32
Mu
ltilevel Q
ueu
e
Read
y q
ue
ue is p
art
itio
ned
in
to s
epa
rate
que
ues:
fore
gro
und
(in
tera
ctive)
backg
rou
nd (
ba
tch)
Each q
ueu
e h
as its
ow
n s
ched
ulin
g a
lgori
thm
fore
gro
und
R
R
backg
rou
nd
F
CF
S
Sch
edu
ling m
ust
be d
on
e b
etw
ee
n t
he
qu
eue
s
Fix
ed p
rio
rity
sch
ed
ulin
g;
(i.e
., s
erv
e a
ll fr
om
fo
reg
rou
nd
th
en
fro
m
backgro
und).
P
ossib
ility
of sta
rvation.
Tim
e s
lice
e
ach
que
ue
gets
a c
ert
ain
am
ou
nt
of
CP
U t
ime w
hic
h it
ca
n
sch
ed
ule
am
on
gst
its p
roce
sses;
i.e
., 8
0%
to
fo
reg
rou
nd
in R
R,
20%
to
b
ackg
rou
nd
in F
CF
S
33
Mu
ltilevel Q
ueu
e S
ch
ed
ulin
g
Use
d w
he
n
pro
ce
sses c
an b
e e
asily
cla
ssifie
d into
gro
ups
each
gro
up h
as a
diffe
ren
t sch
ed
ulin
g r
equ
ire
men
t
Alg
ori
thm
part
itio
n r
ead
y q
ue
ue
into
mu
ltip
le q
ue
ue
s
dete
rmin
e s
om
e s
ch
ed
ulin
g a
lgorith
m f
or
each
pro
ce
sses in e
ach
que
ue
FC
FS
, S
JF,
RR
, e
tc.
dete
rmin
e inte
r-q
ue
ue
sch
ed
ulin
g
fixe
d p
rio
rity
, fixe
d C
PU
utiliz
ation
per
que
ue
, e
tc.
perm
ane
ntly a
ssig
n a
pro
cess t
o a
part
icu
lar
que
ue
34
Mu
ltilevel Q
ueu
e S
ch
ed
ulin
g E
xam
ple
Exam
ple
: fo
regro
und
Vs.
backg
rou
nd
pro
ce
sses
fore
gro
und
are
in
tera
ctive,
backg
rou
nd
are
batc
h p
roce
sses
fore
gro
und
have
priority
over
backg
rou
nd
intr
a-q
ue
ue
sch
ed
ulin
g
fore
gro
und
re
spo
nse
tim
e,
backg
rou
nd
lo
w o
verh
ea
d,
no s
tarv
atio
n
fore
gro
und
R
R,
backg
rou
nd
F
CF
S
sch
ed
ulin
g b
etw
een
th
e q
ue
ue
s
fixe
d p
rio
rity
sch
ed
ulin
g;
fore
gro
und
h
ighe
r p
rio
rity
tim
e s
lice;
80%
to
fo
regro
und in R
R,
20%
to b
ackgro
und in F
CF
S
35
Mu
ltilevel Q
ueu
e S
ch
ed
ulin
g E
xam
ple
(2)
36
Mu
ltilevel F
eed
back Q
ueu
e S
ch
ed
ulin
g
Allo
ws p
roce
ss to m
ove b
etw
een
que
ues
used
to
dyn
am
ically
so
rt p
roce
ss b
ase
d o
n t
heir t
yp
ica
l C
PU
burs
ts
Alg
ori
thm
mu
ltip
le q
ue
ue
s w
ith
diffe
ren
t fixe
d p
rio
ritie
s
roun
d r
obin
at
each
priority
leve
l
run h
igh
est
prio
rity
job
s f
irst,
once
th
ose
fin
ish
, ru
n n
ext
hig
he
st
prio
rity
, e
tc
job
s s
tart
in t
he h
igh
est
prio
rity
que
ue
if t
ime s
lice e
xpires,
dro
p t
he job b
y o
ne level
if t
ime s
lice d
oe
s n
ot
exp
ire
, p
ush
th
e jo
b u
p b
y o
ne
leve
l
37
Exam
ple
of
Mu
ltilevel F
eed
back Q
ueu
es
Prio
rity
0 (
tim
e s
lice =
1):
Prio
rity
1 (
tim
e s
lice
= 2
):
Prio
rity
2 (
tim
e s
lice =
4):
tim
e =
0
Tim
e
AB
C
02
59
38
Exam
ple
of
Mu
ltilevel F
eed
back Q
ueu
es
Prio
rity
0 (
tim
e s
lice =
1):
Prio
rity
1 (
tim
e s
lice
= 2
):
Prio
rity
2 (
tim
e s
lice =
4):
tim
e =
1
Tim
e
A
BC
003
7
A
1
39
Exam
ple
of
Mu
ltilevel F
eed
back Q
ueu
es
Prio
rity
0 (
tim
e s
lice =
1):
Prio
rity
1 (
tim
e s
lice
= 2
):
Prio
rity
2 (
tim
e s
lice =
4):
tim
e =
2
Tim
e
ABC
004
3
A
1
B
40
Exam
ple
of
Mu
ltilevel F
eed
back Q
ueu
es
Prio
rity
0 (
tim
e s
lice =
1):
Prio
rity
1 (
tim
e s
lice
= 2
):
Prio
rity
2 (
tim
e s
lice =
4):
tim
e =
3
Tim
e
AB
C
06
3
A
1
BC
41
Exam
ple
of
Mu
ltilevel F
eed
back Q
ueu
es
Prio
rity
0 (
tim
e s
lice =
1):
Prio
rity
1 (
tim
e s
lice
= 2
):
Prio
rity
2 (
tim
e s
lice =
4):
tim
e =
3
Tim
e
AB
C
06
3
A
1
BC
Suppose A
is b
locked o
n I
/O
42
Exam
ple
of
Mu
ltilevel F
eed
back Q
ueu
es
Prio
rity
0 (
tim
e s
lice =
1):
Prio
rity
1 (
tim
e s
lice
= 2
):
Prio
rity
2 (
tim
e s
lice =
4):
tim
e =
3
Tim
e
A
BC
0
52
A
1
BC
Suppose A
is b
locked o
n I
/O
0
43
Exam
ple
of
Mu
ltilevel F
eed
back Q
ueu
es
Prio
rity
0 (
tim
e s
lice =
1):
Prio
rity
1 (
tim
e s
lice
= 2
):
Prio
rity
2 (
tim
e s
lice =
4):
tim
e =
5
Tim
e
A
B
C
0
3
A
1
BC
Suppose A
now
retu
rns fro
m I
/O
0
44
Exam
ple
of
Mu
ltilevel F
eed
back Q
ueu
es
Prio
rity
0 (
tim
e s
lice =
1):
Prio
rity
1 (
tim
e s
lice
= 2
):
Prio
rity
2 (
tim
e s
lice =
4):
tim
e =
6
Tim
eA
B
C
3
AB
C
0
45
Exam
ple
of
Mu
ltilevel F
eed
back Q
ueu
es
Prio
rity
0 (
tim
e s
lice =
1):
Prio
rity
1 (
tim
e s
lice
= 2
):
Prio
rity
2 (
tim
e s
lice =
4):
tim
e =
8
Tim
eA
B
C
3
AB
C
0
C
46
Exam
ple
of
Mu
ltilevel F
eed
back Q
ueu
es
Prio
rity
0 (
tim
e s
lice =
1):
Prio
rity
1 (
tim
e s
lice
= 2
):
Prio
rity
2 (
tim
e s
lice =
4):
tim
e =
9
Tim
eA
BC
AB
CC
47
Mu
ltilevel F
eed
back Q
ueu
es
Appro
xim
ate
s S
RT
F
a C
PU
-bou
nd
job
dro
ps lik
e a
rock
I/O
-bou
nd
job
s s
tay n
ea
r th
e t
op
Still
unfa
ir fo
r lo
ng r
un
nin
g jo
bs
co
un
ter-
me
asu
re:
Ag
ing
incre
ase
th
e p
riority
of
lon
g r
unn
ing job
s if
they a
re n
ot
se
rvic
ed f
or
a
period
of
tim
e
tric
ky t
o tu
ne
agin
g
48
Lo
ttery
Sch
ed
ulin
g
Adap
tive s
ched
ulin
g a
pp
roa
ch
to a
dd
ress t
he
fair
ne
ss p
roble
m
Alg
ori
thm
each
pro
ce
ss o
wn
s s
om
e t
ickets
on e
ach
tim
e s
lice
, a
tic
ke
t is
ra
ndo
mly
pic
ke
d
on a
vera
ge
, th
e a
llocate
d C
PU
tim
e is p
rop
ort
iona
l to
th
e n
um
ber
of
tickets
g
ive
n t
o e
ach
job
To a
ppro
xim
ate
SJF, short
jobs g
et
more
tic
ke
ts
To a
void
sta
rvation,
each job g
ets
at le
ast one
tic
ke
t
49
Lo
ttery
Sch
ed
ulin
g E
xam
ple
Short
jo
bs: 10 t
icke
ts e
ach
Lon
g jobs: 1 t
icke
t each
# s
ho
rt jo
bs
/#
lon
g jo
bs
% o
f C
PU
fo
r eac
h s
ho
rt jo
b%
of
CP
U f
or
eac
h lo
ng
jo
b
1/1
91
%9
%
0/2
0%
50
%
2/0
50
%0
%
10
/11
0%
1%
1/1
05
0%
5%
50
Th
read
Sch
ed
ulin
g
On s
yste
ms s
uppo
rtin
g t
hre
ads
ke
rne
lth
rea
ds a
re t
he r
eal sch
ed
ulin
g e
ntitie
s
user
thre
ads n
ee
d t
o b
e m
app
ed
to
ke
rne
l th
rea
ds f
or
exe
cutio
n
sch
ed
ulin
g a
ttribu
tes m
ay b
e s
et
at th
rea
d c
reatio
n
Conte
ntion
-scope
PT
HR
EA
D_S
CO
PE
_P
RO
CE
SS
gro
up u
ser
thre
ads t
o c
onte
nd
fo
r co
mm
on
ke
rne
l th
rea
d(s
)
PT
HR
EA
D_S
CO
PE
_S
YS
TE
M
dire
ctly a
ssig
n t
o k
ern
el th
rea
d,
co
nte
nd
s w
ith
oth
er
kern
el th
rea
ds
inheri
tsch
ed
PT
HR
EA
D_IN
HE
RIT
_S
CH
ED
inh
erit
sch
ed
ulin
g p
olic
y a
nd
prio
rity
fro
m p
are
nt
thre
ad
PT
HR
EA
D_E
XP
LIC
IT_S
CH
ED
exp
licitly
sp
ecify s
ch
ed
ulin
g p
olic
y a
nd
prio
rity
of
the n
ew
th
rea
d
51
Th
read
Sch
ed
ulin
g (
2)
sched
polic
y
SC
HE
D_O
TH
ER
regu
lar
non-r
eal-tim
e s
ch
ed
ulin
g
SC
HE
D_R
R
real-tim
e r
oun
d-r
obin
sch
ed
ulin
g
SC
HE
D_F
IFO
real-tim
e F
CF
S s
che
du
ling
sched
para
m
set/
get
the p
riority
of
the t
hre
ad
All
para
mete
rs o
nly
rele
vant
if t
hre
ad lib
rary
su
pp
ort
s n
on
-one-t
o-o
ne
user
level th
rea
ds
for
real-tim
e s
ch
ed
ulin
g
52
Pth
read
Sch
ed
ulin
g E
xam
ple
int main(int argc, char *argv[]){
int i;
pthread t tid[5];
pthread attr t attr;
pthread attr init(&attr); /* get the default attributes */
/* set the scheduling algorithm to PROCESS or SYSTEM */
pthread attr setscope(&attr, PTHREAD SCOPE SYSTEM);
/* set the scheduling policy -
FIFO, RT, or OTHER */
pthread attr setschedpolicy(&attr, SCHED OTHER);
for (i = 0; i < 5; i++)
pthread create(&tid[i],&attr,runner,NULL);
for
(i =
0;
i <
NU
M T
HR
EA
DS
; i+
+)
pth
read join
(tid
[i], N
ULL);
} void
*ru
nner(
void
*para
m){
printf("
I am
a t
hre
ad
\n")
;
pth
read e
xit(0
);
}
53
Mu
ltip
le-P
rocesso
r S
ch
ed
ulin
g Issu
es
Multip
roce
ssor
Sch
edu
ling
asym
me
tric
mu
ltip
roce
ssin
g
only
one
pro
cesso
r a
ccesse
s t
he s
yste
m d
ata
str
uctu
res
sim
ple
sym
me
tric
mu
ltip
roce
ssin
g
(SM
P)
each
pro
ce
ssor
is s
elf-s
ch
ed
ulin
g
nee
d t
o m
ain
tain
sch
ed
ule
r d
ata
str
uctu
res s
yn
chro
niz
atio
n
Pro
cesso
r affin
ity
pro
ce
ss h
as a
ffin
ity f
or
pro
ce
ssor
on w
hic
h it
is c
urr
ently r
unn
ing
redu
ce m
em
ory
and
ca
che
overh
ea
d
me
mory
affin
ity im
po
rtan
t fo
r N
UM
A a
rchite
ctu
res
so
ft a
nd
hard
pro
ce
ssor
affin
ity
how
str
ictly O
S m
ain
tain
s a
ffin
ity p
olic
y
54
NU
MA
an
d C
PU
Sch
ed
ulin
g
55
Mu
ltip
le-P
rocesso
r S
ch
ed
ulin
g Issu
es
Loa
d b
ala
ncin
g
ke
ep
wo
rklo
ad
even
ly d
istr
ibute
d a
cro
ss a
ll C
PU
s
impo
rtan
t if e
ach
pro
ce
ssor
has its
ow
n q
ue
ue
of
read
y p
roce
sses
push
and p
ull
mig
ration
push
or
pu
ll ta
sks to
wa
rds idle
pro
ce
ssors
Multic
ore
pro
ce
ssors
mu
ltip
le p
roce
ssor
co
res o
n s
am
e p
hysic
al ch
ip
unifo
rm m
em
ory
access,
faste
r in
terc
ore
co
mm
un
ica
tion
ma
y b
e s
imu
lta
ne
ou
sly
mu
ltith
rea
de
d (
SM
T, o
r h
ype
rthre
ad
ed
)
instr
uction
s f
rom
mu
ltip
le t
hre
ads s
imulta
ne
ou
sly
liv
e in d
iffe
ren
t p
ipelin
e s
tage
s
OS
giv
en
a v
iew
of
one
pro
cesso
r p
er
hard
wa
re t
hre
ad
ma
y r
edu
ce m
em
ory
sta
lls
ma
y incre
ase
re
sou
rce c
onte
ntio
n
56
Case S
tud
ies:
So
lari
s S
ch
ed
ulin
g
Prio
rity
-based s
ched
ulin
g
Six
cla
sse
s
real tim
e, syste
m,
fair s
hare
, fixe
d p
rio
rity
, tim
e s
ha
r, a
nd inte
ractive
(in o
rde
r o
f p
rio
rity
)
diffe
ren
t p
rio
ritie
s a
nd
sch
ed
ulin
g a
lgorith
ms in d
iffe
ren
t cla
sse
s
The
defa
ult c
lass is tim
e s
hari
ng
uses m
ultile
vel fe
ed
ba
ck q
ue
ue
with
va
ria
ble
tim
e s
lice
s
inve
rse r
ela
tion
ship
betw
een
prio
ritie
s a
nd
tim
e s
lice
s
goo
d r
espo
nse
tim
e f
or
inte
ractive
pro
ce
sses a
nd
goo
d t
hro
ugh
pu
t fo
r C
PU
-bou
nd
pro
ce
sses
se
e t
he d
ispa
tch t
able
57
So
lari
s D
isp
atc
h T
ab
le
58
Case S
tud
ies:
So
lari
s S
ch
ed
ulin
g (
2)
Real-
tim
e s
ched
ulin
g c
lass
hig
he
st
prio
rity
, b
ou
nd
ed
tim
e r
espo
nse
sh
ou
ld b
e c
are
ful be
fore
putt
ing
a p
rocess in
th
is c
lass
Syste
m s
ched
ulin
g c
lass
reserv
ed f
or
ke
rne
l th
rea
ds (
sch
ed
ulin
g a
nd
pa
gin
g d
ae
mon
)
Fix
ed p
riori
ty s
ched
ulin
g c
lass
prio
ritie
s n
ot
dyn
am
ically
adju
ste
d
Fair
sh
are
sch
edu
ling c
lass
base
d o
n lo
tte
ry s
ch
ed
ulin
g
pro
ce
sses g
rou
pe
d into
pro
jects
; e
ach
pro
ject
assig
ne
d s
om
e n
ub
mer
of
lott
ery
to
ken
s
pro
ce
sses w
ith
in e
ach
pro
ject
sh
are
th
e t
oken
fa
irly
Pro
ce
ss c
lass p
rio
ritie
s c
on
ve
rte
d t
o g
lob
al p
rio
ritie
s
59
Map
to
Glo
bal P
rio
riti
es
60
Case S
tud
ies:
Lin
ux S
ch
ed
ulin
g (
v2.5
)
Lin
ux s
ch
ed
ulin
g (
ve
rsio
n 2
.5)
co
nsta
nt
ord
er
O(1
) sch
ed
ulin
g t
ime
su
pp
ort
fo
r S
MP
syste
ms
pro
vid
e f
airne
ss a
nd
su
pp
ort
fo
r in
tera
ctive
ta
sks
Prio
rity
-ba
se
d,
pre
em
ptive
, g
lob
al ro
un
d-r
ob
in s
ch
ed
ulin
g
two
prio
rity
ra
ng
es:
tim
e-s
haring
and
re
al-tim
e
Rea
l-ti
me
ra
ng
e f
rom
0 t
o 9
9 a
nd
nic
e v
alu
e f
rom
100
to
140
num
erica
lly low
er
va
lues h
ave
hig
he
r p
rio
ritie
s
hig
he
r p
rio
rity
ta
sks a
re g
ive lon
ge
r tim
e s
lice
Ta
sk r
un-a
ble
as lon
g a
s t
ime le
ft in t
ime
slic
e
(active)
If n
o t
ime le
ft (
exp
ire
d),
not
run
-able
until a
ll o
the
r ta
sks u
se t
heir
slic
es
se
e m
app
ing f
igure
61
Pri
ori
ties a
nd
Tim
e-s
lice len
gth
62
Case S
tud
ies:
Lin
ux S
ch
ed
ulin
g (
v2.5
)
More
abo
ut L
inux s
ched
ulin
g
ma
inta
ins t
wo
arr
ays:
active
and e
xp
ire
d
tasks lin
ke
d in a
rra
ys a
ccord
ing t
o t
heir p
rio
ritie
s
active t
asks o
n e
xp
ira
tion
of
their t
ime
qua
nta
mo
ve t
o t
he e
xp
ire
d a
rray
real-tim
e t
asks h
ave
sta
tic
prio
ritie
s
norm
al ta
sks h
ave
dyna
mic
prio
ritie
s
63
Case S
tud
ies:
Lin
ux S
ch
ed
ulin
g (
v2.5
)
Even m
ore
abo
ut L
inux s
ched
ulin
g
Aft
er
the t
ime s
lice o
f a
pro
ce
ss is u
se
d u
p,
the p
roce
ss m
ust w
ait u
ntil a
ll re
ad
y p
roce
sses t
o u
se u
p t
heir t
ime
slic
e (
or
be b
locke
d)
roun
d-r
obin
app
roa
ch
no s
tarv
ation p
roble
m
for
a u
ser
pro
cess,
its p
riority
ma
y +
or
5 d
epe
nd
ing w
heth
er
the p
roce
ss
is I
/O-
bou
nd
or
CP
U-b
ound
I/O
bo
un
d p
roce
ss is g
ive
n h
ighe
r p
rio
rity
64
Case S
tud
ies:
Lin
ux S
ch
ed
ulin
g (
v2.6
.23+
)
Co
mp
lete
ly F
air
Sch
ed
ule
r (C
FS
)
Sch
ed
ulin
g c
lasses,
each w
ith s
pecific
priori
ty
schedule
r pic
ks h
ighest priori
ty t
ask in h
ighest schedulin
g c
lass
rath
er
tha
n q
ua
ntu
m b
ase
d o
n fix
ed
tim
e a
llotm
en
ts, b
ase
d o
n p
rop
ort
ion
of C
PU
tim
e
two
sch
ed
ulin
g c
lasse
s in
clu
de
d (
de
fau
lt, re
al-
tim
e),
oth
ers
ca
n b
e a
dd
ed
Qu
an
tum
ca
lcu
late
d b
ase
d o
n n
ice
va
lue
fro
m -
20
to
+19
low
er
valu
e is h
igher
priority
ca
lcu
late
s t
arg
et
late
nc
y
inte
rva
l o
f tim
e d
urin
g w
hic
h ta
sk s
ho
uld
ru
n a
t le
ast o
nce
targ
et la
ten
cy c
an
in
cre
ase
if s
ay n
um
be
r o
f a
ctive
ta
sks in
cre
ases
CF
S s
ch
ed
ule
r m
ain
tain
s p
er
task v
irtu
al ru
n t
ime
in
va
ria
ble
vruntime
associa
ted w
ith d
ecay f
acto
r based o
n p
riority
of
task
low
er
priori
ty is h
igher
decay r
ate
no
rma
l d
efa
ult p
rio
rity
yie
lds v
irtu
al ru
n tim
e =
actu
al ru
n tim
e
task w
ith low
est
vir
tual ru
n t
ime r
uns n
ext
65
Case S
tud
ies:
Lin
ux S
ch
ed
ulin
g (
v2.6
.23+
)
66
Case S
tud
ies:
Su
mm
ary
Basic
id
ea f
or
sched
ule
user
pro
cesse
s is the
sam
e f
or
all
syste
ms
low
er
prio
rity
fo
r C
PU
bou
nd
pro
ce
ss
incre
ase
prio
rity
fo
r I/
O b
ou
nd
pro
ce
ss
The
sched
ulin
g in
Sola
ris / L
inux is m
ore
co
ncern
ed a
bo
ut
fair
ne
ss
mo
re p
op
ula
r a
s t
he O
Ses f
or
se
rvers
The
sched
ulin
g in
Win
do
w X
P is m
ore
co
ncern
ed a
bo
ut u
se
r p
erc
eiv
ed
pe
rfo
rma
nce
mo
re p
op
ula
r a
s t
he O
S fo
r p
ers
ona
l co
mpu
ters
