CSC364 Lecture 08

Introduction to Networked, Distributed, and Parallel Computing
Concurrency Control
(202602)

Transcript

1. Dr. Javier Gonzalez-Sanchez javiergs@calpoly.edu www.javiergs.info o ffi ce: 14 -227 CSC 364 Introduction to Networked, Distributed, and Parallel Computing Lecture 08. Managing Concurrency 1
2. Lock 2
3. 3 Lock (ReentrantLock) A lock is a synchroniz a tion mech a nism used to ensure th a t only one thre a d c a n a ccess a sh a red resource or critic a l section a t a time. Wh a t do they gu a r a ntee? • Mutu a l exclusion → only one thre a d holds the lock a t a time • Visibility → ch a nges a re visible when the lock is rele a sed • Explicit control → lock a cquisition a nd rele a se a re progr a mmer-m a n a ged When to use Locks? • When a dv a nced coordin a tion is required beyond synchronized • When locking a nd unlocking must h a ppen in di ff erent scopes
4. 4 Counter
5. 5 Task
6. 6 Main
7. 7 Lock Hierarchy (java.util.concurrent.locks) • Lock (interf a ce) • Reentr a ntLock is the most common implement a tion of Lock. • St a mpedLock - high-performance, non-reentr a nt lock.
8. Synchronized 8
9. 9 synchronized • Synchronized is a J a v a keyword used to ensure th a t only one thre a d a t a time c a n execute a speci f ic piece of code or method. • It uses a lock implicitly Wh a t do they gu a r a ntee? • Mutu a l exclusion → only one thre a d enters the critic a l section • Visibility → ch a nges a re visible when the lock is rele a sed • Ordering → oper a tions occur in a predict a ble sequence When to use synchronized? • Protecting multiple rel a ted v a ri a bles • Enforcing a ll-or-nothing upd a tes • When code cl a rity a nd correctness m a tter more th a n r a w perform a nce
10. 10 Counter Add Sleeping Time
11. 11 Task
12. 12 Main
13. Atomic 13
14. 14 Atomic An a tomic v a ri a ble is a v a ri a ble th a t c a n be s a fely modi f ied by multiple thre a ds without using locks or synchronized blocks. Wh a t do they gu a r a ntee? • Atomicity → no interle a ving during upd a tes • Visibility → ch a nges a re immedi a tely visible to other thre a ds • No locks → uses low-level CPU comp a re- a nd-sw a p (CAS) When to use Atomic types? • Sh a red counters, f l a gs, or simple st a te • High-contention scen a rios where locks would slow things down • When only one v a ri a ble must be protected
15. 15 Task
16. Main
17. Semaphore 17
18. 18 Semaphore A Sem a phore is a synchroniz a tion mech a nism th a t controls a ccess to a resource by m a int a ining a f ixed number of permits. Thre a ds must a cquire a permit before proceeding a nd rele a se it when f inished. Wh a t do they gu a r a ntee? • Bounded a ccess → only a limited number of thre a ds m a y enter • Visibility → ch a nges a re visible when permits a re rele a sed • No ownership → permits a re not tied to a speci f ic thre a d When to use Sem a phores? • Limiting a ccess to a f inite resource (connections, printers, c a rts) • Throttling concurrency to a void overlo a d
19. 19 Printer
20. 20 Task
21. 21 Main
22. Thread Pools Executor Service 22
23. 23 ExecutorService An ExecutorService is a J a v a component th a t m a n a ges a pool of thre a ds a nd executes t a sks a synchronously without m a nu a lly cre a ting or controlling thre a ds. Wh a t do they provide? • Thre a d reuse → a voids costly thre a d cre a tion • Concurrency control → limits how m a ny thre a ds run a t once • T a sk scheduling → queues a nd executes submitted t a sks • Sc a l a bility → a d a pts to multicore systems When to use ExecutorService? • Running m a ny short-lived t a sks • Limiting p a r a llelism to a v a il a ble CPU cores
24. 24 Task
25. 25 Main
26. 26 Questions
27. Lab 07
28. 28 Concurrency Control St a rter code: • https://github.com/j a viergs/CSC364/tree/m a in/08-control Concurrency mech a nisms explored: • ExecutorService (thre a d pools) • Sem a phore (limited resources) • Reentr a ntLock (exclusive a ccess) • synchronized (sh a red st a te protection)
29. 29 Concurrency Control P a rt 0 – B a seline. Run the progr a m unch a nged. Observe output order a nd termin a tion. Expl a in wh a t e a ch concurrency mech a nism represents P a rt 1 – Sem a phore Experiments (C a p a city Limits). Modify sem a phore permits (1, 2, 3, 10, 50). Observe concurrency, w a iting, a nd execution time. Expl a in why incre a sing permits eventu a lly stops helping P a rt 2 – Removing Sem a phore Protection. Comment out a cquire() / rele a se() c a lls. Observe loss of resource limits. Expl a in the re a l-world bug being simul a ted P a rt 3 – Lock Misuse (De a dlock). Remove unlock() on a Reentr a ntLock. Observe h a ngs a nd incomplete t a sks. Expl a in why a single missing unlock c a n block the system P a rt 4 – Removing synchronized (R a ce Conditions). Remove synchroniz a tion from sh a red st a te upd a tes. Run multiple times a nd observe inconsistent results. Expl a in r a ce conditions using observed beh a vior P a rt 5 – Thre a ds vs CPU Cores. Detect a v a il a ble CPU cores. Experiment with pool sizes (½×, 1×, 2× cores)
30. CSC 364 Introduction to Introduction to Networked, Distributed, and Parallel Computing Javier Gonzalez-Sanchez, Ph.D. javiergs@calpoly.edu Winter 2026 Copyright. These slides can only be used as study material for the class CSC 364 at Cal Poly. They cannot be distributed or used for another purpose. 30