1. pets

CS-206 Concurrency
Lecture 4
Mutual
Exclusion
A
B
cola
Spring 2015
Prof. Babak Falsafi
parsa.epfl.ch/courses/cs206/
Adapted from slides originally developed by Maurice Herlihy and Nir
Shavit from the Art of Multiprocessor Programming, and Babak Falsafi
EPFL Copyright 2015
EPFL CS-206 – Spring 2015
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T
F
6-Mar
u  Mutual
Exclusion in
practice
w  How
do we share?
w  How do we share well?
w Protocols
w Properties
u  Next
week
w  Mutual
Exclusion 2
w Formal definiton
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Recall: Counter in Shared Memory
code
Local
variables
temp = value;
value = temp + 1;
cache
cache
Bus
1
cache
Bus
shared
memory
Need Mutual Exclusion
EPFL CS-206 – Spring 2015
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Sharing requires coordination
u  Illustrated
u  Our
through a fable
Fable
w  Alice
& Bob have pets: a dog and a cat
w  The pets like to play in a pond
w  But, they don’t get a long
w  So, need a protocol to share the pond
w Agree on guidelines as to how to share
w  The protocol would guarantee mutual exclusion
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Mutual Exclusion,
or “Alice & Bob share a pond”
A
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B
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Alice has a pet
A
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B
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Bob has a pet
A
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B
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The Problem
A
B
The pets don’t
get along
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Formalizing the Problem
u  Two
types of formal properties in asynchronous
computation:
u  Safety Properties
w  Nothing
u  Liveness
bad happens ever
Properties
w  Something
EPFL CS-206 – Spring 2015
good happens eventually
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Formalizing our Problem
u  Mutual
Exclusion
w  Both
pets never in pond simultaneously
w  This is a safety property
u  No
Deadlock
w  if
only one wants in, it gets in
w  if both want in, one gets in
w  This is a liveness property
EPFL CS-206 – Spring 2015
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Simple Protocol
u  Idea
w  Just
look at the pond
u  Gotcha
(means “potential problems”)
w  Not
atomic
w  Trees obscure the view
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Interpretation
u  Threads
can’t “see” what other threads are doing
u  Explicit communication required for coordination
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Cell Phone Protocol
u  Idea
w  Alice
calls Bob (or vice-versa)
u  Gotcha
w  Bob
takes shower
w  Alice may need to recharge battery
w  Bob out of cell phone tower reach…
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Interpretation
u  Message-passing
doesn’t work
u  Recipient might not be
w  Listening
w  There
at all
u  Communication
must be
w  Persistent
(like writing)
w  Not transient (like speaking)
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Alice comes up with a clever solution
u  Beer/Cola
cans!
w  Used
to send a signal
w  Sort of like conveying a “ready” bit
w  Fallen can = ready, upright can = not ready
u  One
or more beer cans by Alice’s window (on windowsill)
u  Each with string running to Bob’s house
u  To send a signal, Bob pulls string
u  …and knock the can over
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EPFL CS-206 – Spring 2015
cola
cola
Can Protocol
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Bob conveys a bit
B
cola
A
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Bob conveys a bit
A
B
cola
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Can Protocol
u  Idea
w  Cans
by Alice’s window
w  Strings lead to Bob’s house
w  Bob pulls strings, knocks over cans
u  Gotcha
w  Cans
cannot be reused
w  Bob runs out of cans
w  What if Alice takes a long time to reset (goes on vacation)?
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Interpretation
u  Sending
a signal (interrupt to set a bit)
w  Does
not solve mutual exclusion
w  Sender sets fixed bit in receiver’s space
w  Receiver resets bit when ready
w  Requires unbounded number of interrupt bits
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Flag Protocol
A
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B
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Alice’s Protocol (sort of)
A
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B
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Bob’s Protocol (sort of)
A
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B
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Alice’s Protocol
u  Raise
flag
u  Wait until Bob’s flag is down
u  Unleash pet
u  Lower flag when pet returns
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Bob’s Protocol
u  Raise
flag
u  Wait until Alice’s flag is down
u  Unleash pet
u  Lower flag when pet returns
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Bob’s Protocol (2nd try)
u  Raise
flag
u  While Alice’s flag is up
w  Lower
flag
w  Wait for Alice’s flag to go down
w  Raise flag
u  Unleash
pet
u  Lower flag when pet returns
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Bob’s Protocol
u  Raise
flag
u  While Alice’s flag is up
Bob defers to
Alice
w  Lower
flag
w  Wait for Alice’s flag to go down
w  Raise flag
u  Unleash
pet
u  Lower flag when pet returns
EPFL CS-206 – Spring 2015
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The Flag Principle
u  Raise
the flag
u  Look at other’s flag
u  Flag Principle:
w  If
each raises and looks, then
w  Last to look must see both flags up
EPFL CS-206 – Spring 2015
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Proof of Mutual Exclusion
u  Assume
both pets in pond
w  Derive
a contradiction
w  By reasoning backwards
u  Consider
the last time Alice and Bob each looked before
letting the pets in
u  Without loss of generality assume Alice was the last to
look…
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Proof
Bob last raised flag
Alice last raised her flag
Alice’s last look
Bob’s last look
time
Alice must have seen Bob’s Flag. A Contradiction
EPFL CS-206 – Spring 2015
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Proof of No Deadlock
u  If
only one pet wants in, it gets in
u  Deadlock requires both continually trying to get in
u  If Bob sees Alice’s flag, he gives her priority (a gentleman…)
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Remarks
u  Protocol
w  Bob’s
u  Protocol
w  If
is unfair
pet might never get in
uses waiting
Bob is eaten by his pet, Alice’s pet might never get in
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Moral of Story
u Mutual
Exclusion cannot be solved by
w transient
communication (cell phones)
w interrupts (cans)
u It
can be solved by
w  one-bit
shared variables
w  that can be read or written
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The Fable Continues
u  Alice
and Bob fall in love & marry
u  Then they fall out of love & divorce
w  She
gets the pets
w  He has to feed them
u  Leading
to a new coordination problem:
w  Producer-Consumer
w  Used
everywhere in parallel computing/distributed systems
w  E.g., network buffers (shared between processor & network)
w  E.g., OS scheduler (queues holding active threads)
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Bob Puts Food in the Pond
A
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Alice releases her pets to Feed
mmm…
mmm…
EPFL CS-206 – Spring 2015
B
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Producer/Consumer
u  Alice
and Bob can’t meet
w  Each
has restraining order (by court) on other
w  So he puts food in the pond
w  And later, she releases the pets
u  Avoid
w  Releasing
pets when there’s no food
w  Putting out food if uneaten food remains
EPFL CS-206 – Spring 2015
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Producer/Consumer
u  Need
a mechanism so that
w  Bob
lets Alice know when food has been put out
w  Alice lets Bob know when to put out more food
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Surprise Solution
B
cola
A
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Bob puts food in Pond
B
cola
A
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Bob knocks over Can
A
B
cola
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Alice Releases Pets
A
yum…
yum…
B
cola
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Alice Resets Can when Pets are Fed
B
cola
A
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Pseudocode
while (true) {
while (can.isUp()){};
pet.release();
pet.recapture();
can.reset();
}
Alice’s code
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Pseudocode
while (true) {
while (can.isUp()){};
pet.release();
pet.recapture();
can.reset();
while (true) {
}
while (can.isDown()){};
Bob’s code
pond.stockWithFood();
can.knockOver();
Alice’s code
EPFL CS-206 – Spring 2015
}
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Correctness
u  Mutual
w  Pets
u  No
Exclusion
and Bob never together in pond
Starvation
if Bob always willing to feed, and pets always famished, then pets
eat infinitely often
EPFL CS-206 – Spring 2015
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Correctness
u  Mutual
w  Pets
u  No
Exclusion
safety
and Bob never together in pond
Starvation
liveness
if Bob always willing to feed, and pets always famished, then pets
eat infinitely often.
u  Producer/Consumer
safety
The pets never enter pond unless there is food, and Bob never
provides food if there is unconsumed food.
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Could Also Solve Using Flags
A
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B
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Waiting
u  Both
solutions use waiting
w  while(mumble){}
u  In
some cases waiting is problematic
w  If
one participant is delayed
w  So is everyone else
w  But delays are common & unpredictable
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The Fable drags on …
u  Bob
and Alice still have issues
u  So they need to communicate
u  They agree to use billboards …
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Readers/Writers
u  Alice
& Bob agree to write messages on a billboard
u  Alice starts writing one letter at a time
w  First
she writes “wash the car”
w  Then writes “sell the house”
w  What does Bob read?
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Billboards are Large
B D
A C E
2
3
1
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3
1
Letter
Tiles
From Scrabble™ box
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Write One Letter at a Time …
W A S
4
1
1
H
4
B D
A C E
2
3
1
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3
1
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To post a message
W A S H T H E C A R
4
1
1
4
1
4
1
3
1
1
whew
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Let’s send another message
S E L L
1
1
1
1
L A V A
1
1
4
1
L A
M PS
1
1
3
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3
1
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Uh-Oh
S E L L
1
1
1
1
T H E
1
4
1
C A R
3
1
1
L
1
OK
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Readers/Writers
u  Devise
a protocol so that
w  Writer
writes one letter at a time
w  Reader reads one letter at a time
w  Reader sees “snapshot”
w Old message or new message
w No mixed messages
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Readers/Writers (continued)
u  Easy
with mutual exclusion
u  But mutual exclusion requires waiting
w  One
waits for the other
w  Everyone executes sequentially
u  Remarkably
w  We
can solve R/W without mutual exclusion
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Why do we care?
u  We
want as much of the code as possible to execute
concurrently (in parallel)
u  A larger sequential part implies reduced performance
u  Amdahl’s law: this relation is not linear…
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Parallelizing Readers/Writers
u  Use
two billboards
w  While
Bob reads from one…
w  Alice writes to the other
u  How
do they know where to read/write?
w  Third
billboard
w  Tells Bob which board to read
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Wait-free protocol
S1 E1 L1 L1
T1
H4
2
W4 A1 S1 H4
T1 H4 E1
C3 A1 R1
H4
H
H
H4444
W..a..s..h…
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Wait-free protocol
S1 E1 L1 L1
T1 H4 E1
2
D2 O4 G3
W4 A1 S1 H4
T1 H4 E1
C3 A1 R1
1
Wash the
car! Got it!
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Wait-free protocol
S1 E1 L1 L1
T1 H4 E1
1
D2 O4 G3
W4 A1 S1 H4
T1 H4 E1
C3 A1 R1
Yay, new
message on
board 1!
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Wait-free protocol
S1 E1 L1 L1
T1 H4 E1
1
D2 O4 G3
M4 O4 V4 E4
Sell the dog?
Ok!
EPFL CS-206 – Spring 2015
A4 H4 E1
N4
C3 A1 R1
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Wait-free protocol
u  Is
this protocol entirely wait-free?
u  Is
the protocol correct?
u  How
do you fix it?
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Summary
u  Need
protocols to coordinate
u  Mutual exclusion properties
w  Safety
w  Liveness
w  Fairness
u  Producer/Consumer
sharing
u  Reducing sequential execution/waiting
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