Threads and Critical Sections

Vivek Pai / Kai LiPrinceton University

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GedankenthreadsWhat happens during fork?We need particular mechanisms, but do we have options about what to do?

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MechanicsMidterm grading finished!

Tabulating, recording not doneAvailable this afternoon, really!

Let’s talk a little about the midtermStart threads & critical sectionsReadings will be updated & mentioned in followup (so you’re responsible )

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Thread and Address Space

ThreadA sequential execution stream within a process (also called lightweight process)

Address spaceAll the state needed to run a program Provide illusion that program is running on its own machine (protection)There can be more than one thread per address space

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Concurrency and Threads

I/O devicesOverlap I/Os with I/Os and computation (modern OS approach)

Human usersDoing multiple things to the machine: Web browser

Distributed systemsClient/server computing: NFS file server

MultiprocessorsMultiple CPUs sharing the same memory: parallel program

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Typical Thread APICreation

Fork, Join

Mutual exclusionAcquire (lock), Release (unlock)

Condition variablesWait, Signal, Broadcast

AlertAlert, AlertWait, TestAlert

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User vs. Kernel-Level Threads

QuestionWhat is the difference between user-level and kernel-level threads?

DiscussionsWhen a user-level thread is blocked on an I/O event, the whole process is blockedA context switch of kernel-threads is expensiveA smart scheduler (two-level) can avoid both drawbacks

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Thread Control Block

Shared informationProcessor info: parent process, time, etcMemory: segments, page table, and stats, etcI/O and file: comm ports, directories and file descriptors, etc

Private stateState (ready, running and blocked)RegistersProgram counterExecution stack

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Threads Backed By Kernel Threads

Each thread hasa user stacka private kernel stack

Prosconcurrent accesses to system servicesworks on a multiprocessor

ConsMore memory

Each thread hasa user stacka shared kernel stack with other threads in the same address space

Prosless memoryDoes not work on a multiprocessor

Consserial access to system services

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“Too Much Milk” Problem

Don’t buy too much milkAny person can be distracted at any point

Person A Person B

Look in fridge: out of milkLeave for WawaArrive at WawaBuy milkArrive home

Look in fridge: out of milkLeave for WawaArrive at WawaBuy milkArrive home

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A Possible Solution?

Thread can get context switched after checking milk and note, but before buying milk

if ( noMilk ) { if (noNote) { leave note; buy milk; remove note; }}

if ( noMilk ) { if (noNote) { leave note; buy milk; remove note; }}

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Another Possible Solution?

Thread A switched out right after leaving a note

Thread A

leave noteAif (noNoteB) { if (noMilk) { buy milk }}remove noteA

Thread B

leave noteBif (noNoteA) { if (noMilk) { buy milk }}remove noteB

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Yet Another Possible Solution?

Safe to buyIf the other buys, quit

Thread A

leave noteAwhile (noteB) do nothing;if (noMilk) buy milk;remove noteA

Thread B

leave noteBif (noNoteA) { if (noMilk) { buy milk }}remove noteB

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RemarksThe last solution works, but

Life is too complicatedA’s code is different from B’sBusy waiting is a waste

Peterson’s solution is also complexWhat we want is:

Acquire(lock);if (noMilk) buy milk;Release(lock);

} Critical section

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What Is A Good SolutionOnly one process inside a critical sectionNo assumption about CPU speedsProcesses outside of critical section should not block other processesNo one waits foreverWorks for multiprocessors

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PrimitivesWe want to avoid thinking (repeatedly)So, we want some “contract” that provides certain behaviorLow-level behavior encapsulated in “primitives”Application uses primitives to construct more complex behavior

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The Simplistic Acquire/Release

Kernel cannot let users disable interruptsCritical sections can be arbitrarily longUsed on uniprocessors, but won’t work on multiprocessors

Acquire() { disable interrupts;}

Release() { enable interrupts;}

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Disabling Interrupts

Done right, serializes activityPeople think sequentially – easier to reasonGuarantees code executes without interruption

Delays handling of external events

Used throughout the kernel

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Using Disabling Interrupts

Why do we need to disable interrupts at all?Why do we need to enable interrupts inside the loop in Acquire?

Acquire(lock) { disable interrupts; while (lock != FREE){ enable interrupts; disable interrupts; } lock = BUSY; enable interrupts;}

Release(lock) { disable interrupts; lock = FREE; enable interrupts;}

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Using Disabling Interrupts

When does Acquire re-enable interrupts in going to sleep?

Before enqueue?After enqueue but before block?

Acquire(lock) { disable interrupts; while (lock == BUSY) { enqueue me for lock; block; } else lock = BUSY; enable interrupts;}

Release(lock) { disable interrupts; if (anyone in queue) { dequeue a thread; make it ready; } lock = FREE; enable interrupts;}

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Hardware Support for Mutex

Mutex = mutual exclusionEarly software-only approaches limitedHardware support became commonVarious approaches:

Disabling interruptsAtomic memory load and storeAtomic read-modify-write

L. Lamport, “A Fast Mutual Exclusion Algorithm,” ACM Trans. on Computer Systems, 5(1):1-11, Feb 1987. – use Google to find

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The Big PictureConcurrent Applications

Locks Semaphores Monitors Send/Receive

Load/Store Interrupt disable Test&Set

High-LevelAtomic API

Low-LevelAtomic Ops

Interrupt (timer or I/O completion), Scheduling, Multiprocessor

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Atomic Read-Modify-Write Instructions

Test&Set: Read value and write 1 back to memoryExchange (xchg, x86 architecture)

Swap register and memory

Compare and Exchange (cmpxchg, 486+)If Dest = (al,ax,eax), Dest = SRC;

else (al,ax,eax) = Dest

LOCK prefix in x86Load link and conditional store (MIPS, Alpha)

Read value in one instruction, do some operationsWhen store, check if value has been modified. If not, ok; otherwise, jump back to start

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A Simple Solution with Test&Set

Waste CPU timeLow priority threads may never get a chance to run

Acquire(lock) { while (!TAS(lock)) ;}

Release(lock) { lock = 0;}

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Test&Set, Minimal Busy Waiting

Why does this work?

Acquire(lock) { while (!TAS(lock.guard)) ; if (lock.value) { enqueue the thread; block and lock.guard = 0; } else { lock.value = 1; lock.guard = 0; }}

Release(lock) { while (!TAS(lock.guard)) ; if (anyone in queue) { dequeue a thread; make it ready; } else lock.value = 0; lock.guard = 0;}