Operating Systems

A. Frank - P. WeisbergOperating Systems

The Critical-Section Problem

A. Frank - P. Weisberg

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A. Frank - P. WeisbergCooperating ProcessesIntroduction to Cooperating ProcessesProducer/Consumer ProblemThe Critical-Section ProblemSynchronization HardwareSemaphores


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A. Frank - P. WeisbergThe Critical-Section Problemn processes competing to use some shared data.No assumptions may be made about speeds or the number of CPUs.Each process has a code segment, called Critical Section (CS), in which the shared data is accessed.Problem ensure that when one process is executing in its CS, no other process is allowed to execute in its CS.


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A. Frank - P. WeisbergCS Problem Dynamics (1)When a process executes code that manipulates shared data (or resource), we say that the process is in its Critical Section (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 processors).So each process must first request permission to enter its critical section.


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A. Frank - P. WeisbergCS Problem Dynamics (2)The section of code implementing this request is called the Entry Section (ES).The critical section (CS) might be followed by a Leave/Exit Section (LS). The remaining code is the Remainder Section (RS).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).


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A. Frank - P. WeisbergGeneral structure of process Pi (other is Pj)

do {entry sectioncritical sectionleave sectionremainder section} while (TRUE);Processes may share some common variables to synchronize their actions.


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A. Frank - P. WeisbergSolution to Critical-Section ProblemThere are 3 requirements that must stand for a correct solution:Mutual ExclusionProgressBounded WaitingWe can check on all three requirements in each proposed solution, even though the non-existence of each one of them is enough for an incorrect solution.


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A. Frank - P. WeisbergSolution to CS Problem Mutual ExclusionMutual Exclusion If process Pi is executing in its critical section, then no other processes can be executing in their critical sections.Implications:Critical sections better be focused and short.Better not get into an infinite loop in there. If a process somehow halts/waits in its critical section, it must not interfere with other processes.


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A. Frank - P. WeisbergSolution to CS Problem ProgressProgress If no process is executing in its critical section and there exist some processes that wish to enter their critical section, then the selection of the process that will enter the critical section next cannot be postponed indefinitely:If only one process wants to enter, it should be able to.If two or more want to enter, one of them should succeed.


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A. Frank - P. WeisbergSolution to CS Problem Bounded WaitingBounded Waiting A bound must exist on the number of times that other processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is granted.Assume that each process executes at a nonzero speed.No assumption concerning relative speed of the n processes.


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A. Frank - P. WeisbergTypes of solutions to CS problemSoftware solutions algorithms whos correctness does not rely on any other assumptions.Hardware solutions rely on some special machine instructions.Operating System solutions provide some functions and data structures to the programmer through system/library calls.Programming Language solutions Linguistic constructs provided as part of a language.


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A. Frank - P. WeisbergSoftware SolutionsWe consider first the case of 2 processes:Algorithm 1 and 2/3 are incorrect.Algorithm 4 is correct (Petersons algorithm).Then we generalize to n processes:The Bakery algorithm.Initial notation:Only 2 processes, P0 and P1When usually just presenting process Pi (Larry, I, i), Pj (Jim, J, j) always denotes other process (i != j).


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A. Frank - P. WeisbergInitial Attempts to Solve ProblemGeneral structure of process Pi (other is Pj) do {entry sectioncritical sectionleave sectionremainder section} while (TRUE);Processes may share some common variables to synchronize their actions.


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A. Frank - P. WeisbergAlgorithm 1 Larry/Jim versionShared variables: string turn; initially turn = Larry or Jim (no matter)turn = Larry Larry can enter its critical sectionProcess Larrydo {while (turn != Larry);critical sectionturn = Jim;remainder section} while (TRUE);Jims version is similar but Larry/Jim reversed.


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A. Frank - P. WeisbergAlgorithm 1 Pi/Pj versionShared variables: int turn; initially turn = 0turn = i Pi can enter its critical sectionProcess Pido {while (turn != i);critical sectionturn = j;remainder section} while (TRUE);Satisfies mutual exclusion and bounded waiting, but not progress.


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A. Frank - P. WeisbergAlgorithm 2 Larry/Jim versionShared variablesboolean flag-larry, flag-jim; initially flag-larry = flag-jim = FALSEflag-larry= TRUE Larry ready to enter its critical sectionProcess Larrydo { while (flag-jim);flag-larry = TRUE; critical sectionflag-larry = FALSE;remainder section} while (TRUE);


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A. Frank - P. WeisbergAlgorithm 2 Pi/Pj versionShared variablesboolean flag[2]; initially flag [0] = flag [1] = FALSEflag [i] = TRUE Pi ready to enter its critical sectionProcess Pido { while (flag[j]);flag[i] = TRUE; critical sectionflag [i] = FALSE;remainder section} while (TRUE);Satisfies progress, but not mutual exclusion and bounded waiting requirements.


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A. Frank - P. WeisbergAlgorithm 3 Larry/Jim versionShared variablesboolean flag-larry, flag-jim; initially flag-larry = flag-jim = FALSEflag-larry= TRUE Larry ready to enter its critical sectionProcess Larrydo {flag-larry = TRUE; while (flag-jim); critical sectionflag-larry = FALSE; remainder section} while (TRUE);


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A. Frank - P. WeisbergAlgorithm 3 Pi/Pj versionShared variablesboolean flag[2]; initially flag [0] = flag [1] = FALSEflag [i] = TRUE Pi wants to enter its critical sectionProcess Pido {flag[i] = TRUE; while (flag[j]);critical sectionflag [i] = FALSE;remainder section} while (TRUE);Satisfies mutual exclusion, but not progress and bounded waiting (?) requirements.


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A. Frank - P. WeisbergAlgorithm 4 Larry/Jim versionCombined shared variables of algorithms 1 and 2/3.Process Larrydo {flag-larry = TRUE; turn = Jim; while (flag-jim and turn == Jim);critical sectionflag-larry = FALSE;remainder section} while (TRUE);


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A. Frank - P. WeisbergAlgorithm 4 Pi/Pj versionCombined shared variables of algorithms 1 and 2/3.Process Pido {flag [i] = TRUE; turn = j; while (flag [j] and turn == j);critical sectionflag [i] = FALSE;remainder section} while (TRUE);Meets all three requirements; solves the critical-section problem for two processes.


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A. Frank - P. WeisbergAlgorithm 5 Larry/Jim versionLike Algorithm 4, but with the first 2 instructions of the entry section swapped is it still a correct solution? Process Larrydo { turn = Jim; flag-larry = TRUE; while (flag-jim and turn == Jim);critical sectionflag-larry = FALSE;remainder section} while (TRUE);


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A. Frank - P. WeisbergBakery Algorithm (1)Critical Section for n processes:Before entering its critical section, a process receives a number (like in a bakery). Holder of the smallest number enters the critical section.The numbering scheme here always generates numbers in increasing order of enumeration; i.e., 1,2,3,3,3,3,4,5...If processes Pi and Pj receive the same number, if i < j, then Pi is served first; else Pj is served first (PID assumed unique).


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A. Frank - P. WeisbergBakery Algorithm (2)Choosing a number:max (a0,, an-1) is a number k, such that k ai for i = 0, , n 1Notation for lexicographical order (ticket #, PID #) (a,b) < (c,d) if a < c or if a == c and b < dShared data:boolean choosing[n];int number[n]; Data structures are initialized to FALSE and 0, respectively.


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A. Frank - P. WeisbergBakery Algorithm for Pi do { choosing[i] = TRUE;number[i] = max(number[0], , number[n 1]) +1;choosing[i] = FALSE;for (j = 0; j < n; j++) { while (choosing[j]) ; while ((number[j] != 0) && ((number[j],j) < (number[i],i))) ;}critical sectionnumber[i] = 0;remainder section} while (TRUE);


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A. Frank - P. WeisbergWhat about process failures?If all 3 criteria (ME, progress, bounded waiting) are satisfied, then a valid solution will provide robustness against failure of a process in its remainder section (RS).since failure in RS is just like having an infinitely long RS.However, no valid solution can provide robustness against a process failing in its critical section (CS).A process Pi that fails in its CS does not signal that fact to other processes: for them Pi is still in its CS.


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A. Frank - P. WeisbergDrawbacks of software solutionsSoftware solutions are very delicate .Processes that are requesting to enter their critical section are busy waiting (consuming processor time needlessly).If critical sections are long, it would be more efficient to block processes that are waiting.


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