Question

In order to prevent deadlocks, let a system adopt the policy of forcing processes to request resources in ascending order of request type rank. In particular, when a process is requesting a resource of a certain type, the process cannot be holding other higher ranked resources.

Consider four resource types with the following profile in this system:

Resource Type	Rank	Number of Instances
RT1	4	2
RT2	7	4
RT3	8	3
RT4	10	2

Let the following sequence of resource requests be processed by the system (note that REQ(A, B, k) represents the request of k resources of type B by process A and REL(A, B, k) represents process A releasing k resources of type B):

• REQ(P4, RT2, 3)
• REQ(P3, RT3, 2)
• REQ(P2, RT4, 1)
• REL(P4, RT2, 3)
• REQ(P4, RT1, 2)
• REL(P2, RT4, 1)
• REL(P3, RT3, 2)
• REQ(P3, RT2, 3)
• REQ(P2, RT3, 2)
• REQ(P1, RT4, 2)

Note that all of the above request and release operations can be processed without any violation of the deadlock-prevention policy of the system.

For each of the following sequences of request/release operations, examine if it can or cannot be processed without violating the deadlock-prevention scheme above. Identify the sequence that is allowed by the scheme and consequently can be processed without leading to system deadlock?

• A. REL(P2, RT3, 2), REQ(P4, RT4, 2), REQ(P2, RT4, 2), REQ(P1, RT1, 2)
• B. REQ(P2, RT4, 2), REL(P1, RT4, 2), REQ(P4, RT4, 2), REQ(P1, RT1, 2)
• C. REL(P2, RT3, 2), REQ(P3, RT4, 2), REQ(P2, RT1, 2), REQ(P1, RT2, 2)
• D. REL(P3, RT2, 3), REQ(P2, RT4, 2), REQ(P4, RT2, 2), REQ(P1, RT3, 2)

Answer 1

ven- Resource Rank No. of Instances RT1 RT 2 It RT3 8 . 2- . RT4 T 10 Processing Given Roquests first REQ(P4, RT2,3): Allocate 3 Instances of RT 9 to P4. Then I instance of RTa PsRemoins @ 132 2. REO (P3, RT3,2)=- Allocate a Instances of RT3 To Pz' 3. REQ (P22 RT4, 1) =- Allocate of Rty to Pai - Allocate 1 Instance t . I Nowi RT RT2 RT3 RT4 4- REL.(PU, RT213)--; Pre Releases 3 Instances of RTE: Now; RT I RT2 RT 3 | RTY 5- REQ (P4, R1, ,12)=- Py is not holding ariy Resource. It already Releases -Xesource allocated to its so No problema - Allocate a instances of RT to Py. :

now, JRT RT RT RTul . 6. REL(P2, RT4; 1): Pa' releases I instance of Rty allocated to it Prough | RT_RTI R 7. RELCP3, RT3 12) Pz releases. g instances of RT3 allocated to its . voidy RTI | RTE T3 | रा५ । 8. REQ (R3, R12,3)-- Pz is not holding any resource. It already releases resource allocated to it. So.: No problem. Allocate 3 instances of RTX to :: Now, let, RT2 RTS | RTP TOL3 & q. RES(frRT3YQ). Pz pas not holding any resource.So No poblm . Allocate a instances of RTG to P25 :: IRT /RT2 RT3 RT4] To 11 2 Allocate 2 instances of Rty to Pin NÓW, heti tanto Now; 10 Real P1, RT4, 2) =

After completion of given request release operations, we have Available :- Resource Rank No of Instances RT RT2 77 1 RT 2 1 8 1 1 | RTY 10 lo AND 2- Py holding RT, (a instances) Pz holding RT2_(3 instances). Pa holding RT3 (a instances) Pi holding Rty (a instances). A REL (P2, RT3,2), REG (P4, RT4,2), REO (PARTY, 3); RED(P4, RT112):- Ki REL (P2, RT312)-- S P2 releases. & instances Of Rte. 6 Now , ERTI RT2 RT3 RTY 12. RES (PY, RT412):- E Py is holding Rti į requesting for Rly. Rank (RTU) < Rank CRTY). Not violating the given Scheme: Pue is requesting resources in the ascending order of request type rank. So no prblm- Allocate'ą instances. Of RTY to Py: But we don't have any instances of RTy available motor So leave the request & proces

13. REO (P2, R14, 2)2- P₂ holding RP3 & requesting RTY Rank (RT2)'s Rank(Rty). not violating guli cam allocate: But RTy res his not available. ource . 14. REQ (P1,RT1,2)= pa holding RTU & requesting RT Rank (RTU) > Rank CRTA). violating the scheme. Not allowed. 5.) No request can be granted. This leads to deadlock violating the scheme. ." B) RES (Pay RT4,3), REL(P4, RT4, 3) · RES (P4, R74,2), REB (P1, RT!,252- 11. RED (P21R74,274- B is holding RT3 & fequesting Rtyn Rank (RT3) < Rank (RTU). Not violating the scheme, ção allocate. But : RTY resource is not available. R REICP4, RT412) P1 releagės a instances. of RTY. Rank (RED = 10 Now, 10 717 13. RED (Py, RT42) - Pu PS holding ATI e requesting Rtyu Rank CRTD < Rank CRT). Not violating Silloreta instances of RTy to py:

NOW, TRT1 RT RT3 RTY 0110 Po Holding RT3 (2. instances) P2 Pg » RT2 (3 instances) py → RTI (2 Pnstances); RTU (ZPnstances). 4. RED (PI, RT1,2)== PA didn't holding any resource. So Not violating schemei can allocate. But RTL resource is not available: Pie P P₂ P3 n-allocated in To ( requesting қта, ки / RT3 КТg : Pu holding Rty, RT, & Pi requesting RTE P2 requesting , RT4: · If Py releases a instances of RTU.BE & instances of Rt. Then they can be a allocated to pipe according to their requirements • No request I release operation iſ violating the scheme. • But due to request of Palla, the system leads to deadlock, BCO2 P1 i P2 can't be completed until 84 releases RTU, RTE

O REL(P2, RT3,2?, RES(PIRTY, 2), REB (PZ/RT13), REES (P1, RT3139:- I CI REL(P2, RT312) : : Po releases a instances of RT3. Now, IRT I RI 2 RT3 RT4 / 01.13,10 12- RED(P3, RTY, 272- P3 holding RT 2 & requesting Rty. Not violating scheme. can allocate' But RTA Po not available 13. RES (PART 1333- P zPe not holding any resourcer soʻcam allocate RT to P2 But RT resource is not L available 147 REES (PI, RT2,2)-- - Pi is holding Rly & requesting RT1. Rank (RTU) > Ranki (RTI). violating the scheme. cant ge granted No other request can be granted so Dead lockar so violating the scheme also. P P , Ps Pu DRT >rti RTY K

D) RELCP3, RT2;3), REB (P2, RT412) REQ (PY, R 12,2), REQ (P4, RT3,2). 1. REL(P3, RT2,3)=- Ps releases 3. instances of RT2: . . Now, RT RT2 RT3 RTY 12: RED (P21 RTY, 2)-- Pz po holding Ritz & requesting Rty.. Rank CRTE) < Rank CRTY)- Not violating scheme. can allocater But no instance of RTU is available i 3. RED (P4, RT2;272- Pu holding RM & requesting RT2 can allocate Rank (RTU) <Rank (RT3) ieinot violating scheme- . so allocate a instances of RT2 to Py? . Now; Rti | RT2 | RT3 | Rty / 14 REQ (P1, R13 12) = L Pe holding RTÜ te requesting RT3 Ramn (RTU) > Rank ('RDE)** 3 ) violating the scheme No request can be granted =1) Deadlock ē. violating the "Scheme alson