pptx

CSCI
-
1680

Network Programming II


Rodrigo Fonseca

Today

•
Network programming

–
Programming Paradigms

–
Programming libraries

•
Final project


Low
-
level Sockets

•
Address Family AF_PACKET

–
Socket type: SOCK_RAW

•
See link
-
layer (Ethernet) headers. Can send broadcast
on a LAN. Can get/create non
-
IP packets

–
Socket type: SOCK_DGRAM

•
See IP headers. Can get protocols other than
TCP/UDP: ICMP, SCTP, DCCP, your own…

•
Can cook your own IP packets

–
Must have root privileges to play with these


Building High Performance
Servers

The need for concurrency

•
How to improve throughput?

–
Decrease latency (throughput
α

1/latency)

–
Hard to do!

•
Optimize code (this you should try!)

•
Faster processor (no luck here, recently)

•
Speed of light isn’t changing anytime soon…

•
Disks have to deal with things like inertia!

–
Do multiple things at once

•
Concurrency

–
Allows overlapping of computation and I/O

–
Allows use of multiple cores, machines

High
-
performance Servers

Common Patterns

Read
Request
Find
File
Read File
Send Data
Get
Conn
Read
Request
Find
File
Read File
Send Data
Accept
Conn
Get
Conn
Send
Header
Process 1
Read
Request
Find
File
Read File
Send Data
Get
Conn
Read
Request
Find
File
Read File
Send Data
Accept
Conn
Get
Conn
Send
Header
Process N
Fi
g
u
r
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2
:
M
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t
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-
Pr
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-
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Send Data
Accept
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Header
Fi
g
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3
:
M
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3.
2
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(
MT
)
s
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F
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3
,
em
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m
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om
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.
g.
,
F
r
e
e
B
S
D
2.
2.
6)
pro
vi
de
onl
y
us
e
r
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l
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e
l
t
hre
a
d
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se
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2
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in
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n
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at
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l
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k
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t
h
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B
S
D
UNIX
select
or
t
he
S
ys
t
e
m
V
poll
is
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s
e
d
to
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h
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k
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ope
ra
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ons
t
ha
t
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om
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t
e
d.
F
i
gure
4
de
pi
c
t
s
th
e
S
P
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D
a
r
c
h
ite
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tu
r
e
.
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w
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r
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k
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onne
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om
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ha
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da
t
a
or
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v
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s
pa
c
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i
n
t
he
i
r
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e
nd
b
uf
f
e
r
s
.
)
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n
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w
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e
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t
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s
a
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ul
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t
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a
r
c
hi
t
e
c
t
ur
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s
a
r
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r
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p
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m
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o
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c
h
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s
d
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sk
o
p
e
r
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t
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o
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s.
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t
he
s
e
ope
ra
t
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ng
s
ys
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m
s
,
non-bl
oc
ki
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read
an
d
write
ope
ra
t
i
ons
w
ork
a
s
e
xpe
c
t
e
d
on
ne
t
w
ork
s
oc
k-
et
s
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d
p
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b
u
t
m
ay
act
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al
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y
b
l
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ck
w
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s
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o
n
d
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s
k
fil
e
s
.
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s
a
r
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s
u
l
t
,
s
u
p
p
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e
d
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y
n
o
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b
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k
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g
read
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ra
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tio
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s
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n
fi
le
s
m
a
y
s
till
b
lo
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k
th
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lle
r
w
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ile
d
is
k
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/O
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in
p
r
o
g
r
e
s
s
.
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o
th
o
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e
r
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s
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te
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s
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s
e
d
in
o
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r
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x
p
e
r
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me
n
t
s
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h
i
b
i
t
t
h
i
s
b
e
h
a
v
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o
r
(
F
r
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e
B
S
D
2
.2
.6
a
n
d
S
o
l
a
r
i
s
2.
6).
T
o
t
he
be
s
t
of
our
kno
w
l
e
dge
,
t
he
s
a
m
e
i
s
t
r
ue
for
mo
s
t
v
e
r
s
i
o
n
s
o
f
U
N
I
X
.
Ma
n
y
U
N
I
X
s
y
s
t
e
m
s
p
r
o
v
i
d
e
a
l
t
e
r
n
a
t
e
A
P
I
s
t
h
a
t
i
m
-
pl
e
m
e
nt
t
rue
a
s
ync
hronous
di
s
k
I/
O
,
b
ut
t
he
s
e
A
P
Is
a
r
e
ge
ne
r
a
l
l
y
not
i
nt
e
gr
a
t
e
d
w
i
t
h
t
he
select
ope
ra
t
i
on.
Th
i
s
m
a
k
e
s
i
t
d
i
f
fi
c
u
l
t
o
r
i
m
p
o
s
s
i
b
l
e
t
o
s
i
m
u
l
t
a
n
e
o
u
s
l
y
ch
eck
f
o
r
co
m
p
l
et
i
o
n
o
f
n
et
w
o
r
k
an
d
d
i
s
k
I
/
O
e
v
en
t
s
i
n
an
ef
fi
ci
en
t
m
an
n
er
.
M
o
r
eo
v
er
,
o
p
er
at
i
o
n
s
s
u
ch
as
open
an
d
stat
on
fi
l
e
de
s
c
r
i
pt
or
s
m
a
y
s
t
i
l
l
be
bl
oc
ki
ng.
Fo
r
t
h
e
s
e
r
e
a
s
o
n
s
,
e
x
i
s
t
i
n
g
S
P
E
D
s
e
r
v
e
r
s
d
o
n
o
t
u
s
e
th
e
s
e
s
p
e
c
ia
l
a
s
y
n
c
h
r
o
n
o
u
s
d
is
k
in
te
r
f
aces
.
A
s
a
r
es
u
l
t
,
fil
e
read
ope
r
a
t
i
ons
t
ha
t
do
not
hi
t
i
n
t
he
fi
l
e
c
a
c
he
m
a
y
cau
s
e
t
h
e
m
ai
n
s
er
v
er
t
h
r
ead
t
o
b
l
o
ck
,
cau
s
i
n
g
s
o
me
l
o
s
s
in
c
o
n
c
u
r
r
e
n
c
y
a
n
d
p
e
r
f
o
r
m
a
n
c
e
.
3.
4
A
symme
t
r
ic
M
u
lt
i-
Pr
oc
e
s
s
E
ve
n
t
-
D
r
i
ve
n
Th
e
A
s
y
m
m
e
t
r
i
c
M
u
l
t
i
-
P
r
o
c
e
s
s
Ev
e
n
t
-
D
r
i
v
e
n
(A
M
P
E
D
)
a
rc
h
i
t
e
c
t
u
re
,
i
l
l
u
s
t
ra
t
e
d
i
n
F
i
g
u
re
5
,
c
o
m
b
i
n
e
s
Read
Request
Find
File
Read File
Send Data
Get
Conn
Read
Request
Find
File
Read File
Send Data
Accept
Conn
Get
Conn
Send
Header
Process 1
Read
Request
Find
File
Read File
Send Data
Get
Conn
Read
Request
Find
File
Read File
Send Data
Accept
Conn
Get
Conn
Send
Header
Process N
Fi
g
u
r
e
2
:
M
u
l
t
i
-
Pr
o
c
e
s
s
-
I
n
t
h
e
M
P
m
o
d
e
l
,
each
s
er
v
er
pr
oc
e
s
s
ha
ndl
e
s
one
r
e
que
s
t
a
t
a
t
i
m
e
.
P
r
oc
e
s
s
e
s
e
x
e
c
ut
e
th
e
p
r
o
c
e
s
s
in
g
s
ta
g
e
s
s
e
q
u
e
n
tia
lly
.
Read
Request
Find
File
Read File
Send Data
Get
Conn
Read
Request
Find
File
Read File
Send Data
Accept
Conn
Get
Conn
Send
Header
Fi
g
u
r
e
3
:
M
u
l
t
i
-
T
h
r
e
a
d
e
d
-
T
h
e
M
T
m
o
d
e
l
u
s
e
s
a
s
i
n
g
l
e
ad
d
r
es
s
s
p
ace
w
i
t
h
m
u
l
t
i
p
l
e
co
n
cu
r
r
en
t
t
h
r
ead
s
o
f
e
x
ecu
-
tio
n
.
E
a
c
h
th
r
e
a
d
h
a
n
d
le
s
a
r
e
q
u
e
s
t.
Si
n
c
e
e
a
c
h
p
r
o
c
e
s
s
h
a
s
i
t
s
o
w
n
p
r
i
v
a
t
e
a
d
d
r
e
s
s
s
p
a
c
e
,
no
s
ync
hroni
z
a
t
i
on
i
s
ne
c
e
s
s
a
ry
t
o
ha
ndl
e
t
he
proc
e
s
s
i
ng
of
di
f
f
e
r
e
nt
H
T
T
P
re
que
s
t
s
2
.H
o
w
e
v
e
r
,
i
t
m
a
y
b
e
m
o
r
e
di
f
fi
c
ul
t
t
o
pe
r
f
or
m
opt
i
m
i
z
a
t
i
ons
i
n
t
hi
s
a
r
c
hi
t
e
c
t
ur
e
t
ha
t
re
l
y
o
n
g
l
o
b
a
l
i
n
fo
rm
a
t
i
o
n
,
s
u
c
h
a
s
a
s
h
a
re
d
c
a
c
h
e
o
f
va
l
i
d
U
R
L
s
.
F
i
g
u
r
e
2
i
l
l
u
s
t
r
a
t
e
s
t
h
e
M
P
a
r
c
h
i
t
e
c
t
u
r
e
.
3.
2
M
u
l
t
i
-
t
h
r
e
ad
e
d
Mu
l
t
i
-
t
h
r
e
a
d
e
d
(
MT
)
s
e
r
v
e
r
s
,
d
e
p
i
c
t
e
d
i
n
F
i
g
u
r
e
3
,
em
p
l
o
y
m
u
l
t
i
p
l
e
i
n
d
ep
en
d
en
t
t
h
r
ead
s
o
f
co
n
t
r
o
l
o
p
er
at
-
in
g
w
ith
in
a
s
in
g
le
s
h
a
r
e
d
a
d
d
r
e
s
s
s
p
a
c
e
.
E
a
c
h
th
r
e
a
d
pe
r
f
or
m
s
a
l
l
t
he
s
t
e
ps
a
s
s
oc
i
a
t
e
d
w
i
t
h
one
H
T
T
P
r
e
-
que
s
t
be
f
or
e
a
c
c
e
pt
i
ng
a
ne
w
r
e
que
s
t
,
s
i
m
i
l
a
r
t
o
t
he
M
P
mo
d
e
l
’
s
u
s
e
o
f
a
p
r
o
c
e
s
s
.
Th
e
p
r
i
mar
y
d
i
f
f
er
en
ce
b
et
w
een
t
h
e
M
P
an
d
t
h
e
M
T
ar
ch
i
t
ect
u
r
e,
h
o
w
e
v
er
,
i
s
t
h
at
al
l
t
h
r
ead
s
can
s
h
ar
e
g
l
o
b
al
va
r
i
a
b
l
e
s
.
T
h
e
u
s
e
o
f
a
s
i
n
g
l
e
s
h
a
r
e
d
a
d
d
r
e
s
s
s
p
a
c
e
l
e
n
d
s
its
e
lf
e
a
s
ily
to
o
p
tim
iz
a
tio
n
s
th
a
t
r
e
ly
o
n
s
h
a
r
e
d
s
ta
te
.
Ho
we
v
e
r
,
t
h
e
t
h
r
e
a
d
s
m
u
s
t
u
s
e
s
o
m
e
f
o
r
m
o
f
s
y
n
c
h
r
o
-
ni
z
a
t
i
on
t
o
c
ont
r
ol
a
c
c
e
s
s
t
o
t
he
s
ha
r
e
d
da
t
a
.
Th
e
M
T
m
o
d
e
l
r
e
q
u
i
r
e
s
t
h
a
t
t
h
e
o
p
e
r
a
t
i
n
g
s
y
s
t
e
m
pro
vi
de
s
s
upport
f
or
ke
rne
l
t
hre
a
ds
.
T
ha
t
i
s
,
w
he
n
one
th
r
e
a
d
b
lo
c
k
s
o
n
a
n
I
/O
o
p
e
r
a
tio
n
,
o
th
e
r
r
u
n
n
a
b
le
th
r
e
a
d
s
wi
t
h
i
n
t
h
e
s
a
m
e
a
d
d
r
e
s
s
s
p
a
c
e
m
u
s
t
r
e
m
a
i
n
e
l
i
g
i
b
l
e
for
e
x
e
c
ut
i
on.
S
om
e
ope
ra
t
i
ng
s
ys
t
e
m
s
(e
.
g.
,
F
r
e
e
B
S
D
2.
2.
6)
pro
vi
de
onl
y
us
e
r
-
l
e
v
e
l
t
hre
a
d
l
i
bra
ri
e
s
w
i
t
hout
ke
rne
l
s
upport
.
S
uc
h
s
ys
t
e
m
s
c
a
nnot
e
f
fe
c
t
i
v
e
l
y
s
upport
MT
se
r
v
e
r
s.
2
Sy
n
c
h
r
o
n
i
z
a
t
i
o
n
i
s
n
e
c
e
s
s
a
r
y
i
n
s
i
d
e
t
h
e
O
S
t
o
a
c
c
e
p
t
i
n
c
o
m
i
n
g
connect
i
ons,
s
i
n
ce
t
h
e
accept
queue
i
s
shar
ed
3.
3
S
in
gle
-
p
r
oc
e
s
s
e
ve
n
t
-
d
r
i
ve
n
Th
e
s
i
n
g
l
e-
p
r
o
ces
s
e
v
en
t
-
d
r
i
v
en
(
S
P
E
D
)
ar
ch
i
t
ect
u
r
e
us
e
s
a
s
i
ngl
e
e
v
e
nt
-dri
v
e
n
s
e
r
v
e
r
proc
e
s
s
t
o
pe
rform
co
n
cu
r
r
en
t
p
r
o
ces
s
i
n
g
o
f
m
u
l
t
i
p
l
e
H
T
T
P
r
eq
u
es
t
s
.
T
h
e
se
r
v
e
r
u
se
s
n
o
n
-
b
l
o
c
k
i
n
g
sy
st
e
m
s
c
a
l
l
s
t
o
p
e
r
f
o
r
m
a
sy
n
-
ch
r
o
n
o
u
s
I
/
O
o
p
er
at
i
o
n
s
.
A
n
o
p
er
at
i
o
n
l
i
k
e
t
h
e
B
S
D
UNIX
select
or
t
he
S
ys
t
e
m
V
poll
is
u
s
e
d
to
c
h
e
c
k
for
I
/
O
ope
ra
t
i
ons
t
ha
t
ha
v
e
c
om
pl
e
t
e
d.
F
i
gure
4
de
pi
c
t
s
th
e
S
P
E
D
a
r
c
h
ite
c
tu
r
e
.
A S
P
E
D
s
e
r
v
e
r
c
a
n
b
e
t
h
o
u
g
h
t
o
f
a
s
a
s
t
a
t
e
m
a
c
h
i
n
e
th
a
t
p
e
r
f
o
r
m
s
o
n
e
b
a
s
ic
s
te
p
a
s
s
o
c
ia
te
d
w
ith
th
e
s
e
r
v
in
g
of
a
n
H
T
T
P
re
que
s
t
a
t
a
t
i
m
e
,
t
hus
i
nt
e
rl
e
a
vi
ng
t
he
pro-
ces
s
i
n
g
s
t
ep
s
as
s
o
ci
at
ed
w
i
t
h
m
an
y
H
T
T
P
r
eq
u
es
t
s
.
I
n
each
i
t
er
at
i
o
n
,
t
h
e
s
er
v
er
p
er
f
o
r
m
s
a
select
to
c
h
e
c
k
for
c
om
pl
e
t
e
d
I/
O
e
v
e
nt
s
(
ne
w
c
onne
c
t
i
on
a
rri
v
a
l
s
,
c
om
-
pl
e
t
e
d
fi
l
e
ope
r
a
t
i
ons
,
c
l
i
e
nt
s
oc
ke
t
s
t
ha
t
ha
v
e
r
e
c
e
i
v
e
d
da
t
a
or
ha
v
e
s
pa
c
e
i
n
t
he
i
r
s
e
nd
b
uf
f
e
r
s
.
)
W
he
n
a
n
I
/
O
ev
e
n
t
i
s
r
e
a
d
y
,
i
t
c
o
m
p
l
e
t
e
s
t
h
e
c
o
r
r
e
s
p
o
n
d
i
n
g
b
a
s
i
c
s
t
e
p
an
d
i
n
i
t
i
at
es
t
h
e
n
e
x
t
s
t
ep
as
s
o
ci
at
ed
w
i
t
h
t
h
e
H
T
T
P
r
e-
que
s
t
,
i
f
a
ppropri
a
t
e
.
In
p
ri
n
c
i
p
l
e
,
a
S
P
E
D
s
e
rv
e
r
i
s
a
b
l
e
t
o
o
v
e
rl
a
p
t
h
e
CP
U
,
d
i
s
k
a
n
d
n
e
t
w
o
r
k
o
p
e
r
a
t
i
o
n
s
a
s
s
o
c
i
a
t
e
d
w
i
t
h
t
h
e
se
r
v
i
n
g
o
f
m
a
n
y
H
T
T
P
r
e
q
u
e
st
s,
i
n
t
h
e
c
o
n
t
e
x
t
o
f
a
si
n
-
gl
e
proc
e
s
s
a
nd
a
s
i
ngl
e
t
hre
a
d
of
c
ont
rol
.
A
s
a
r
e
s
ul
t
,
th
e
o
v
e
r
h
e
a
d
s
o
f
c
o
n
te
x
t
s
w
itc
h
in
g
a
n
d
th
r
e
a
d
s
y
n
c
h
r
o
-
ni
z
a
t
i
on
i
n
t
he
M
P
a
nd
M
T
a
r
c
hi
t
e
c
t
ur
e
s
a
r
e
a
v
oi
de
d.
Ho
we
v
e
r
,
a
p
r
o
b
l
e
m
a
s
s
o
c
i
a
t
e
d
wi
t
h
S
P
E
D
s
e
r
v
e
r
s
i
s
t
h
a
t
ma
n
y
c
u
r
r
e
n
t
o
p
e
r
a
t
i
n
g
s
y
s
t
e
ms
d
o
n
o
t
p
r
o
v
i
d
e
s
u
i
t
a
b
l
e
su
p
p
o
r
t
f
o
r
a
sy
n
c
h
r
o
n
o
u
s
d
i
sk
o
p
e
r
a
t
i
o
n
s.
In
t
he
s
e
ope
ra
t
i
ng
s
ys
t
e
m
s
,
non-bl
oc
ki
ng
read
an
d
write
ope
ra
t
i
ons
w
ork
a
s
e
xpe
c
t
e
d
on
ne
t
w
ork
s
oc
k-
et
s
an
d
p
i
p
es
,
b
u
t
m
ay
act
u
al
l
y
b
l
o
ck
w
h
en
u
s
ed
o
n
d
i
s
k
fil
e
s
.
A
s
a
r
e
s
u
l
t
,
s
u
p
p
o
s
e
d
l
y
n
o
n
-
b
l
o
c
k
i
n
g
read
ope
ra
-
tio
n
s
o
n
fi
le
s
m
a
y
s
till
b
lo
c
k
th
e
c
a
lle
r
w
h
ile
d
is
k
I
/O
is
in
p
r
o
g
r
e
s
s
.
B
o
th
o
p
e
r
a
tin
g
s
y
s
te
m
s
u
s
e
d
in
o
u
r
e
x
p
e
r
i-
me
n
t
s
e
x
h
i
b
i
t
t
h
i
s
b
e
h
a
v
i
o
r
(
F
r
e
e
B
S
D
2
.2
.6
a
n
d
S
o
l
a
r
i
s
2.
6).
T
o
t
he
be
s
t
of
our
kno
w
l
e
dge
,
t
he
s
a
m
e
i
s
t
r
ue
for
mo
s
t
v
e
r
s
i
o
n
s
o
f
U
N
I
X
.
Ma
n
y
U
N
I
X
s
y
s
t
e
m
s
p
r
o
v
i
d
e
a
l
t
e
r
n
a
t
e
A
P
I
s
t
h
a
t
i
m
-
pl
e
m
e
nt
t
rue
a
s
ync
hronous
di
s
k
I/
O
,
b
ut
t
he
s
e
A
P
Is
a
r
e
ge
ne
r
a
l
l
y
not
i
nt
e
gr
a
t
e
d
w
i
t
h
t
he
select
ope
ra
t
i
on.
Th
i
s
m
a
k
e
s
i
t
d
i
f
fi
c
u
l
t
o
r
i
m
p
o
s
s
i
b
l
e
t
o
s
i
m
u
l
t
a
n
e
o
u
s
l
y
ch
eck
f
o
r
co
m
p
l
et
i
o
n
o
f
n
et
w
o
r
k
an
d
d
i
s
k
I
/
O
e
v
en
t
s
i
n
an
ef
fi
ci
en
t
m
an
n
er
.
M
o
r
eo
v
er
,
o
p
er
at
i
o
n
s
s
u
ch
as
open
an
d
stat
on
fi
l
e
de
s
c
r
i
pt
or
s
m
a
y
s
t
i
l
l
be
bl
oc
ki
ng.
Fo
r
t
h
e
s
e
r
e
a
s
o
n
s
,
e
x
i
s
t
i
n
g
S
P
E
D
s
e
r
v
e
r
s
d
o
n
o
t
u
s
e
th
e
s
e
s
p
e
c
ia
l
a
s
y
n
c
h
r
o
n
o
u
s
d
is
k
in
te
r
f
aces
.
A
s
a
r
es
u
l
t
,
fil
e
read
ope
r
a
t
i
ons
t
ha
t
do
not
hi
t
i
n
t
he
fi
l
e
c
a
c
he
m
a
y
cau
s
e
t
h
e
m
ai
n
s
er
v
er
t
h
r
ead
t
o
b
l
o
ck
,
cau
s
i
n
g
s
o
me
l
o
s
s
in
c
o
n
c
u
r
r
e
n
c
y
a
n
d
p
e
r
f
o
r
m
a
n
c
e
.
3.
4
A
symme
t
r
ic
M
u
lt
i-
Pr
oc
e
s
s
E
ve
n
t
-
D
r
i
ve
n
Th
e
A
s
y
m
m
e
t
r
i
c
M
u
l
t
i
-
P
r
o
c
e
s
s
Ev
e
n
t
-
D
r
i
v
e
n
(A
M
P
E
D
)
a
rc
h
i
t
e
c
t
u
re
,
i
l
l
u
s
t
ra
t
e
d
i
n
F
i
g
u
re
5
,
c
o
m
b
i
n
e
s
Event Dispatcher
Read
Request
Read
Request
Find
File
Find
File
Get
Conn
Accept
Conn
Send Header
Read File
Send Data
Read File
Send Data
Send Header
Fi
g
u
r
e
4
:
Si
n
g
l
e
Pr
o
c
e
s
s
E
v
e
n
t
D
r
i
v
e
n
-
T
h
e
SPE
D
mo
d
e
l
u
s
e
s
a
s
i
n
g
l
e
p
r
o
c
e
s
s
t
o
p
e
r
f
o
r
m
a
l
l
c
l
i
e
n
t
p
r
o
c
e
s
s
-
in
g
a
n
d
d
is
k
a
c
ti
v
ity
in
a
n
e
v
e
n
t-
d
r
i
v
e
n
m
a
n
n
e
r
.
th
e
e
v
e
n
t-
d
r
i
v
e
n
a
p
p
r
o
a
c
h
o
f
th
e
S
P
E
D
a
r
c
h
ite
c
tu
r
e
wi
t
h
m
u
l
t
i
p
l
e
he
l
pe
r
proc
e
s
s
e
s
(
or
t
hre
a
ds
)
t
ha
t
ha
ndl
e
bl
oc
ki
ng
di
s
k
I/
O
ope
ra
t
i
ons
.
B
y
de
f
a
ul
t
,
t
he
m
a
i
n
ev
e
n
t
-
d
r
i
v
e
n
p
r
o
c
e
s
s
h
a
n
d
l
e
s
a
l
l
p
r
o
c
e
s
s
i
n
g
s
t
e
p
s
a
s
s
o
-
ci
at
ed
w
i
t
h
H
TTP
r
e
q
u
e
s
t
s
.
Wh
en
a
d
i
s
k
o
p
e
r
a
t
i
o
n
i
s
ne
c
e
s
s
a
r
y
(
e
.
g.
,
be
c
a
us
e
a
fi
l
e
i
s
r
e
que
s
t
e
d
t
ha
t
i
s
not
lik
e
ly
to
b
e
in
th
e
m
a
in
m
e
m
o
r
y
fi
le
c
a
c
h
e
)
,
th
e
m
a
in
se
r
v
e
r
p
r
o
c
e
ss
i
n
st
r
u
c
t
s
a
he
l
pe
r
vi
a
a
n
i
nt
e
r
-
pr
oc
e
s
s
co
m
m
u
n
i
cat
i
o
n
(
I
P
C
)
ch
an
n
el
(
e.
g
.
,
a
p
i
p
e)
t
o
p
er
f
o
r
m
th
e
p
o
te
n
tia
lly
b
lo
c
k
in
g
o
p
e
r
a
tio
n
.
O
n
c
e
th
e
o
p
e
r
a
tio
n
co
m
p
l
et
es
,
t
h
e
h
el
p
er
r
et
u
r
n
s
a
n
o
t
i
fi
cat
i
o
n
v
i
a
I
P
C
;
t
h
e
ma
i
n
s
e
r
v
e
r
p
r
o
c
e
s
s
l
e
a
r
n
s
o
f
t
h
i
s
e
v
e
n
t
l
i
k
e
a
n
y
o
t
h
e
r
I/
O
c
om
pl
e
t
i
on
e
v
e
nt
vi
a
select
.
Th
e
A
M
P
ED
ar
ch
i
t
ect
u
r
e
s
t
r
iv
es
t
o
p
r
es
er
v
e
t
h
e
ef
fi
-
ci
en
c
y
o
f
t
h
e
S
P
E
D
ar
ch
i
t
ect
u
r
e
o
n
o
p
er
at
i
o
n
s
o
t
h
er
t
h
an
di
s
k
re
a
ds
,
b
ut
a
v
oi
ds
t
he
pe
rform
a
nc
e
probl
e
m
s
s
uf
fe
re
d
by
S
P
E
D
due
t
o
i
na
ppropri
a
t
e
s
upport
f
or
a
s
ync
hronous
di
s
k
r
e
a
ds
i
n
m
a
n
y
ope
r
a
t
i
ng
s
ys
t
e
m
s
.
A
M
P
E
D
a
c
hi
e
v
e
s
th
is
u
s
in
g
o
n
ly
s
u
p
p
o
r
t
th
a
t
is
w
id
e
ly
a
v
a
ila
b
le
in
m
o
d
e
r
n
ope
ra
t
i
ng
s
ys
t
e
m
s
.
In
a
U
N
I
X
s
ys
t
e
m
,
A
M
P
E
D
us
e
s
t
he
s
t
a
nda
rd
non-
bl
oc
ki
ng
read
,
write
,a
n
d
accept
sy
st
e
m
c
a
l
l
s
o
n
so
c
k
e
t
s
a
n
d
p
i
p
e
s,
a
n
d
t
h
e
select
sy
st
e
m
c
a
l
l
t
o
t
e
st
f
o
r
I/
O
c
om
pl
e
t
i
on.
T
he
mmap
ope
r
a
t
i
on
i
s
us
e
d
t
o
a
c
c
e
s
s
da
t
a
from
t
he
fi
l
e
s
ys
t
e
m
a
nd
t
he
mincore
ope
ra
t
i
on
i
s
us
e
d
t
o
c
he
c
k
i
f
a
fi
l
e
i
s
i
n
m
a
i
n
m
e
m
or
y
.
No
t
e
t
h
a
t
t
h
e
h
e
l
p
e
r
s
c
a
n
b
e
i
m
p
l
e
m
e
n
t
e
d
e
i
t
h
e
r
a
s
ke
r
ne
l
t
hr
e
a
ds
w
i
t
hi
n
t
he
m
a
i
n
s
e
r
v
e
r
pr
oc
e
s
s
or
a
s
s
e
p-
ar
at
e
p
r
o
ces
s
es
.
Ev
en
w
h
en
h
el
p
er
s
ar
e
i
m
p
l
emen
t
ed
as
se
p
a
r
a
t
e
p
r
o
c
e
sse
s,
t
h
e
u
se
o
f
mmap
al
l
o
w
s
t
h
e
h
el
p
er
s
to
initia
te
the
r
e
a
d
ing
o
f
a
fi
l
e
from
d
is
k
w
ithout
intro-
duc
i
ng
a
ddi
t
i
ona
l
da
t
a
c
opyi
ng.
In
t
hi
s
c
a
s
e
,
bot
h
t
he
mai
n
s
er
v
er
p
r
o
ces
s
an
d
t
h
e
h
el
p
er
mmap
a r
e
q
u
e
s
t
e
d
fi
l
e
.
Th
e
h
e
l
p
e
r
t
o
u
c
h
e
s
a
l
l
t
h
e
p
a
g
e
s
i
n
i
t
s
m
e
m
o
r
y
m
a
p
p
i
n
g
.
On
c
e
fi
n
i
s
h
e
d
,
i
t
n
o
t
i
fi
e
s
t
h
e
m
a
i
n
s
e
r
v
e
r
p
r
o
c
e
s
s
t
h
a
t
i
t
i
s
no
w
s
a
f
e
t
o
t
ra
ns
m
i
t
t
he
fi
l
e
w
i
t
hout
t
he
r
i
s
k
of
bl
oc
ki
ng.
4
D
e
s
i
gn
c
o
m
p
ar
i
s
o
n
In
t
h
i
s
s
e
c
t
i
o
n
,
w
e
p
re
s
e
n
t
a
q
u
a
l
i
t
a
t
i
v
e
c
o
m
p
a
ri
s
o
n
of
t
he
pe
r
f
or
m
a
nc
e
c
ha
r
a
c
t
e
r
i
s
t
i
c
s
a
nd
pos
s
i
bl
e
opt
i
m
i
z
a
-
tio
n
s
in
th
e
v
a
r
io
u
s
W
e
b
s
e
r
v
e
r
a
r
c
h
ite
c
tu
r
e
s
p
r
e
s
e
n
te
d
in
th
e
p
r
e
v
io
u
s
s
e
c
tio
n
.
Event Dispatcher
Read
Request
Read
Request
Find
File
Find
File
Get
Conn
Accept
Conn
Send Header
Read File
Send Data
Read File
Send Data
Send Header
Helper 1
Helper 2
Helper k
Fi
g
u
r
e
5
:
A
s
y
m
m
e
t
r
i
c
M
u
l
t
i
-
Pr
o
c
e
s
s
E
v
e
n
t
D
r
i
v
e
n
-
T
h
e
AM
P
E
D
m
o
d
e
l
u
s
e
s
a
s
i
n
g
l
e
p
r
o
c
e
s
s
f
o
r
e
v
e
n
t
-
d
r
i
v
e
n
r
e
-
que
s
t
proc
e
s
s
i
ng,
b
ut
ha
s
ot
he
r
he
l
pe
r
proc
e
s
s
e
s
t
o
ha
n-
dl
e
s
om
e
di
s
k
ope
ra
t
i
ons
.
4
.
1
P
e
r
f
o
r
ma
n
c
e
c
h
a
r
a
c
t
e
r
i
s
t
i
c
s
Di
s
k
o
p
e
r
a
t
i
o
n
s
- T
h
e
c
o
s
t
o
f h
a
n
d
l
i
n
g
d
i
s
k
a
c
t
i
v
i
t
y
va
r
i
e
s
b
e
t
w
e
e
n
t
h
e
a
r
c
h
i
t
e
c
t
u
r
e
s
b
a
s
e
d
o
n
w
h
a
t
,
i
f
a
ny
,
ci
r
cu
m
s
t
an
ces
cau
s
e
al
l
r
eq
u
es
t
p
r
o
ces
s
i
n
g
t
o
s
t
o
p
w
h
i
l
e
a d
i
s
k
o
p
e
r
a
t
i
o
n
i
s
i
n
p
r
o
g
r
e
s
s
.
I
n
t
h
e M
P
a
n
d
M
T
m
o
d
-
el
s
,
o
n
l
y
t
h
e
p
r
o
ces
s
o
r
t
h
r
ead
t
h
at
cau
s
es
t
h
e
d
i
s
k
ac-
ti
v
ity
is
b
lo
c
k
e
d
.
I
n
A
M
P
E
D
,
th
e
h
e
lp
e
r
p
r
o
c
e
s
s
e
s
a
r
e
us
e
d
t
o
pe
r
f
or
m
t
he
bl
oc
ki
ng
di
s
k
a
c
t
i
ons
,
s
o
w
hi
l
e
t
he
y
ar
e
b
l
o
ck
ed
,
t
h
e
s
er
v
er
p
r
o
ces
s
i
s
s
t
i
l
l
a
v
ai
l
ab
l
e
t
o
h
an
-
dl
e
o
t
h
e
r
re
que
s
t
s
.
T
h
e
e
xt
ra
c
o
s
t
i
n
t
h
e
A
MP
E
D
m
ode
l
is
d
u
e
to
th
e
in
te
r
-
p
r
o
c
e
s
s
c
o
m
m
u
n
ic
a
tio
n
b
e
tw
e
e
n
th
e
se
r
v
e
r
a
n
d
t
h
e
h
e
l
p
e
r
s.
I
n
S
P
E
D
,
o
n
e
p
r
o
c
e
ss
h
a
n
d
l
e
s
a
l
l
cl
i
en
t
i
n
t
er
act
i
o
n
as
w
el
l
as
d
i
s
k
act
i
v
i
t
y
,
s
o
al
l
u
s
er
-
l
e
v
el
proc
e
s
s
i
ng
s
t
ops
w
he
ne
v
e
r
a
n
y
r
e
que
s
t
re
qui
re
s
di
s
k
a
c
-
ti
v
ity
.
Mem
o
ry
ef
f
ect
s
- T
h
e
s
e
r
v
e
r
’
s
m
e
m
o
r
y
c
o
n
s
u
m
p
t
i
o
n
af
f
ect
s
t
h
e
s
p
ace
a
v
ai
l
ab
l
e
f
o
r
t
h
e
fi
l
es
y
s
t
em
cach
e.
Th
e
S
P
E
D
ar
ch
i
t
ect
u
r
e
h
as
s
m
al
l
m
emo
r
y
r
eq
u
i
r
emen
t
s
,
si
n
c
e
i
t
h
a
s
o
n
l
y
o
n
e
p
r
o
c
e
ss
a
n
d
o
n
e
st
a
c
k
.
W
h
e
n
co
m
p
ar
ed
t
o
S
P
E
D
,
t
h
e
M
T
m
o
d
el
i
n
cu
r
s
s
o
m
e
ad
d
i
-
tio
n
a
l
m
e
m
o
r
y
c
o
n
s
u
m
p
tio
n
a
n
d
k
e
r
n
e
l
r
e
s
o
u
r
c
e
s
,
p
r
o
-
port
i
ona
l
t
o
t
he
num
be
r
of
t
hre
a
ds
e
m
pl
oye
d
(
i
.
e
.
,
t
he
ma
x
i
ma
l
n
u
mb
e
r
o
f
c
o
n
c
u
r
r
e
n
t
l
y
s
e
r
v
e
d
H
T
T
P
r
e
q
u
e
s
t
s
)
.
AM
P
E
D’
s
h
e
l
p
e
r
p
r
o
c
e
s
s
e
s
c
a
u
s
e
a
d
d
i
t
i
o
n
a
l
o
v
e
r
h
e
a
d
,
bu
t
t
h
e
h
e
l
p
e
r
s
h
a
ve
s
m
a
l
l
a
p
p
l
i
c
a
t
i
o
n
-
l
e
ve
l
m
e
m
o
r
y
d
e
-
ma
n
d
s
a
n
d
a
h
e
l
p
e
r
i
s
n
e
e
d
e
d
o
n
l
y
p
e
r
c
o
n
c
u
r
r
e
n
t
d
i
s
k
ope
r
a
t
i
on,
not
f
or
e
a
c
h
c
onc
ur
r
e
nt
l
y
s
e
r
v
e
d
H
T
T
P
r
e
-
que
s
t
.
T
he
MP
m
ode
l
i
nc
urs
t
he
c
o
s
t
of
a
s
e
p
a
ra
t
e
p
ro-
ces
s
p
er
co
n
cu
r
r
en
t
l
y
s
er
v
ed
H
T
T
P
r
eq
u
es
t
,
w
h
i
ch
h
as
su
b
st
a
n
t
i
a
l
m
e
m
o
r
y
a
n
d
k
e
r
n
e
l
o
v
e
r
h
e
a
d
s.
Di
s
k
u
t
i
l
i
z
a
t
i
o
n
- T
h
e
n
u
m
b
e
r o
f
c
o
n
c
u
r
r
e
n
t
d
i
s
k
r
e
-
que
s
t
s
t
ha
t
a
s
e
r
v
e
r
c
a
n
ge
ne
r
a
t
e
a
f
f
e
c
t
s
w
he
t
he
r
i
t
c
a
n
be
ne
fi
t
f
r
om
m
ul
t
i
pl
e
di
s
ks
a
nd
di
s
k
he
a
d
s
c
he
dul
i
ng.
Th
e
M
P
/
M
T
m
o
d
e
l
s
c
a
n
c
a
u
s
e
o
n
e
d
i
s
k
r
e
q
u
e
s
t
p
e
r
p
r
o
-
ces
s
/
t
h
r
ead
,
w
h
i
l
e
t
h
e
A
M
P
E
D
m
o
d
el
can
g
en
er
at
e
o
n
e
re
que
s
t
pe
r
he
l
pe
r
.
In
c
ont
ra
s
t
,
s
i
nc
e
a
l
l
us
e
r
-
l
e
v
e
l
pro-
ces
s
i
n
g
s
t
o
p
s
i
n
t
h
e
S
P
E
D
ar
ch
i
t
ect
u
r
e
w
h
en
e
v
er
i
t
ac-
ces
s
es
t
h
e
d
i
s
k
,
i
t
can
o
n
l
y
g
en
er
at
e
o
n
e
d
i
s
k
r
eq
u
es
t
at
a
Event Dispatcher
Read
Request
Read
Request
Find
File
Find
File
Get
Conn
Accept
Conn
Send Header
Read File
Send Data
Read File
Send Data
Send Header
Fi
g
u
r
e
4
:
Si
n
g
l
e
Pr
o
c
e
s
s
E
v
e
n
t
D
r
i
v
e
n
-
T
h
e
SPE
D
mo
d
e
l
u
s
e
s
a
s
i
n
g
l
e
p
r
o
c
e
s
s
t
o
p
e
r
f
o
r
m
a
l
l
c
l
i
e
n
t
p
r
o
c
e
s
s
-
in
g
a
n
d
d
is
k
a
c
ti
v
ity
in
a
n
e
v
e
n
t-
d
r
i
v
e
n
m
a
n
n
e
r
.
th
e
e
v
e
n
t-
d
r
i
v
e
n
a
p
p
r
o
a
c
h
o
f
th
e
S
P
E
D
a
r
c
h
ite
c
tu
r
e
wi
t
h
m
u
l
t
i
p
l
e
he
l
pe
r
proc
e
s
s
e
s
(
or
t
hre
a
ds
)
t
ha
t
ha
ndl
e
bl
oc
ki
ng
di
s
k
I/
O
ope
ra
t
i
ons
.
B
y
de
f
a
ul
t
,
t
he
m
a
i
n
ev
e
n
t
-
d
r
i
v
e
n
p
r
o
c
e
s
s
h
a
n
d
l
e
s
a
l
l
p
r
o
c
e
s
s
i
n
g
s
t
e
p
s
a
s
s
o
-
ci
at
ed
w
i
t
h
H
TTP
r
e
q
u
e
s
t
s
.
Wh
en
a
d
i
s
k
o
p
e
r
a
t
i
o
n
i
s
ne
c
e
s
s
a
r
y
(
e
.
g.
,
be
c
a
us
e
a
fi
l
e
i
s
r
e
que
s
t
e
d
t
ha
t
i
s
not
lik
e
ly
to
b
e
in
th
e
m
a
in
m
e
m
o
r
y
fi
le
c
a
c
h
e
)
,
th
e
m
a
in
se
r
v
e
r
p
r
o
c
e
ss
i
n
st
r
u
c
t
s
a
he
l
pe
r
vi
a
a
n
i
nt
e
r
-
pr
oc
e
s
s
co
m
m
u
n
i
cat
i
o
n
(
I
P
C
)
ch
an
n
el
(
e.
g
.
,
a
p
i
p
e)
t
o
p
er
f
o
r
m
th
e
p
o
te
n
tia
lly
b
lo
c
k
in
g
o
p
e
r
a
tio
n
.
O
n
c
e
th
e
o
p
e
r
a
tio
n
co
m
p
l
et
es
,
t
h
e
h
el
p
er
r
et
u
r
n
s
a
n
o
t
i
fi
cat
i
o
n
v
i
a
I
P
C
;
t
h
e
ma
i
n
s
e
r
v
e
r
p
r
o
c
e
s
s
l
e
a
r
n
s
o
f
t
h
i
s
e
v
e
n
t
l
i
k
e
a
n
y
o
t
h
e
r
I/
O
c
om
pl
e
t
i
on
e
v
e
nt
vi
a
select
.
Th
e
A
M
P
ED
ar
ch
i
t
ect
u
r
e
s
t
r
iv
es
t
o
p
r
es
er
v
e
t
h
e
ef
fi
-
ci
en
c
y
o
f
t
h
e
S
P
E
D
ar
ch
i
t
ect
u
r
e
o
n
o
p
er
at
i
o
n
s
o
t
h
er
t
h
an
di
s
k
re
a
ds
,
b
ut
a
v
oi
ds
t
he
pe
rform
a
nc
e
probl
e
m
s
s
uf
fe
re
d
by
S
P
E
D
due
t
o
i
na
ppropri
a
t
e
s
upport
f
or
a
s
ync
hronous
di
s
k
r
e
a
ds
i
n
m
a
n
y
ope
r
a
t
i
ng
s
ys
t
e
m
s
.
A
M
P
E
D
a
c
hi
e
v
e
s
th
is
u
s
in
g
o
n
ly
s
u
p
p
o
r
t
th
a
t
is
w
id
e
ly
a
v
a
ila
b
le
in
m
o
d
e
r
n
ope
ra
t
i
ng
s
ys
t
e
m
s
.
In
a
U
N
I
X
s
ys
t
e
m
,
A
M
P
E
D
us
e
s
t
he
s
t
a
nda
rd
non-
bl
oc
ki
ng
read
,
write
,a
n
d
accept
sy
st
e
m
c
a
l
l
s
o
n
so
c
k
e
t
s
a
n
d
p
i
p
e
s,
a
n
d
t
h
e
select
sy
st
e
m
c
a
l
l
t
o
t
e
st
f
o
r
I/
O
c
om
pl
e
t
i
on.
T
he
mmap
ope
r
a
t
i
on
i
s
us
e
d
t
o
a
c
c
e
s
s
da
t
a
from
t
he
fi
l
e
s
ys
t
e
m
a
nd
t
he
mincore
ope
ra
t
i
on
i
s
us
e
d
t
o
c
he
c
k
i
f
a
fi
l
e
i
s
i
n
m
a
i
n
m
e
m
or
y
.
No
t
e
t
h
a
t
t
h
e
h
e
l
p
e
r
s
c
a
n
b
e
i
m
p
l
e
m
e
n
t
e
d
e
i
t
h
e
r
a
s
ke
r
ne
l
t
hr
e
a
ds
w
i
t
hi
n
t
he
m
a
i
n
s
e
r
v
e
r
pr
oc
e
s
s
or
a
s
s
e
p-
ar
at
e
p
r
o
ces
s
es
.
Ev
en
w
h
en
h
el
p
er
s
ar
e
i
m
p
l
emen
t
ed
as
se
p
a
r
a
t
e
p
r
o
c
e
sse
s,
t
h
e
u
se
o
f
mmap
al
l
o
w
s
t
h
e
h
el
p
er
s
to
initia
te
the
r
e
a
d
ing
o
f
a
fi
l
e
from
d
is
k
w
ithout
intro-
duc
i
ng
a
ddi
t
i
ona
l
da
t
a
c
opyi
ng.
In
t
hi
s
c
a
s
e
,
bot
h
t
he
mai
n
s
er
v
er
p
r
o
ces
s
an
d
t
h
e
h
el
p
er
mmap
a r
e
q
u
e
s
t
e
d
fi
l
e
.
Th
e
h
e
l
p
e
r
t
o
u
c
h
e
s
a
l
l
t
h
e
p
a
g
e
s
i
n
i
t
s
m
e
m
o
r
y
m
a
p
p
i
n
g
.
On
c
e
fi
n
i
s
h
e
d
,
i
t
n
o
t
i
fi
e
s
t
h
e
m
a
i
n
s
e
r
v
e
r
p
r
o
c
e
s
s
t
h
a
t
i
t
i
s
no
w
s
a
f
e
t
o
t
ra
ns
m
i
t
t
he
fi
l
e
w
i
t
hout
t
he
r
i
s
k
of
bl
oc
ki
ng.
4
D
e
s
i
gn
c
o
m
p
ar
i
s
o
n
In
t
h
i
s
s
e
c
t
i
o
n
,
w
e
p
re
s
e
n
t
a
q
u
a
l
i
t
a
t
i
v
e
c
o
m
p
a
ri
s
o
n
of
t
he
pe
r
f
or
m
a
nc
e
c
ha
r
a
c
t
e
r
i
s
t
i
c
s
a
nd
pos
s
i
bl
e
opt
i
m
i
z
a
-
tio
n
s
in
th
e
v
a
r
io
u
s
W
e
b
s
e
r
v
e
r
a
r
c
h
ite
c
tu
r
e
s
p
r
e
s
e
n
te
d
in
th
e
p
r
e
v
io
u
s
s
e
c
tio
n
.
Event Dispatcher
Read
Request
Read
Request
Find
File
Find
File
Get
Conn
Accept
Conn
Send Header
Read File
Send Data
Read File
Send Data
Send Header
Helper 1
Helper 2
Helper k
Fi
g
u
r
e
5
:
A
s
y
m
m
e
t
r
i
c
M
u
l
t
i
-
Pr
o
c
e
s
s
E
v
e
n
t
D
r
i
v
e
n
-
T
h
e
AM
P
E
D
m
o
d
e
l
u
s
e
s
a
s
i
n
g
l
e
p
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s
s
f
o
r
e
v
e
n
t
-
d
r
i
v
e
n
r
e
-
que
s
t
proc
e
s
s
i
ng,
b
ut
ha
s
ot
he
r
he
l
pe
r
proc
e
s
s
e
s
t
o
ha
n-
dl
e
s
om
e
di
s
k
ope
ra
t
i
ons
.
4
.
1
P
e
r
f
o
r
ma
n
c
e
c
h
a
r
a
c
t
e
r
i
s
t
i
c
s
Di
s
k
o
p
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r
a
t
i
o
n
s
- T
h
e
c
o
s
t
o
f h
a
n
d
l
i
n
g
d
i
s
k
a
c
t
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t
y
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r
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n
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a
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i
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s
b
a
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e
d
o
n
w
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a
t
,
i
f
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ny
,
ci
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cu
m
s
t
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s
e
al
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s
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o
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w
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a d
i
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o
p
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r
a
t
i
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n
i
s
i
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o
g
r
e
s
s
.
I
n
t
h
e M
P
a
n
d
M
T
m
o
d
-
el
s
,
o
n
l
y
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e
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ead
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i
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ity
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d
.
I
n
A
M
P
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D
,
th
e
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e
lp
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r
p
r
o
c
e
s
s
e
s
a
r
e
us
e
d
t
o
pe
r
f
or
m
t
he
bl
oc
ki
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di
s
k
a
c
t
i
ons
,
s
o
w
hi
l
e
t
he
y
ar
e
b
l
o
ck
ed
,
t
h
e
s
er
v
er
p
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ces
s
i
s
s
t
i
l
l
a
v
ai
l
ab
l
e
t
o
h
an
-
dl
e
o
t
h
e
r
re
que
s
t
s
.
T
h
e
e
xt
ra
c
o
s
t
i
n
t
h
e
A
MP
E
D
m
ode
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is
d
u
e
to
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in
te
r
-
p
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o
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e
s
s
c
o
m
m
u
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ic
a
tio
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b
e
tw
e
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n
th
e
se
r
v
e
r
a
n
d
t
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e
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e
l
p
e
r
s.
I
n
S
P
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D
,
o
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e
p
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o
c
e
ss
h
a
n
d
l
e
s
a
l
l
cl
i
en
t
i
n
t
er
act
i
o
n
as
w
el
l
as
d
i
s
k
act
i
v
i
t
y
,
s
o
al
l
u
s
er
-
l
e
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el
proc
e
s
s
i
ng
s
t
ops
w
he
ne
v
e
r
a
n
y
r
e
que
s
t
re
qui
re
s
di
s
k
a
c
-
ti
v
ity
.
Mem
o
ry
ef
f
ect
s
- T
h
e
s
e
r
v
e
r
’
s
m
e
m
o
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y
c
o
n
s
u
m
p
t
i
o
n
af
f
ect
s
t
h
e
s
p
ace
a
v
ai
l
ab
l
e
f
o
r
t
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e
fi
l
es
y
s
t
em
cach
e.
Th
e
S
P
E
D
ar
ch
i
t
ect
u
r
e
h
as
s
m
al
l
m
emo
r
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r
eq
u
i
r
emen
t
s
,
si
n
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e
i
t
h
a
s
o
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l
y
o
n
e
p
r
o
c
e
ss
a
n
d
o
n
e
st
a
c
k
.
W
h
e
n
co
m
p
ar
ed
t
o
S
P
E
D
,
t
h
e
M
T
m
o
d
el
i
n
cu
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s
s
o
m
e
ad
d
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tio
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a
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e
m
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p
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d
k
e
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e
l
r
e
s
o
u
r
c
e
s
,
p
r
o
-
port
i
ona
l
t
o
t
he
num
be
r
of
t
hre
a
ds
e
m
pl
oye
d
(
i
.
e
.
,
t
he
ma
x
i
ma
l
n
u
mb
e
r
o
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r
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n
t
l
y
s
e
r
v
e
d
H
T
T
P
r
e
q
u
e
s
t
s
)
.
AM
P
E
D’
s
h
e
l
p
e
r
p
r
o
c
e
s
s
e
s
c
a
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d
d
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d
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t
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r
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s
m
a
l
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a
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e
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e
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ma
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s
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d
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e
l
p
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d
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e
n
t
d
i
s
k
ope
r
a
t
i
on,
not
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or
e
a
c
h
c
onc
ur
r
e
nt
l
y
s
e
r
v
e
d
H
T
T
P
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e
-
que
s
t
.
T
he
MP
m
ode
l
i
nc
urs
t
he
c
o
s
t
of
a
s
e
p
a
ra
t
e
p
ro-
ces
s
p
er
co
n
cu
r
r
en
t
l
y
s
er
v
ed
H
T
T
P
r
eq
u
es
t
,
w
h
i
ch
h
as
su
b
st
a
n
t
i
a
l
m
e
m
o
r
y
a
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d
k
e
r
n
e
l
o
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e
r
h
e
a
d
s.
Di
s
k
u
t
i
l
i
z
a
t
i
o
n
- T
h
e
n
u
m
b
e
r o
f
c
o
n
c
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r
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e
n
t
d
i
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que
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ne
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w
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t
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a
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be
ne
fi
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f
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om
m
ul
t
i
pl
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s
ks
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s
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a
d
s
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dul
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ng.
Th
e
M
P
/
M
T
m
o
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e
l
s
c
a
n
c
a
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e
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-
ces
s
/
t
h
r
ead
,
w
h
i
l
e
t
h
e
A
M
P
E
D
m
o
d
el
can
g
en
er
at
e
o
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e
re
que
s
t
pe
r
he
l
pe
r
.
In
c
ont
ra
s
t
,
s
i
nc
e
a
l
l
us
e
r
-
l
e
v
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pro-
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s
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o
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i
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t
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e
S
P
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D
ar
ch
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t
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u
r
e
w
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e
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er
i
t
ac-
ces
s
es
t
h
e
d
i
s
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,
i
t
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o
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l
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er
at
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o
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d
i
s
k
r
eq
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es
t
at
a
Figures from
Pai
, et al., 1999 “Flash: An efficient and portable Web server”

Multiple processes

Multiple Threads

Single Process Event Driven

Single Process Event Driven with Helpers

Threads

•
Usual model for achieving concurrency

•
Uniform abstraction for single and multiple
cores

•
Concurrency with locks/
mutexes

–
Threads may block, hold locks for long time

•
Easy to reason about

–
Each thread has own stack

•
Strong support from OS, libraries,
debuggers

•
Traditionally, problems with more than a few
100 threads

–
Memory overhead,
O(n
) operations

Performance, Thread
-
based
server

From Welsh, et al., SOSP 2001 “SEDA: An Architecture for Well
-
Conditioned, Scalable
Internet Services

Events

•
Small number of threads, one per CPU

•
Threads do one thing:

while(1) {

get event from queue

Handle event to completion

}

•
Events are network, I/O readiness and
completion, timers, signals

–
Remember select()?

•
Assume event handlers never block

–
Helper threads handle blocking calls, like disk
I/O

Events

•
Many works in the early 2000’s claimed
that events are needed for high
performance servers

–
E.g., Flash,
thttpd
, Zeus, JAWS web servers

•
Indeed, many of today’s fastest servers
are

event
-
driven

–
E.g.,
OKCupid
,
lighttpd
,
nginx
, tornado



Lighttpd
: “Its event
-
driven architecture is optimized for a large number of
parallel connections”



Performance, Event
-
Driven Web
server

From Welsh, et al., SOSP 2001 “SEDA: An Architecture for Well
-
Conditioned, Scalable
Internet Services

Flash Web Server

•
Pai
,
Drushel
,
Zwaenepoel
, 1999

•
Influential work

•
Compared four architectures

–
Multi
-
process servers

–
Multi
-
threaded servers

–
Single
-
process event
-
driven

–
Asymmetric Multi
-
process event driven

•
AMPED was the fastest

Events (cont)

•
Highly efficient code

–
Little or no switching overhead

–
Easy concurrency control

•
Common complaint: hard to program and
reason about

–
For people and tools

•
Main reason:
stack ripping

Events criticism: control flow

Accept

Conn.

Write

Response

Read

File

Read

Request

Pin

Cache

Web Server

Exit

Threads

Events

thread_main(int sock) {


struct session s;


accept_conn(sock, &s);


read_request(&s);


pin_cache(&s);


write_response(&s);


unpin(&s);

}


pin_cache(struct session *s) {


pin(&s);


if( !in_cache(&s) )


read_file(&s);

}

CacheHandler(struct session *s) {


pin(s);


if( !in_cache(s) ) ReadFileHandler.enqueue(s);


else ResponseHandler.enqueue(s);

}

RequestHandler(struct session *s) {


…; CacheHandler.enqueue(s);

}

. . .

ExitHandlerr(struct session *s) {


…; unpin(&s); free_session(s);

}

AcceptHandler(event e) {


struct session *s = new_session(e);


RequestHandler.enqueue(s); }

•
Events obscure control flow

–
For programmers
and
tools

Events criticism: Exceptions

•
Exceptions complicate control flow

–
Harder to understand program flow

–
Cause bugs in cleanup code

Accept

Conn.

Write

Response

Read

File

Read

Request

Pin

Cache

Web Server

Exit

Threads

Events

thread_main(int sock) {


struct session s;


accept_conn(sock, &s);


if( !read_request(&s) )


return;


pin_cache(&s);


write_response(&s);


unpin(&s);

}


pin_cache(struct session *s) {


pin(&s);


if( !in_cache(&s) )


read_file(&s);

}

CacheHandler(struct session *s) {


pin(s);


if( !in_cache(s) ) ReadFileHandler.enqueue(s);


else ResponseHandler.enqueue(s);

}

RequestHandler(struct session *s) {


…; if( error )

return;

CacheHandler.enqueue(s);

}

. . .

ExitHandlerr(struct session *s) {


…;
unpin(&s); free_session(s);


}

AcceptHandler(event e) {


struct session *s = new_session(e);


RequestHandler.enqueue(s); }

Events criticism: State
Management

Threads

Events

thread_main(int sock) {


struct session s;


accept_conn(sock, &s);


if( !read_request(&s) )


return;


pin_cache(&s);


write_response(&s);


unpin(&s);

}


pin_cache(struct session *s) {


pin(&s);


if( !in_cache(&s) )


read_file(&s);

}

CacheHandler(struct session *s) {


pin(s);


if( !in_cache(s) ) ReadFileHandler.enqueue(s);


else ResponseHandler.enqueue(s);

}

RequestHandler(struct session *s) {


…; if( error )

return; CacheHandler.enqueue(s);

}

. . .

ExitHandlerr(struct session *s) {


…; unpin(&s);

free_session(s);


}

AcceptHandler(event e) {


struct session *s = new_session(e);


RequestHandler.enqueue(s); }

Accept

Conn.

Write

Response

Read

File

Read

Request

Pin

Cache

Web Server

Exit

•
Events require manual state management

•
Hard to know when to free

–
Use GC or risk bugs

Usual Arguments

•
Events:

–
Hard to program (stack ripping)

–
Easy to deal with concurrency (cooperative task
management)

•
Shared state is more explicit

–
High performance (low overhead, no switching, no
blocking)

•
Threads

–
Easy to reason about flow, state (automatic stack
management)

–
Hard to deal with concurrency (preemptive task
management)

•
Everything is shared

–
Lower performance (thread switching cost, memory
overhead)


Capriccio (2003)

•
Showed threads can
perform as well as
events

–
Avoid
O(n
) operations

–
Cooperative lightweight
user
-
level threads

•
(still one kernel thread
per core)

–
Asynchronous I/O

•
Handled by the library

–
Variable
-
length stacks

–
The thread library runs
an event
-
based system
underneath!


Requests / Second
Concurrent Tasks
Event-Based Server
Threaded Server
20000
30000
40000
50000
60000
70000
80000
90000
100000
110000
1
10
100
1000
10000
100000
1e+06
Artificial Dichotomy!

•
Old debate! Lauer and Needham, 78

–
Duality between process
-
based and message
-
passing

–
Updated by the Capriccio folks, 2003






•
Performance should be similar

–
No inherent reason for threads to be worse

–
Implementation is key

Threads

Events



Monitors



Exported functions



Call/return and fork/join



Wait on condition variable



Event handler & queue



Events accepted



Send message / await reply



Wait for new messages

Artificial Dichotomy


•
Threads

–
Preemptive multitasking

–
Automatic stack management

•
Events

–
Cooperative multitasking

–
Manual stack management (stack ripping)

•
Adya
, 2002: you can choose your
features!

–
They show that you can have cooperative
multitasking with automatic stack
managment

Adya
, A. et al., 2002. “Cooperative Task Management without Manual Stack
Managementor
, Event
-
driven Programming is Not the Opposite of Threaded
Programming

Threads vs. Events

•
Today you still have to mostly choose
either style (complete packages)

–
Thread
-
based servers very dependent on OS,
threading libraries

•
Some promising directions!

–
TAME allows you to write sequential C++ code
(with some annotations), converts it into event
-
based

–
Scala

(
oo
/functional language that runs on the
JVM) makes threaded and event
-
based code
look almost identical

Popular Event
-
Based
Frameworks

•
libevent

•
libasync

(SFS, SFS
-
light)

•
Javascript

–
All browser code

–
Node.js

at the server side

•
GUI programming

Some available libraries

With material from Igor
Ganichev

Python

•
Rich standard library

–
url/http/ftp/pop/imap/smtp/telnet

–
SocketServer
,
HTTPServer
,
DocXMLRPCServer
,
etc

•
Twisted

–
Very popular

–
Has
a lot

of stuff, but quite modular

–
Event
-
driven, many design patterns. Steep
learning curve…

–
Well maintained and documented

Java

•
Mature RPC library: RMI

•
River: RMI + service discovery, mobile
code

•
Java.NIO

–
High
-
level wrapping of OS primitives

•
Select
-
> Selector . Socket
-
> Channel

–
Good, efficient buffer abstraction

•
Jetty

–
Extensible, event
-
driven framework

–
High
-
performance

–
Avoid unnecessary copies

–
Other side doesn’t have to be in Java


C

•
Sockets!

•
Direct access to what the OS provides

•
Libevent

–
Simple, somewhat portable abstraction of
select() with uniform access to events: I/O,
timers, signals

–
Supports /dev/poll, kqueue(2), event ports,
select(2), poll(2) and epoll(4).

–
Well maintained, actively developed

–
Behind many very high
-
performance servers

•
Memcached

C++

•
Boost.ASIO

–
Clean, lightweight, portable abstraction of
sockets and other features

–
Not a lot of higher
-
level protocol support

–
Has support for both synchronous and
asynchronous operations, threads (from other
parts of Boost)

•
Others: ACE, POCO

ICE

•
Cross
-
language middleware + framework

–
Think twisted + protocol buffers

•
Open source but owned by a company

•
SSL, sync/
async
, threads, resource
allocation, firewall traversal, event
distribution, fault tolerance

•
Supports many languages

–
C++, Java, .NET
-
languages (such as C# or
Visual Basic), Objective
-
C, Python, PHP, and
Ruby

Other “cool” approaches

•
Erlang
,
Scala
, Objective C

–
Support the Actor model: program is a bunch of
actors sending messages to each other

–
Naturally extends to multi
-
core and multiple
machines, as sending messages is the same

•
Go

–
Built for concurrency, uses ‘
Goroutines
’, no
shared state

–
“Don’t share memory to communicate,
communicate to share memory”

Node.js

•
Javascript

server framework

•
Leverages highly efficient Chrome V8
Javascript

JIT runtime

•
Completely event
-
based

•
Many high
-
level libraries

var

http =
require('http
');

http.createServer(function

(
req
, res) {


res.writeHead(200, {'Content
-
Type': 'text/plain'});


res.end('Hello

World
\
n');

}).listen(8124, "127.0.0.1");

console.log('Server

running at http://127.0.0.1:8124/');

Final Assignment

Final Project

•
Tethering IP over 3G

•
Problem: Laptop in need of internet, no Wi
-
Fi
available.

•
On hand: Smartphone with 3G connection.

•
Native applications don’t always allow
custom network programming.

–
iOS

App Store guidelines.


Custom Tethering Solution

•
Websockets

to the rescue!

–
Implemented in browsers.

–
Bi
-
directional, full
-
duplex connection over a
single TCP socket.

–
Modern smartphone browsers have implemented
websockets
.

Implementation

Some questions

•
How to connect phone to laptop?

•
How to encode data?

•
Virtual interfaces: TUN or TAP?

•
Client: setting up routes

•
Server: what to do with the packets you
receive?



Some Resources

•
TUN/TAP Interfaces

–
TunTap

package for Mac OSX

•
Websocket

Server

–
Twisted

•
NAT

–
Scapy

•
Base64 Encoding

–
http://www.ietf.org/rfc/rfc3548.txt