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Deadlock

Deadlock. Examples. P1 requests most of real memory Disk block mgr is swapped out to make room for P1's requests P1 requests disk block 1, which is held by p2 (& vice-versa for another block) Deadlock: disk block mgr cannot come in P1 cannot complete to get out. System Approaches.

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Deadlock

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  1. Deadlock

  2. Examples • P1 requests most of real memory • Disk block mgr is swapped out to make room for P1's requests • P1 requests disk block 1, which is held by p2 (& vice-versa for another block) • Deadlock: • disk block mgr cannot come in • P1 cannot complete to get out

  3. System Approaches • Prevention • Avoidance • Detection & Recovery • Manual mgmt

  4. Conditions for Deadlock • Mutual exclusion on R1 • Hold R1 & request on R2 • No preemption – once a R is requested, the request can't be retracted (because the app is now blocked! • Circularity • All 4 must apply simultaneously • Necessary, but NOT sufficient

  5. Prevention • Design resource mgrs to always prevent at least ONE such condition • Easy in batch systems • Hard/impossible in other systems • Hard to apply to EVERY Rmgr

  6. Avoidance • Predict effects of requests • refuse if deadlock could occur • Underutilizes R • Easy for batch systems • all requests are pre-defined

  7. Detection & Recovery • Periodic (or manual) check for deadlock • implied by response time • expensive • non-productive until D fixed • D is indicated by non-occurrence • is it deadlocked or just waiting normally? • analysis of resource types (I/O vs code) • Recovery • preempt R from holder • delete offending process

  8. Manual mgmt • Contemporary O/S's include detection & prevention algorithms • Not all R are covered due to cost (implementation or to users) • Often, simplest method is reboot

  9. Resource State Diagrams A process P A resource R A request for R R is held by P

  10. The State Transition Model • 3 possible events, E • request - ri • allocate - ai • deallocate - di • Pi  P in sj S  sk S due to x  E sj sk x

  11. A blocked process (P2) Circles are states, not processes. Subscripts represent processes.Arrows are transitions. a1 sj r3 r1 Transitions can occur OUT of Sjonly via the requests from P1 & P3or the allocation to P1.

  12. Creating a complete state diagram Start with 1 process, P1, and only one R at a time may be requested. d d S0 S1 S2 S3 S4 r a r a Now duplicate this diagram for P2. Result is a complex diagram showing all possible states for P1 with all possible states for P2, as well as all the possible transitions.

  13. Prevention : Hold & Wait • Must prevent holding followed by request • Two ways: • request everything at once • release all before making new requests

  14. Prevention: Circular Wait • Draw system transition diagram or graph • Look for a prospective cycle • Disallow allocations that cause the cycle

  15. Prevention: Allow preemption • Pn can "back-out" of a request • This is known as preempting the request State transitions e.g.; a move is possible from Sj to Sk when request n is honored rn sj sk dn rm sm Resource Request Graph

  16. Avoidance • Similar to Prevention • Allows transition if guaranteed to be OK • Analyze new state before entering • System always safe • Unsafe state: no guarantee that deadlock won't occur

  17. The Banker's Algorithm • maxc [ i, j ] is max claim for Rj by pi • alloc [ i, j ] is units of Rj held by pi • cj is the # of units of j in the whole system • Can always compute • avail [ j ] = cj - S0 i < nalloc [ i, j ] • and hence Rj available • Basically examine and enumerate all transitions Classic avoidance algorithm

  18. Banker's Algorithm - Steps 1& 2 • // 4 resource types • C=# avail=<8, 5, 9, 7> • Compute units of R still available (C - col_sum) • avail [0] = 8 - 7 = 1 • avail [1] = 5 - 3 = 2 • avail [2] = 9 - 7 = 2 • avail [3] = 7 - 5 = 2 Step 1:allocalloc' Step 2:computations above yield: avail=<1,2,2,2> Current (safe) Allocation

  19. Banker's Algorithm - Step 3 • Avail=<1,2,2,2> = # currently available for all Rj • Compute: maxc - alloc for each Pi (look for any satisfiable) • alloc' for P2 is <4,0,0,3> (from prev. table) • maxc[2, 0] - alloc'[2,0] = 5 - 4 = 1 ≤ avail[0] ≡ 1 • maxc[2, 1] - alloc'[2,1] = 1 - 0 = 1 ≤ avail[1] ≡ 2 • etc If no Pi satisfies: maxc - alloc' <= avail,then unsafe <stop> If alloc'=0 for all Pi <Stop> Maximum Claims

  20. Banker's algorithm for P0 • maxc[0, 0] - alloc'[0,0] = 3 - 2 = 1 ≤ avail[0] ≡ 1 • maxc[0, 1] - alloc'[0,1] = 2 - 0 = 1 ≤ avail[1] ≡ 2 • maxc[0, 2] - alloc'[0,2] = 1 - 1 = 0 ≤ avail[2] ≡ 2 • maxc[0, 3] - alloc'[0,3] = 4 - 1 = 3 ≤ avail[3] ≡ 2 • Therefore P0 cannot make a transition to a safe state from the current state. • Likewise for P1

  21. Banker's Algorithm - Step 4 • So P2 can claim, use and release all its Rigiving a new availability vector: avail2[0]=avail[0]+alloc'[2,0]=1+4=5 avail2[1]=avail[1]+alloc'[2,1]=2+0=2 avail2[2]=avail[2]+alloc'[2,2]=2+0=2 avail2[3]=avail[3]+alloc'[2,3]=2+3=5 avail2=<5,2,2,5> so at least one P can get its max claim satisfied

  22. Serially Reusable Resources e.g.; disks, networks, CPUs P holds R (1 unit)  A resource A process P requests R (1 unit)  A resource A process

  23. State Transitions due to Requests • In Sj, Pi is allowed to request qch units of Rh, provided pi has no outstanding requests. • Sj Sk, where the RRG for Sk is derived from Sj by adding q request edges from pi to Rh q edges pi Rh pi Rh pi requests q units State Sk State Sj of Rh RRG=Resource Request Graph

  24. Transitions P0 P0 • subscript on state indicates who the requestor was. • r1 is a transition: request for the resource by P1 r1     P1 P1 s00 s01

  25. Consumable Resource Graphs P produces R  P requests R (1 unit)  P requests R (2 unit) 

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