Threads & Synchronization, Conclusion

Threads & Synchronization, Conclusion

Vivek PaiNov 20, 2001

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MechanicsMechanics

Read Birrell’s paper – future useI’ll send out a link via e-mail

Next time: basic networking, IPCSome readings already listed, maybe more in e-mail

Now: wrap up threads, critical sections Also – quiz 3 graded, working on current grades

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Big Picture on SynchronizationBig Picture on Synchronization

Why do we need it?We have multiple things runningThey perform read/write sharing of data

Can we avoid synchronization?Eliminate shared dataPerform read-only sharingEliminate parallel execution

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Synchronization PrimitivesSynchronization Primitives Why so many?

Different mechanisms for different situationsConvenient for programmers

Can we have fewer?Most can be built from “first principles”Reinvention error-prone and time-consuming

How do you decide?Analysis or Idiom

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Primitives and PurposesPrimitives and Purposes Locks

One holder, maybe lots of waiters Semaphores

Possibly lots of holders, lots of waiters Barriers

Wait until everyone catches up Condition variables

Waiters block, signal everyone when some condition occurs Monitors

Expresses synchronization at a high level

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Continuing on SynchronizationContinuing on Synchronization

So far, we’ve seen “Spinning” on lock during entire critical section Disabling interrupts for critical section (bad) Queue associated with each lock & blocking System calls for locking – possibly blocking

Since system calls exist, is everything solved?Assume shared variable “count”Lock(&mutex); count++; Unlock(&mutex);

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Cost of Protecting a Shared VariableCost of Protecting a Shared Variable

Making lock system callPushing parameter, sys call # onto stackGenerating trap/interrupt to enter kernel

System call in kernelJump to appropriate function in kernelVerify process passed in valid pointer to mutexDo locking operation, block process if needed

Actually change count – load/modify/store System call again to release mutex

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What is Lock Contention?What is Lock Contention? Competition for a lock

Uncontended = rarely in use by someone elseContended = often used by someone elseHeld = currently in use by someone

Question: what do these combinations do?Spinning on low-contention lockSpinning on high-contention lockBlocking on low-contention lockBlocking on high-contention lock

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Things to PonderThings to Ponder

If critical section is just “count++;”, what is the overhead of the synchronization

Is there some other way of doing this?

What if you don’t know how long the critical section will be?

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What If You Have the FollowingWhat If You Have the Following

Test-and-set – works at either user or kernel

System calls for block/unblockBlock takes some token and goes to sleepUnblock “wakes up” a waiter on token

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User-Level Acquire/Release using Block and Unblock

User-Level Acquire/Release using Block and Unblock

In what scenarios is this scheme appropriate? Where should it not be used?

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Semaphores (Dijkstra, Semaphores (Dijkstra, 1965)1965) Down or “P”

AtomicWait for semaphore to become positive and then decrement by 1

P(s) { if (--s < 0) Block(s);}

V(s) { if (++s <= 0) Unblock(s);}

• Up or “V”– Atomic

– Increment semaphore by 1 wake up a waiting P if any

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Bounded Buffer Bounded Buffer (Consumer-Producer)(Consumer-Producer) Example:

grep vivek access.log | more

Producer Consumer

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Bounded Buffer w/ Bounded Buffer w/ SemaphoresSemaphoresmutex = 1emptyCount = N;fullCount = 0;

producer() { while (1) { produce an item P(emptyCount);

P(mutex); put the item in buffer V(mutex);

V(fullCount); }}

consumer() { while (1) { P(fullCount);

P(mutex); take an item from buffer V(mutex);

V(emptyCount); consume the item }}

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Implementing General SemaphoresImplementing General Semaphores

Need a mutex for each semaphore Block and Unblock need to release mutex after

entering their critical sectionP(s) { Acquire(s.mutex); if (--s.value < 0) Block(s); else Release(s.mutex)}

V(s) { Acquire(s.mutex); if (++s.value <= 0) Unblock(s); else Release(s.mutex)}

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Implement General Implement General Semaphores Semaphores with Acquire/Releasewith Acquire/Release

Kotulski (1988)Two processes call P(s) (when s.value is 0) and preempted after Release(s.mutex)Two other processes call V(s)

P(s) { Acquire(s.mutex); if (--s.value < 0) { Release(s.mutex); Acquire(s.delay); } else Release(s.mutex);}

V(s) { Acquire(s.mutex); if (++s.value <= 0) Release(s.delay); Release(s.mutex);}

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Hemmendinger’s Solution (1988)

The idea is not to release s.mutex and turn it over individually to the waiting process

P and V are executing in lockstep

P(s) { Acquire(s.mutex); if (--s.value < 0) { Release(s.mutex); Acquire(s.delay); } Release(s.mutex);}

V(s) { Acquire(s.mutex); if (++s.value <= 0) Release(s.delay); else Release(s.mutex);}

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Kearns’s Solution (1988)

P(s) { Acquire(s.mutex); if (--s.value < 0) { Release(s.mutex); Acquire(s.delay); Acquire(s.mutex); if (--s.wakecount > 0) Release(s.delay); } Release(s.mutex);}

V(s) { Acquire(s.mutex); if (++s.value <= 0) { s.wakecount++; Release(s.delay); } Release(s.mutex);}

Two Release( s.delay) calls are also possible

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Hemmendinger’s Correction (1989)P(s) { Acquire(s.mutex); if (--s.value < 0) { Release(s.mutex); Acquire(s.delay); Acquire(s.mutex); if (--s.wakecount > 0) Release(s.delay); } Release(s.mutex);}

V(s) { Acquire(s.mutex); if (++s.value <= 0) { s.wakecount++; if (s.wakecount == 1) Release(s.delay); } Release(s.mutex);}

Correct but a complex solution

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Hsieh’s Solution (1989)

Use Acquire(s.delay) to block processes Correct but still a constrained implementation

P(s) { Acquire(s.delay); Acquire(s.mutex); if (--s.value > 0) Release(s.delay); Release(s.mutex);}

V(s) { Acquire(s.mutex); if (++s.value == 1) Release(s.delay); Release(s.mutex);}

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Definition TimeDefinition Time What’s a semaphore?

OED says signaling mechanism using flags, especially used for rail and sea

What’s a monitor? Is itA device to watch a signal without disrupting itA person who watches, enforces, etcA programming-language construct for exclusionA giant lizard

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Answer: All of the AboveAnswer: All of the Above

Up to 6.5 feet long Thought to alert for the presence of crocodiles

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Natural Habitat of the MonitorNatural Habitat of the Monitor

The Mesa systemXerox PARC (Palo Alto Research Center)– Cool place– Lots of neat stuff developed there

Digital SRCExodus of people from PARCLanguage, parallelism focus

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Motivation

What we wantDequeue(q) blocks until q is not empty

Semaphores are difficult to use: orders are important

Enqueue(q, item){ Acquire(mutex); put item into q; Release(mutex);}

Dequeue(q){ Acquire(mutex); take an item from q; Release(mutex); return item;}

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Think About Critical SectionsThink About Critical Sections

What are they?Pieces of code in the parallel environment

What makes code a critical section?Correctness constraints, ultimatelyBut really, what makes code a critical section?

Is there some way to take advantage of this?If so, when?

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Answer – Push It To The CompilerAnswer – Push It To The Compiler

Easier on programmer Compiler gets it right once

Programmer gets it wrong often Information available at compile-time Small amount of programmer annotation

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Monitor Hides Mutual Exclusion Procedures are

mutually exclusive Shareddata

...

Queue of waiting processestrying to enter the monitor

procedures

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Condition Variables in A Monitor

Wait( condition )Block on “condition”

Signal( condition )Wakeup a blocked process on “condition”

Conditions are not “sticky”

Shareddata

...Entry queue

operations

xy

Queues associatedwith x, y conditions

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Producer-Consumer with Monitorsmonitor ProdCons condition full, empty;

procedure Enter; begin if (the queue is full) wait(full); put item into buffer; if (only one item) signal(empty); end; procedure Remove; begin if (buffer is empty) wait(empty); remove an item; if (buffer was full) signal(full); end;

procedure Producerbegin while true do begin produce an item ProdCons.Enter(); end;end;

procedure Consumerbegin while true do begin ProdCons.Remove(); consume an item; end;end;

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Wow, This Looks Like Cake!Wow, This Looks Like Cake!

Well, the language/compiler has to support it One problem – what happens on wakeup?

Only one thing can be inside monitorWakeup implies signaller, waiter in monitor

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Options of the SignalerOptions of the Signaler Exit the monitor (Hansen) Relinquishes control to the awaken process and suspend

the current one (Hoare)Complex if the signaler has other work to toTo make sure there is no work to do is difficult because the signal implementation is not aware how it is usedIt is easy to prove things

Continues its execution (Mesa)Easy to implementBut, the condition may not be true when the awaken process actually gets a chance to run

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Mesa Style “Monitor”(Birrell’s Paper) Acquire and Release Wait( lock, condition )

Atomically unlock the mutex and enqueued on the condition variable (block the thread)Re-lock the lock when it is awaken

Signal( condition )Noop if there is no thread blocked on the condition variableWake up at least one if there are threads blocked

Broadcast( condition )Wake up all

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Example Add an item to the queue

Acquire( mutex );add an item to the queue;Signal( nonEmptyCond );Release( mutex );

Remove an item from a queueAcquire( mutex );while ( queue is empty )

Wait( mutex, nonEmptyCond );remove an item;Release( mutex );

Question: Can “while” be replaced by “if”

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Mesa-Style vs. Hoare-Style Monitor Mesa-style

Signaller keeps lock and CPUWaiter simply put on ready queue, with no special priority

Hoare-styleSignaller gives up lock and waiter runs immediatelyWaiter gives lock and CPU back to signaller when it exits critical section or if it waits again

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Condition Variables PrimitivesCondition Variables Primitives Wait( mutex, cond )

Enter the critical section (min busy wait) Release mutexPut my PCB to cond’s queueCall schedulerExit the critical sectionAcquire mutex

• Signal( cond )– Enter the critical section

(min busy wait)

– Wake up one PCB in cond’s queue

– Exit the critical section

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Are Monitors Alive Today?Are Monitors Alive Today? Actual monitors were fairly dead

Java resurrected them What killed the monitor?

Man encroached on their environment – just kiddingLanguage support a real dead weightC kills everything not-C

But they still exist, sort ofCondition variables, etc., used heavily