5advance( E )Signals occurrence of an eventUpdate eventcount value
6read( E ) Returns value of eventcount May or may not count events in progress
7await( E, v ) Similar to read( E ) Waits for value v to be reached May not return immediately once the vth advance is executed
8Producer/Consumer Example N-cell ring bufferbuffer[0:N –1]Eventcounts IN and OUTproduce() to generate items
9Single producer code Procedure producer() begin integer i; for i:= 1 to infinity dobeginawait( OUT, i – N);buffer[i mod N] := produce( );advance( IN );end
10Single consumer code Procedure consumer() begin integer i; Procedure producer()begin integer i;for i:= 1 to infinity dobeginawait( OUT, i – N);buffer[i mod N] := produce( );advance( IN );endProcedure consumer()begin integer i;for i := 1 to infinitybeginawait( IN, i );consume( buffer[i mod N]);advance( OUT );end
11Possible Situations Fast Producer Fast Consumer Producer will wait until item it is trying to overwrite is consumed.Fast ConsumerConsumer will wait until the producer has added the value.
12EventCount observations How is this solution different than semaphores?Relative ordering rather than mutual exclusion.Producer & consumer can be concurrent.Never does a process have to wait due to synchronization.
13Sequencers Used when synchronization requires arbitration Used to order the eventsUseful with Eventcounts but not on its ownNon-decreasing integer value
14Sequencer operations ticket(S) Value returned is the process’s ordering.Two calls to ticket( S ) will always return different values.
15Producer/Consumer Example Same as with Eventcounts but multiple producers nowSequencer TUse ticket(T) to synchronize with other producers
16buffer[t+1mod N] := produce( ); advance( IN ); end Procedure producer()begin integer i;for i:= 1 to infinity dobeginawait( OUT, i – N);buffer[i mod N] := produce( );advance( IN );endProcedure producer()begin integer t;do foreverbegint := ticket(T);await( IN, t);await( OUT, t – N + 1 )buffer[t+1mod N] := produce( );advance( IN );end
17Relation to semaphores Lower level than SemaphoresSemaphores can be built from Eventcounts and Sequencers
18Semaphore Example Semaphore S EventCount is S.E Sequencer is S.T Initial value of S is S.I
19Semaphore Wait Procedure P(S) begin integer t; t := ticket( S.T ); await( S.E, t – S.I );end
20Semaphore SignalProcedure V( S )beginadvance( S.E )end
21Deadlock Free Simultaneous P 2 Semaphores R and SGlobal semaphore G to synchronize part of operation
22Procedure Pboth( R, S )begin integer g, r, s;g := ticket( G.T );await( G.E, g );r := ticket( R.T );s := ticket( S.T );advance( G.E );await( R.E, r – R.I );await( S.E, s – S.I );end
23Observations G is used for obtaining tickets await operation could be deferredUseful in solving the Dining Philosophers Problem
24Observations Process destroyed while holding 1 ticket Keep the window during which a process has an unredeemed a ticket shortDon’t allow destroying the process during the window
25Flow of information operations are: Observer or Signaler unlike the semaphore waitEasily adapted to permissionsObserver permission(advance).signaler permission(read,await).Useful in secure systems
26Secure Readers - Writers Shared databaseReaders have observer permissionWriters have observer and signaler permissionWriter priorityS and C are eventcountsT is a sequencer
27Reader code Procedure reader() begin integer w; abort: w := read(S); await(C, w);“read DB”if read(S) ≠ w then goto abort;end
28Writer code Procedure writer() begin integer t; advance( S ); t := ticket( T );await( C, t );advance( C );end
29Conclusion new mechanism for synchronization. not based on mutual exclusion.Provides an interface between processes.Information flow paths are clear.Effective in secure systems.Unnecessary serialization avoided.