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Synchronization with Eventcounts and Sequencers David P. Reed Rajendra K. Kanodia.

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Presentation on theme: "Synchronization with Eventcounts and Sequencers David P. Reed Rajendra K. Kanodia."— Presentation transcript:

1 Synchronization with Eventcounts and Sequencers David P. Reed Rajendra K. Kanodia

2 Introduction What is it used for? Synchronizing the use of shared resources How is it different from semaphores and monitors? Mutual exclusion

3 EventCounts What is an eventcount? Tracks number of events Non-decreasing integer variable

4 EventCount operations advance(E) read(E) await(E, v)

5 advance( E ) Signals occurrence of an event Update eventcount value

6 read( E ) Returns value of eventcount May or may not count events in progress

7 await( E, v ) Similar to read( E ) Waits for value v to be reached May not return immediately once the vth advance is executed

8 Producer/Consumer Example N-cell ring buffer buffer[0:N – 1] Eventcounts IN and OUT produce() to generate items

9 Single producer code Procedure producer() begin integer i; for i:= 1 to infinity do begin await( OUT, i – N); buffer[i mod N] := produce( ); advance( IN ); end

10 Single consumer code Procedure consumer() begin integer i; for i := 1 to infinity begin await( IN, i ); consume( buffer[i mod N]); advance( OUT ); end Procedure producer() begin integer i; for i:= 1 to infinity do begin await( OUT, i – N); buffer[i mod N] := produce( ); advance( IN ); end

11 Possible Situations Fast Producer Producer will wait until item it is trying to overwrite is consumed. Fast Consumer Consumer will wait until the producer has added the value.

12 EventCount 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.

13 Sequencers Used when synchronization requires arbitration Used to order the events Useful with Eventcounts but not on its own Non-decreasing integer value

14 Sequencer operations ticket(S) Value returned is the process s ordering. Two calls to ticket( S ) will always return different values.

15 Producer/Consumer Example Same as with Eventcounts but multiple producers now Sequencer T Use ticket(T) to synchronize with other producers

16 Procedure producer() begin integer t; do forever begin t := ticket(T); await( IN, t); await( OUT, t – N + 1 ) buffer[t+1mod N] := produce( ); advance( IN ); end Procedure producer() begin integer i; for i:= 1 to infinity do begin await( OUT, i – N); buffer[i mod N] := produce( ); advance( IN ); end

17 Relation to semaphores Lower level than Semaphores Semaphores can be built from Eventcounts and Sequencers

18 Semaphore Example Semaphore S EventCount is S.E Sequencer is S.T Initial value of S is S.I

19 Semaphore Wait Procedure P(S) begin integer t; t := ticket( S.T ); await( S.E, t – S.I ); end

20 Semaphore Signal Procedure V( S ) begin advance( S.E ) end

21 Deadlock Free Simultaneous P 2 Semaphores R and S Global semaphore G to synchronize part of operation

22 Procedure 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

23 Observations G is used for obtaining tickets await operation could be deferred Useful in solving the Dining Philosophers Problem

24 Observations Process destroyed while holding 1 ticket Keep the window during which a process has an unredeemed a ticket short Don t allow destroying the process during the window

25 Flow of information operations are: Observer or Signaler unlike the semaphore wait Easily adapted to permissions Observer permission(advance). signaler permission(read,await). Useful in secure systems

26 Secure Readers - Writers Shared database Readers have observer permission Writers have observer and signaler permission Writer priority S and C are eventcounts T is a sequencer

27 Reader code Procedure reader() begin integer w; abort: w := read(S); await(C, w); read DB if read(S) w then goto abort; end

28 Writer code Procedure writer() begin integer t; advance( S ); t := ticket( T ); await( C, t ); advance( C ); end

29 Conclusion 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.


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