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Concurrency Control and Deadlocks in Distributed Systems

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Understand race conditions, need for concurrency control and solution requirements. Explore CC primitives like semaphores, monitors, and locks, and learn about deadlocks and resolutions.

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Concurrency Control and Deadlocks in Distributed Systems

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  1. Learning Objectives • Understanding of race conditions and the need for concurrency control. • Good solution requirements. • CC primitives: semaphores, monitors and locks. • Deadlocks and deadlock resolutions. Concurrency control

  2. Concurrency Control • Concurrency control is necessary to achieve data consistency in systems that share data, in particular it is essential in distributed systems. • ? Give a couple of examples where lack of concurrency control may lead to system inconsistency (or race conditions). Concurrency control

  3. Mutual Exclusion and Critical Section • Mutual exclusion assures that at most process accesses a shared piece in a given time moment. • Parts of the code that deal access shared data is called critical section (region). • In order to achieve Mutual Exclusion two or more processes should be in their critical sections simultaneously. Concurrency control

  4. “Good” Solution Requirements • Req. 1: Mutually exclusive access to critical section. • Req. 2: No blocking or interference of other processes (not interested in entering critical section). • Req. 3: No starvation. Concurrency control

  5. Semaphores • Semaphores are shared variables that control access to critical section. • There are two ATOMIC operations defined for semaphores: Down (Lower) and Up (Raise). • ? Study the algorithm for semaphores in Box 5.1. p. 107 and trace it for 2 processes and initial value of semaphore equal to 1 and then equal to 0. Study Box 5.2 p. 109 first. Concurrency control

  6. Semaphores and Deadlocks • Correctness of the system relies on a programmer and his/her correct use of Down and Up operations. • ? Write a code for two processes that share semaphores (eg. S1 and S2), which execution leads to a deadlock? Concurrency control

  7. Semaphore Evaluation • ? Does the Semaphore primitive fulfill the three requirements for a “good” solution to the critical section problem. • “Good” Solution Requirements • ? Solve problems 5.3. and 5.4. p. 125. Concurrency control

  8. Monitors • High level synchronization (concurrency control) primitives. • A monitor is a construct that allows for grouping of data structures in a separate module or package. • Monitors assure that no more than one process can be active within a monitor at any given time. Concurrency control

  9. Monitor Implementation • Monitors are often implemented using “binary” semaphores. • However, the semaphores are used by the compiler not a programmer thus reducing a chance of semaphore misuse. • Monitors use condition variables: wait (suspends a process) and signal (resumes a process). Concurrency control

  10. Monitor Evaluation • Study box 5.4 on page 111 for Java Monitors. • ? Do the Monitors fulfill the three requirements for a “good” solution to the critical section problem? • “Good” Solution Requirements Concurrency control

  11. Locks • Locks are another primitive to achieve mutual exclusion. • Lock control access to critical section by having two states: locked and unlocked. • If a process wants to access critical section then if checks the value of lock. Only if the lock is in unlocked state access to critical section is granted. ( see box 5.5 p. 112 for problems with implementing Locks) Concurrency control

  12. Other Approaches to Locks • Taking turns • busy/waiting or spinning • ? Is this a good solution? • Hardware support • Indivisible (atomic hardware instructions): • Test-and-Set-lock (TSL) (or check and set) • SWAP (register, memory) • ?Are TSL and SWAP a “good” solution? Concurrency control

  13. Software Lock Control • Centralized lock manager • Client (C) /Server (S) approach • messages: request (C  S), queued (C  S), granted (C  S), release(C  S) • Distributed lock manager • peer to peer approach (R-requester, P-other participant) • messages: request (R  Ps), queued (R  Ps), granted (R  Ps) Concurrency control

  14. Token Passing • Token is a special type of a message. • Token passing solution relies on a single token traveling through all the processors creating a logical ring. • Only the process that possesses a token can access critical section. Concurrency control

  15. Evaluation of Software Lock Control • ?Are the proposed methods: • centralized lock manager • distributed lock manager • token passing “good” solutions to critical section problem? • ? Solve problems 5.7 and 5.8 p. 126. ? Concurrency control

  16. Deadlocks • Four necessary conditions for deadlocks: • mutual exclusion of resources • non-preemption • hold and wait • circular wait • Back to Semaphores and Deadlocks • To Deadlock Prevention Concurrency control

  17. Resource Allocation Graphs • Resource Allocation Graphs or RAGs are used to model possible deadlocks in distributed systems. • ?Study figure 5.5 p.120 and answer the following question. Does a cycle in RAG imply a deadlock? Concurrency control

  18. Distributed Deadlocks • Distributed Deadlocks often involve a shared buffer space used for IPC. • Such deadlocks are commonly referred to as communication deadlocks. • Types of communication deadlocks: • direct store and forward (two locations) • indirect store and forward (more than two locations) Concurrency control

  19. Deadlock Resolution • Deadlock Prevention • Deadlock Avoidance • Deadlock Ignorance • Deadlock Detection Concurrency control

  20. Deadlock Prevention • Deadlock Prevention relies on designing out one of the four necessary conditions. • Sample methods: • acquire all resources or release those possessed after a timeout (?Which condition is eliminated?) • acquire resources in order (Which condition is eliminated?) • use seniority rules (time stamps) (Which condition is eliminated?) Concurrency control

  21. Deadlock Avoidance • Deadlock avoidance relies on resource allocation that moves the system from a “safe” state to a “safe” state. • The system is always in a safe state thus it never deadlocks. • Due to unrealistic assumptions and its high complexity this approach is not very practical. Concurrency control

  22. Deadlock Ignorance • Ignore Deadlocks altogether. • ?What happens if deadlock occurs in a system that ignores it? • ? Why this approach is “O.K.” in general purpose distributed systems but not acceptable in real-time systems? Concurrency control

  23. Deadlock Detection • Relies on detecting deadlocks AFTER they occur. • Once deadlock is detected, the necessary steps need to be taken in order to eliminate deadlock. • ? What would be the simples approach to eliminating a deadlock? Concurrency control

  24. Distributed Deadlock Detection • A process that is blocked waiting for a resource sends a probe (a special message) to the process that is being blocked by. • A probe consists of the following fields: • originator, sender, receiver • If the receiving process is blocked it forwards the probe to the process that is being blocked by but modifies sender and receiver fields in the probe. • If the receiving process is not blocked is discards the probe. • If originator=receiver then there is a deadlock. Concurrency control

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