Notes

Outline

Concurrency: Mutual Exclusion and Synchronization Chapter 5

Problems with concurrent execution Concurrent processes (or threads) often need to share data (maintained either in shared memory or files) and resources If there is no controlled access to shared data, some processes will obtain an inconsistent view of this data The action performed by concurrent processes will then depend on the order in which their execution is interleaved

An example Process P1 and P2 are running this same procedure and have access to the same variable “a” Processes can be interrupted anywhere If P1 is first interrupted after user input and P2 executes entirely Then the character echoed by P1 will be the one read by P2 !!

The critical section problem When a process executes code that manipulates shared data (or resource), we say that the process is in it’s critical section (CS) (for that shared data) The execution of critical sections must be mutually exclusive: at any time, only one process is allowed to execute in its critical section (even with multiple CPUs) Then each process must request the  permission to enter it’s critical section (CS)

The critical section problem The section of code implementing this request is called the entry section The critical section (CS) might be followed by an exit section The remaining code is the remainder section The critical section problem is to design a protocol that the processes can use so that their action will not depend on the order in which their execution is interleaved (possibly on many processors)

Framework for analysis of solutions Each process executes at nonzero speed but no assumption on the relative speed of n processes General structure of a process: many CPU may be present but memory hardware prevents simultaneous access to the same memory location No assumption about order of interleaved execution For solutions: we need to specify entry and exit sections

Requirements for a valid solution to the critical section problem Mutual Exclusion At any time, at most one process can be in its critical section (CS) Progress Only processes that are in their entry section can be selected to enter their CS. This selection cannot be postponed indefinitely

Requirements for a valid solution to the critical section problem Bounded Waiting After a process has made a request to enter it’s CS, there is a bound on the number of times that the other processes are allowed to enter their CS otherwise the process will suffer from starvation

Types of solutions Software solutions algorithms who’s correctness does not rely on any other assumptions (see framework) Hardware solutions rely on some special machine instructions Operation System solutions provide some functions and data structures to the programmer

Software solutions We consider first the case of 2 processes Algorithm 1 and 2 are incorrect Algorithm 3 is correct (Peterson’s algorithm) Then we generalize to n processes the bakery algorithm Notation We start with 2 processes: P0 and P1 When presenting process Pi, Pj always denotes the other process (i != j)

Algorithm 1 The shared variable turn is initialized (to 0 or 1) before executing any Pi Pi’s critical section is executed iff turn = i Pi is busy waiting if Pj is in CS: mutual exclusion is satisfied Progress requirement is not satisfied since it requires strict alternation of CSs Ex: if turn=0 and P0 is in it’s RS, P1 can never enter it’s CS

Algorithm 2 Keep 1  Bool variable for each process: flag[0] and flag[1] Pi signals that it is ready to enter it’s CS by: flag[i]:=true Mutual Exclusion is satisfied but not the progress requirement If we have the sequence: T0: flag[0]:=true T1: flag[1]:=true Both process will wait forever to enter their CS: we have a deadlock

Algorithm 3 (Peterson’s algorithm) Initialization: flag[0]:=flag[1]:=false turn:= 0 or 1 Willingness to enter CS specified by flag[i]:=true If both processes attempt to enter their CS simultaneously, only one turn value will last Exit section: specifies that Pi is unwilling to enter CS

Algorithm 3: proof of correctness Mutual exclusion is preserved since: P0 and P1 are both in CS only if flag[0] = flag[1] = true and only if turn = j for each Pi (impossible) We now prove that the progress and bounded waiting requirements are satisfied: Pi cannot enter CS only if stuck in while() with condition flag[j] = true and turn = j. If Pj is not ready to enter CS then flag[j] = false and Pi can then enter its CS

Algorithm 3: proof of correctness (cont.) If Pj has set flag[j]=true and is in its while(), then either turn=i or turn=j If turn=i, then Pi enters CS. If turn=j then Pj enters CS but will then reset flag[j]=false on exit: allowing Pi to enter CS but if Pj has time to reset flag[j]=true, it must also set turn=i since Pi does not change value of turn while stuck in while(), Pi will enter CS after at most one CS entry by Pj (bounded waiting)

n-process solution: bakery algorithm Before entering their CS, each Pi receives a number. Holder of smallest number enter CS (like in bakeries, ice-cream stores...) When Pi and Pj receives same number: if i<j then Pi is served first, else Pj is served first Pi resets its number to 0 in the exit section Notation: (a,b) < (c,d) if a < c or if a = c and b < d max(a0,...ak) is a number b such that b >= ai for i=0,..k

The bakery algorithm (cont.) Shared data: choosing: array[0..n-1] of boolean; initialized to false number: array[0..n-1] of integer; initialized to 0 Correctness relies on the following fact: If Pi is in CS and Pk has already chosen its number[k]!= 0, then (number[i],i) < (number[k],k) but the proof is somewhat tricky...

The bakery algorithm (cont.)

Drawbacks of software solutions Processes that are requesting to enter in their critical section are busy waiting (consuming processor time needlessly) If CSs are long, it would be more efficient to block processes that are waiting...

Hardware solutions: interrupt disabling On a uniprocessor: mutual exclusion is preserved but efficiency of execution is degraded: while in CS, we cannot interleave execution with other processes that are in RS On a multiprocessor: mutual exclusion is not preserved Generally not an acceptable solution

Hardware solutions: special machine instructions Normally, access to a memory location excludes other access to that same location Extension: designers have proposed machines instructions that perform 2 actions atomically (indivisible) on the same memory location (ex: reading and testing) The execution of such an instruction is mutually exclusive (even with multiple CPUs) They can be used simply to provide mutual exclusion but need more complex algorithms for satisfying the 3 requirements of the CS problem

The test-and-set instruction A C++  description of test-and-set: An algorithm that uses testset for Mutual Exclusion: Shared variable b is initialized to 0 Only the first Pi who sets b enter CS

The test-and-set instruction (cont.) Mutual exclusion is preserved: if Pi enter CS, the other Pj are busy waiting When Pi exit CS, the selection of the Pj who will enter CS is arbitrary: no bounded waiting. Hence starvation is possible Processors (ex: Pentium) often provide an atomic xchg(a,b) instruction that swaps the content of a and b. But xchg(a,b) suffers from the same drawbacks as test-and-set

Using xchg for mutual exclusion Shared variable b is initialized to 0 Each Pi has a local variable k The only Pi that can enter CS is the one who finds b=0 This Pi excludes all the other Pj by setting b to 1

Semaphores Synchronization tool (provided by the OS) that do not require busy waiting A semaphore S is an integer variable that, apart from initialization, can only be accessed through 2 atomic and mutually exclusive operations: wait(S) signal(S) To avoid busy waiting: when a process has to wait, it will be put in a blocked queue of processes waiting for the same event

Semaphores Hence, in fact, a semaphore is a record (structure): When a process must wait for a semaphore S, it is blocked and put on the semaphore’s queue The signal operation removes (acc. to a fair policy like FIFO) one process from the queue and puts it in the list of ready processes

Semaphore’s operations (atomic)

Semaphores: observations When S.count >=0:  the number of processes that can execute wait(S) without being blocked = S.count When S.count<0: the number of processes waiting on S is = |S.count| Atomicity and mutual exclusion: no 2 process can be in wait(S) and signal(S) (on the same S) at the same time (even with multiple CPUs) This is a critical section problem: the critical sections are the blocks of code defining wait(S) and signal(S)

Semaphores: observations Hence: the critical sections here are very short: typically 10 instructions Solutions: uniprocessor: disable interrupts during these operations (ie: for a very short period). This does not work on a multiprocessor machine. multiprocessor: use previous software or hardware schemes. The amount of busy waiting should be small.

Using semaphores for solving critical section problems For n processes Initialize S.count to 1 Then only 1 process is allowed into CS (mutual exclusion) To allow k processes into CS, we initialize S.count to k

Using semaphores to synchronize processes We have 2 processes: P1 and P2 Statement S1 in P1 needs to be performed before statement S2 in P2 Then define a semaphore “synch” Initialize synch to 0 Proper synchronization is achieved by having in P1: S1; signal(synch); And having in P2: wait(synch); S2;

The producer/consumer problem A producer process produces information that is consumed by a consumer process Ex1: a print program produces characters that are consumed by a printer Ex2: an assembler produces object modules that are consumed by a loader We need a buffer to hold items that are produced and eventually consumed A common paradigm for cooperating processes

P/C: unbounded buffer We assume first an unbounded buffer  consisting of a linear array of elements in points to the next item to be produced out points to the next item to be consumed

P/C: unbounded buffer We need a semaphore S to perform mutual exclusion on the buffer: only 1 process at a time can access the buffer We need another semaphore N to synchronize producer and consumer on the number N (= in - out) of items in the buffer an item can be consumed only after it has been created

P/C: unbounded buffer The producer is free to add an item into the buffer at any time: it performs wait(S) before appending and signal(S) afterwards  to prevent customer access It also performs signal(N) after each append to increment N The consumer must first do wait(N) to see if there is an item to consume and use wait(S)/signal(S) to access the buffer

Solution of P/C: unbounded buffer

P/C: unbounded buffer Remarks: Putting signal(N) inside the CS of the producer (instead of outside) has no effect since the consumer must always wait for both semaphores before proceeding The consumer must perform wait(N) before wait(S), otherwise deadlock occurs if consumer enter CS while the buffer is empty Using semaphores is a difficult art...

P/C: finite circular buffer of size k can consume only when number N of (consumable) items is at least 1 (now: N!=in-out) can produce only when number E of empty spaces is at least 1

P/C: finite circular buffer of size k As before: we need a semaphore S to have mutual exclusion on buffer access we need a semaphore N to synchronize producer and consumer on the number of consumable items In addition: we need a semaphore E to synchronize producer and consumer on the number of empty spaces

Solution of P/C: finite circular buffer of size k

The Dining Philosophers Problem 5 philosophers who only eat and think each need to use 2 forks for eating we have only 5 forks a classical synchronization problem illustrates the difficulty of allocating resources among process without deadlock and starvation

The Dining Philosophers Problem Each philosopher is a process One semaphore per fork: fork: array[0..4] of semaphores Initialization: fork[i].count:=1 for i:=0..4 A first attempt: Deadlock if each philosopher start by picking his left fork!

The Dining Philosophers Problem A solution: admit only 4 philosophers at a time that tries to eat Then 1 philosopher can always eat when the other 3 are holding 1 fork Hence, we can use another semaphore T that would limit at 4 the numb. of philosophers “sitting at the table” Initialize: T.count:=4

Problems with Semaphores Semaphores provide a powerful tool for enforcing mutual exclusion and coordinate processes But wait(S) and signal(S) are scattered among several processes. Hence, difficult to understand their effects Usage must be correct in all the processes One bad (or malicious) process can fail the entire collection of processes

Monitors Are high-level language constructs that provide equivalent functionality to that of semaphores but are easier to control Found in many concurrent programming languages Concurrent Pascal, Modula-3, uC++, Java... Can be implemented by semaphores...

Monitor Is a software module containing: one or more procedures an initialization sequence local data variables Characteristics: local variables accessible only by monitor’s procedures a process enters the monitor by invoking one of it’s procedures only one process can be in the monitor at any one time

Monitor The monitor ensures mutual exclusion: no need to program this constraint explicitly Hence, shared data are protected by placing them in the monitor The monitor locks the shared data on process entry Process synchronization is done by the programmer by using condition variables that represent conditions a process may need to wait for before executing in the monitor

Condition Variables are local to the monitor (accessible only within the monitor) can be access and changed only by two functions: cwait(a): blocks execution of the calling process on condition (variable) a csignal(a): resume execution of some process blocked on condition (variable) a. If several such process exists: choose any one If no such process exists: do nothing

Monitor Awaiting processes are either in the entrance queue or in a condition queue A process puts itself into condition queue cn by issuing cwait(cn) csignal(cn) brings into the monitor 1 process in condition cn queue Hence csignal(cn) blocks the calling process and puts it in the urgent queue (unless csignal is the last operation of the monitor procedure)

Producer/Consumer problem Two processes: producer consumer Synchronization is now confined within the monitor append(.) and take(.) are procedures within the monitor: are the only means by which P/C can access the buffer If these procedures are correct, synchronization will be correct for all participating processes

Monitor for the bounded P/C problem Monitor needs to hold the buffer: buffer: array[0..k-1] of items; needs two condition variables: notfull: true when buffer is not full notemty: true when buffer is not empty needs buffer pointers and counts: nextin: points to next item to be appended nextout: points to next item to be taken count: holds the number of items in buffer

Monitor for the bounded P/C problem

Message Passing Is a general method used for interprocess communication (IPC) for processes inside the same computer for processes in a distributed system Yet another mean to provide process synchronization and mutual exclusion We have at least two primitives: send(destination, message) received(source, message) In both cases, the process may or may not be blocked

Synchronization in message passing For the sender: it is more natural not to be blocked after issuing send(.,.) can send several messages to multiple dest. but sender usually expect acknowledgment of message receipt (in case receiver fails) For the receiver: it is more natural to be blocked after issuing receive(.,.) the receiver usually needs the info before proceeding but could be blocked indefinitely if sender process fails before send(.,.)

Synchronization in message passing Hence other possibilities are sometimes offered Ex: blocking send, blocking receive: both are blocked until the message is received occurs when the communication link is unbuffered (no message queue) provides tight synchronization (rendez-vous)

Addressing in message passing direct addressing: when a specific process identifier is used for source/destination but it might be impossible to specify the source ahead of time (ex: a print server) indirect addressing (more convenient): messages are sent to a shared mailbox which consists of a queue of messages senders place messages in the mailbox, receivers pick them up

Mailboxes and Ports A mailbox can be private to one sender/receiver pair The same mailbox can be shared among several senders and receivers the OS may then allow the use of message types (for selection) Port: is a mailbox associated with one receiver and multiple senders used for client/server applications: the receiver is the server

Ownership of ports and mailboxes A port is usually owned and created by the receiving process The port is destroyed when the receiver terminates The OS creates a mailbox on behalf of a process (which becomes the owner) The mailbox is destroyed at the owner’s request or when the owner terminates

Enforcing mutual exclusion with message passing create  a mailbox mutex shared by n processes send() is non blocking receive() blocks when mutex is empty Initialization: send(mutex, “go”); The first Pi who executes receive() will enter CS. Others will be blocked until Pi resends msg.

The bounded-buffer P/C problem with message passing We will now make use of messages The producer place items (inside messages) in the mailbox mayconsume mayconsume acts as our buffer: consumer can consume item when at least one message is present Mailbox mayproduce is filled initially with k null messages (k= buffer size) The size of mayproduce shrinks with each production and grows with each consumption can support multiple producers/consumers

The bounded-buffer P/C problem with message passing