INFT1331073310 Section 8 Concurrency II: Mutual Exclusion

1
INFT13-310/73-310Section 8Concurrency
IIMutual Exclusion
http//www-dse.doc.ic.ac.uk/concurrency/
Using material from Magee and Kramer
2
Modelling Concurrency

• Issues
• How to model execution speed?
• arbitrary
• abstract away time
• How to model concurrent execution?
• actions in each process ordered
• arbitrary relative ordering between processes
  (interleaving)
• Result
• general (asynchronous) model independent of
  scheduling

• Parallel Composition
• If P and Q are processes, (PQ) represents the
  concurrent execution of P and Q

E.g., PONDER (think -gt STOP). DINNER (talk
-gt eat -gt STOP). PONDINNER (PONDER
DINNER). Traces think -gt talk -gt eat. talk -gt
think -gt eat. talk -gt eat -gt think.
Laws of composition (P Q) (Q P) (P (Q
R)) ((P Q) R) (P Q R)
3
Modelling Interaction

• Shared actions
• Processes in an interaction with actions in
  common share the actions
• unshared actions can be arbitrarily interleaved
• shared actions are executed at the same time by
  all sharing processes
• E.g.,
• MAKER (make -gt ready -gt MAKER).
• USER (ready -gt use -gt USER).
• MAKER_USER (MAKER USER).

• Handshake
• An action acknowledged by another
• E.g.,
• MAKER2 (make-gtready-gtused-gtMAKER2).
• USER2 (ready-gtuse-gtused-gtUSER2).
• MAKER_USER2 (MAKER2 USER2).

How many states? How many traces?
4
Modelling Multi-party Synchronisation

• Handshake An action acknowledged by another
• E.g.,
• MAKER_A (makeA-gtready-gtused-gtMAKER_A).
• MAKER_B (makeB-gtready-gtused-gtMAKER_B).
• ASSEMBLE (ready-gtassemble-gtused-gtASSEMBLE).
• FACTORY (MAKER_A MAKER_B ASSEMBLE).

How many states? How many traces?
makeA
makeB
makeA
ready
assemble
5
0
1
2
3
4
makeB
used
5
Guten Jour!
public static void displayMessage(
RepeatedMessage rm ) throws InterruptedException
for (int i 0 i lt rm.message.length()
i) System.out.print(rm.message.charAt(i)
) sleep(50) System.out.println()

Sample output BGount ejno uTra!g !B on
jour! BoGnu tjeonu rT!a Bgo!n jour! BonG
ujtoeunr !T Baogn! jour! BGount ejno
uTra!g B!o n jouGru!t en Tag!

• HOMME (B-gto-gtn-gtj-gto-gtu-gtr-gtHOMME).
• DEUTSCHER (G-gtu-gtt-gte-gtn-gtT-gta-gtg-gtDEUTSCHER).
• MESS (HOMME DEUTSCHER).

6
Bon Tag!
public synchronized static void displayMessage(
RepeatedMessage rm ) throws InterruptedException
for (int i 0 i lt rm.message.length()
i) System.out.print(rm.message.charAt(i)
) sleep(50) System.out.println()

Sample output Bon jour! Guten Tag! Bon
jour! Guten Tag! Bon jour! Bon jour! Guten Tag!

• HOMME (f_go-gtB-gto-gtn-gtj-gto-gtu-gtr-gtf_end-gtHOMME).
  
• DEUTSCHER (g_go-gtG-gtu-gtt-gte-gtn-gtT-gta-gtg-gtg_end
  -gtDEUTSCHER).
• SYSTEM (f_go -gtf_end -gtSYSTEM g_go -gtg_end
  -gtSYSTEM).
• NO_MESS (HOMME DEUTSCHER SYSTEM).

7
Interference

• Interference destructive update caused by
  arbitrary interleaving of read and write actions

• ETURN
• (ein-gtereadCntec-gtewriteCntec1-gtETURN
• eout-gtereadCntec-gtewriteCntec-1-gtETURN).
  
• WTURN
• (win-gtwreadCntwc-gtwwriteCntwc1-gtWTURN
• wout-gtwreadCntwc-gtwwriteCntwc-1-gtWTURN)
  .
• COUNTERcount
• (readCntcount-gtCOUNTERcount
• writeCntnewCount-gtCOUNTERnewCount).
• PARK (ETURN WTURN COUNTER).

• E.g., Theme park
• two gates counting entrants
• central record of number of people in park

Park
Does it work?
East
people
West
Turnstile
Turnstile
8
Interference

• Possible state

• Possible element of trace
• Suppose count 10
• ...-gtereadec (ec 10)
• -gtwreadwc (wc 10)
• -gtwwritewc1 (wc1 11 count)
• -gtewriteec1 (ec1 11 count)
• -gt
• count 11, but 2 more people in park

How?
Does it work? means Does it work for all
traces?
9
Atomic Actions

• An atomic action is an action that executes
  uninterrupted from start to finish on a processor
• (In our CSP/FSP notation, events represent atomic
  actions)

• Note! Very few statements in a high-level
  programming language are atomic
• E.g., assignment is not atomic
• Java x 10
• Assembler
• LOAD R1 10
• LOAD R2 ADDR_X
• STORE _at_R2 R1

10
Interference and Mutual Exclusion

• Interference bugs are extremely difficult to
  track
• General solution
• Identify the critical section of the code
• Ensure only one process is allowed to execute
  during its critical section
• The critical sections are the accesses to the
  shared memory/resource

• Mutually exclusive access to critical section
  through locking
• LOCK (acquire-gtrelease-gtLOCK).
• ETURN (ein-gtINC-gtETURN
• eout-gtDEC-gtETURN).
• INC (acquire-gtreadx-gtwritex1-gtrelease).
• DEC (acquire-gtreadx-gtwritex-1-gtrelease).
• COUNTERcount
• (readcount-gtCOUNTERcount
• writenewCount-gtCOUNTERnewCount).

11
Locking in Java

• Keyword synchronized
• Attached to method system ensures method
  executes to completion with no interleaving
• Can also be attached to an object
• synchronized (object)
• // code
• Object object is locked for duration of code
• code cannot be executed if another process has
  the lock on object

• class Counter
• public synchronized void increment( )
  count

or
class Turnstile Counter counter public
void enter( ) synchronized (counter)
counter.increment() // Remember!
Locks object
12
Buffering

• Locking (synchronisation) limits the number of
  possible traces to the safe ones
• If not used carefully, locking will force the
  system into a sequential execution
• E.g., the theme park
• ein-gtINC-gtwin-gtINC-gtein-gtINC-gt

• One technique to improve concurrency in many
  systems is buffering
• Dont lock the central object
• Have each thread communicate with central control
  through its own buffer
• buffer is a list (e.g., a Vector)
• buffers are locked for adding and removing, but
  while doing other work, processes are concurrent

13
Monitors

• Synchronisation (and locking)
• someone must be controlling the locks
• processes will be waiting to access locked
  objects
• Who is controlling?
• Where do processes wait?

• A Monitor is a method of managing synchronisation
• A Monitor provides
• encapsulation of data and methods
• mutually exclusive execution of monitored methods
• condition synchronisation
• a wait queue of processes waiting to use the
  monitored resources

14
Monitors

• Condition synchronisation
• Java provides a wait queue per object
• final void wait() throws InterruptedException
• puts thread on wait queue
• thread releases synchronisation lock on monitor
• thread waits until notified by another thread
• when awoken, thread must still wait to reacquire
  synchronisation lock on monitor
• final void notify()
• wake up a single thread on the wait queue
• final void notifyAll()
• wake up all threads on the wait queue

• Java provides support for treating any object as
  a monitor (through keyword synchronized)
• Typical usage scenario
• Active entities (initiating actions) implemented
  as threads
• Passive entities (responding to actions)
  controlling shared resources implemented as
  monitors

15
Monitor and Condition Synchronisation Example

• Carpark
• cars can enter carpark when its not full
• cars can leave carpark when its not empty
• Processes
• arrivals
• departures
• controller

• CONTROLspaces
• ( when (spaces gt 0) arrive -gt CONTROLspaces-1
• when (spaces lt MAX) depart -gt
  CONTROLspaces1 ).
• ARRIVALS (arrive -gt ARRIVALS).
• DEPARTURES (depart -gt DEPARTURES).
• CARPARK ( CONTROL ARRIVALS DEPARTURES).

• Implementation
• ARRIVALS and DEPARTURES threads
• CONTROL monitor

16
Typical Implementation of Condition
Synchronisation

• CSP/FSP
• when cond act -gt NEWSTATE

Java public synchronized void act( ) throws
InterruptedException while (!cond) wait(
) // modify monitor state notifyAll( )
Notification necessary to awaken threads that may
be waiting to enter the monitor now that its
state has changed
17
Carpark in Java

• class CarParkControl
• protected int spaces
• protected int capacity
• public synchronized void arrive()
• throws InterruptedException
• while (spaces0) wait()
• spaces - -
• notify()
• public synchronized void depart()
• throws InterruptedException
• while (spacescapacity) wait()
• spaces
• notify()

• class Arrivals implements Runnable
• protected CarParkControl carpark
• public Arrivals(CarParkControl c)
• carpark c
• public void run()
• try
• while (true)
• carpark.arrive()
• // delay
• catch (InterruptedException e)