Course Syllabus 1. Introduction - History; Views; Concepts; Structure 2. Process Management - Processes; State + Resources; Threads; Unix implementation of Processes 3. Scheduling – Paradigms; Unix; Modeling 4. Synchronization - Synchronization primitives and their equivalence; Deadlocks 5. Memory Management - Virtual memory; Page replacement algorithms; Segmentation 6. File Systems - Implementation; Directory and space management; Unix file system; Distributed file systems (NFS) 7. Virtualization – Virtual machines, type I and II hypervisors, classic virtualization, sensitive and privileged instructions, binary translation, memory virtualization 7. Distributed Synchronization (if there's time) Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

Mutual exclusion: motivation Race Conditions: Example: Spooler directory with slots; index variables; two processes attempt concurrent access. In points to the next empty slot, Out points to entry holding name of file to print next. Process A 4 5 6 7 . . . . . . abc.doc f.ps paper.ps In=7 Process B Out = 4 Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

Code using reads/writes only Add-file-to-print-spool (char *name) Local-in := In StoreName(Spool-dir[local-in], name) In := (local-in+1) mod DIR-LEN A scenario proving the above code wrong Process A performs line 1 Process A is preempted by Process B Process B performs Add-file-to-print-spool to completion Process A is re-scheduled, completes lines 2-3. Process B's file is never printed. Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

The mutual exclusion problem (Dijkstra, 1965) How can we avoid such race conditions? We need to devise a protocol that guarantees mutually exclusive access by processes to a shared resource (such as a file, printer, etc.) or, more generally, a critical section of code Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

The problem model Multiple processes Processes can apply read, write, or stronger operations to shared memory Completely asynchronous We assume processes do not fail-stop Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

Mutual exclusion: formal definition loop forever Remainder code Entry code Critical section (CS) Exit code end loop Remainder code Entry code CS Exit code Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

Mutex Requirements Mutual exclusion: No two processes are at the critical section (CS) at the same time Deadlock-freedom: If processes are trying to enter their critical section, then some process eventually enters the critical section Starvation-freedom: If a process is trying to enter its critical section, then this process must eventually enter its critical section Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

Mutual exclusion using critical regions Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

Brute force solution: disabling interrupts Disable interrupts Do your stuff Enable interrupts Problems User processes are not allowed to disable interrupts Disabling interrupts must be done for a very short period of time Does not solve the problem in a multi-processor system Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

2-process Mutual Exclusion with a lock variable initially: lock=0 await c: repeatedly evaluate condition c until true Program for both processes await lock=0 lock:=1 CS lock:=0 No Does the algorithm satisfy mutex? Does it satisfy deadlock-freedom? Does it satisfy starvation-freedom? Yes No Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

Mutual Exclusion: strict alternation initially: turn=0 Program for process 0 await turn=0 CS turn:=1 Program for process 1 await turn=1 CS turn:=0 Yes Does the algorithm satisfy mutex? Does it satisfy deadlock-freedom? Does it satisfy starvation-freedom? No No Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

Mutual Exclusion: flag array bool flags[2] initially {false, false} Program for process 0 flags[0]:=true await flags[1]=false CS flags[0]:=false Program for process 1 flags[1]:=true await flags[0]=false CS flags[1]:=false Yes Does the algorithm satisfy mutex? Does it satisfy deadlock-freedom? Does it satisfy starvation-freedom? No No Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

Peterson’s 2-process algorithm (Peterson, 1981) initially: b[0]:=false, b[1]:=false, initial value of turn immaterial Program for process 0 b[0]:=true turn:=0 await (b[1]=false or turn=1) CS b[0]:=false Program for process 1 b[1]:=true turn:=1 await (b[0]=false or turn=0) CS b[1]:=false Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

Schema for Peterson’s 2-process algorithm Indicate participation b[i]:=true Barrier turn:=i no, maybe Is there contention? b[1-i]=true? yes First to cross barrier? turn=1-i? no yes Critical Section Exit code b[i]:=false Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili Synchronization Algorithms and Concurrent Programming , Gadi Taubenfeld © 2006

Peterson’s 2-process algorithm satisfies both mutual-exclusion and starvation-freedom Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

Mutual exclusion for n processes: A tournament tree Level 2 1 2 3 4 5 6 7 Level 1 Level 0 Processes A tree-node is identified by: [level, node#] Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili Synchronization Algorithms and Concurrent Programming , Gadi Taubenfeld © 2006

N-process mutual exclusion: a tournament tree Variables Per node: b[level, 2node], b[level, 2node+1], turn[level,node] Per process (local): level, node, id. Program for process i node:=i For level = 0 to log n-1 do id:=node mod 2 node:= node/2 b[level,2node+id]:=true turn[level,node]:=id await (b[level,2node+1-id]=false or turn[level,node]=1-id) end do CS for level=log n –1 downto 0 do node:=  i/2level b[level,2node+id]:=false // id is value set at this node in entry Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

Fairness: First in First Out (FIFO) Mutual Exclusion Deadlock-freedom Starvation-freedom remainder doorway waiting entry code critical section exit code FIFO: if process p is waiting and process q has not yet started the doorway, then q will not enter the CS before p Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili Synchronization Algorithms and Concurrent Programming , Gadi Taubenfeld © 2006

Lamport’s Bakery Algorithm 1 2 3 4 5 n remainder 1 3 2 2 4 doorway entry waiting 1 3 2 4 2 1 2 2 CS time 1 2 2 exit Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili Synchronization Algorithms and Concurrent Programming , Gadi Taubenfeld © 2006

Implementation 1 code of process i , i  {1 ,..., n} number[i] := 1 + max {number[j] | (1  j  n)} for j := 1 to n (<> i) { await (number[j] = 0)  (number[j] > number[i]) } critical section number[i] := 0 1 2 3 4 n number integer Does this implementation work? Answer: No, it can deadlock! Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili Synchronization Algorithms and Concurrent Programming , Gadi Taubenfeld © 2006

Implementation 1: deadlock 2 3 4 5 n remainder 1 2 2 doorway entry waiting 1 2 2 1 deadlock CS time 1 exit Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili Synchronization Algorithms and Concurrent Programming , Gadi Taubenfeld © 2006

Implementation 2 code of process i , i  {1 ,..., n} number[i] := 1 + max {number[j] | (1  j  n)} for j := 1 to n (<> i) { await (number[j] = 0)  (number[j],j) < number[i],i) // lexicographical order } critical section number[i] := 0 1 2 3 4 n number integer Does this implementation work? Answer: It does not satisfy mutual exclusion! Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili Synchronization Algorithms and Concurrent Programming , Gadi Taubenfeld © 2006

Implementation 2: no mutual exclusion 1 2 3 4 5 n remainder 1 2 2 doorway entry waiting 1 2 2 1 2 2 CS time 1 exit Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili Synchronization Algorithms and Concurrent Programming , Gadi Taubenfeld © 2006

The Bakery Algorithm code of process i, i  {1 ,..., n} 1: choosing[i] := true 2: number[i] := 1 + max {number[j] | (1  j  n)} 3: choosing[i] := false 4: for j := 1 to n do 5: await choosing[j] = false 6: await (number[j] = 0)  (number[j],j)  (number[i],i) 7: od 8: critical section 9: number[i] := 0 Doorway Waiting Bakery 1 2 3 4 n choosing false false false false false false bits number integer Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

Lamport’s bakery algorithm satisfies mutual-exclusion, starvation-freedom, and FIFO Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

The test-and-set instruction initially: v:=0 Test-and-set(w) do atomically prev:=w w:=1 return prev Program for process i await test&set(v) = 0 Critical Section v:=0 Mutual exclusion? Yes Deadlock-freedom? Yes Starvation-freedom? No Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

Starvation-free mutex using test-and-set boolean lock initially 0, interested[n] initially false program for process i interested[i]:=true await ( (test-and-set(lock) = 0) || (interested[i]=false) ) CS interested[i]:=false j:=(i+1) % n while (j != i && !interested[j]) j:=++j % n if (j=i) lock:=0 else interested[j]:=false Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili