Concurrency Intro

1. Concurrency Intro • Interleaving (multiprogramming) produces many problems • Relative speed of two processes is not generally predictable • Must be careful when sharing global resources • O.S. Data Structures, Files, etc. • Protecting system from bad processes • Difficult to allocate resources optimally if a process is blocked or suspended • Errors tend to be difficult to find • Often dependent on exact timing • Example: Interrupt A requires 1ms to process, but error occurs if interrupt B happens during that period • Multiprocessing increases problems, but not drastically

2. Definitions • Mutual Exclusion – Making sure two processes can’t both have a resource at the same time • Critical Section – The area of a program where a resource is being used. (Processes must have mutual exclusion while executing a critical section.)

3. Definitions • Deadlock – When two or more processes halt, unable to proceed • P1 is using A, needs B • P2 is using B, needs A Both wait for one another indefinitely!! (i.e. the wait is mutual) • Starvation – A process is denied a resource indefinitely • P1 using A, P2 and P3 wait for A • P2 gets A when P1 done • P1 comes in, P1 and P3 wait for A • And so on …

4. Concurrency Requirements • Mutual Exclusion must be enforced • Only one process at a time may be accessing the critical section • A process can halt outside the critical section without harm • No deadlock or starvation (a process that wants to will eventually get into critical section)

5. Concurrency Requirements • If no process is in a critical section, a process requesting entry must be allowed to enter without delay • No assumptions about number or relative speed of processes or processors • A process remains inside its critical section for a limited period of time • Can be solved by software or hardware, with or without O.S. support

6. Process P1 -- Start CS -- Load r, n Add r, 1 Store r, n -- End CS -- Process P2 -- Start CS – Load r, n Add r, 1 Store r, n -- End CS -- Use of critical section avoids race conditions!

7. Software Approaches • Turn variable (figure 5.2a) P0 P1 while(turn!=0)/**/; while(turn!=1)/**/; (Do nothing loop) (Do nothing loop) critical section critical section turn = 1; turn = 0; • Shared variable turn indicates who is allowed to enter next • can enter if turn = me • On exit, point variable to other process • Starvation if other process never enters

8. Software Approaches • “Busy” Flag (figure 5.2b) P0 P1 while(flag[1])/**/; while(flag[0]) /**/; flag[0] = true; flag[1] = true; critical section critical section flag[0] = false; flag[1] = false; • Each process has a flag to indicate it is in the critical section • Fails mutual exclusion if processes are in lockstep *** Somebody explain this!!! ***

9. Software Approaches • Busy Flag Modified (figure 5.2c) P0 P1 flag[0] = true; flag[1] = true; while ( flag[1] ) /**/; while ( flag[0] ) /**/; critical section critical section flag[0] = false; flag[1] = false; • Deadlocks if processes are in lockstep *** Explain how !! ***

10. Software Approaches • Busy Flag Again (figure 5.2d) P0 P1 flag[0] = true; flag[1] = true; while ( flag[1] ) while ( flag[0] ) { { flag[0] = false; flag[1] = false; delay delay flag[0] = true; flag[1] = true; } } critical section critical section flag[0] = false; flag[1] = false; -- Defer if other process wants the CS • Livelock if processes are in lockstep

11. Software Approaches • Dekker’s Algorithm (figure 5.3a) • Use flags for mutual exclusion, turn variable to break deadlock • Handles mutual exclusion, deadlock, and starvation

12. Software Approaches • Peterson’s Algorithm (figure 5.3b) P0 P1 flag[0] = true; flag[1] = true; turn = 1; turn = 0; while ( flag[1] && while ( flag[0] && turn == 1 ) /**/; turn == 0 ) /**/; critical section critical section flag[0] = false; flag[1] = false; • What if we have more than two processes? -- A process must check everyone else’s flag

13. Hardware Support • Disable Interrupts • Only works for uniprocessors • Delays response to external events • Special Instructions • Must appear as a indivisible unit to other processors • Test and Set – If V is 0, set to 1 and return true, else return false • May let user specify test/set values • Exchange – Swap reg and memory • Example uses: figure 5.5a, b • Properties • Simple • One variable per critical section • Often use busy waiting • Possibility of deadlock or starvation

14. Semaphores • First defined by Dijkstra • Semaphore is an integer with a non-negative initial value • Two atomic operations are defined: • Wait (S) S --; if S<0 then block the process else continue • Signal (S) S++; if S <0, wake one blocked process

15. Binay Semaphore • Can only take on values 0 and 1. • Initial value is 1 • Ideal for implementing Critical sections S: Binare semaphore:= 1; Process p1 Process p2 Wait (S) Wait (S) -- CS -- -- CS -- Signal (S) Signal (S)

16. Semaphores • Provide a very useful form of mutual exclusion • Initialize semaphore s to 1 do forever: wait(s) critical section signal(s) • Implementing Wait/Signal • Hardware primitives (Fig. 5.17, pg 229) • Busy wait only occurs inside wait/signal • Software methods • Semaphore internals are the “critical section” for the low-level methods • Generally these are short enough to be acceptable • Meaning of count: • if count ³ 0 : how many processes can call wait() without blocking • if count < 0 : how many processes are currently blocked on this semaphore

17. Producers & Consumers • One process produces some type of data • The other process consumes that data • Data stored in a shared buffer (infinite size) • Require mutual exclusion to access buffer -- A Full buffer must not be overwritten! -- An empty buffer must not be consumed!!

18. Producers & Consumers s: bin sem, delay: sem, n=0; • Producer do forever produce item wait(s) append to queue n++ if n = 1 then signal(delay) signal(s) • Consumer wait(delay) do forever wait(s) remove from queue n-- m = n signal(s) if m = 0 then wait(delay)