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DATE 2015-04-01

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MESSAGE
DATE 2015-04-13
FROM Ruben Safir
SUBJECT Subject: [LIU Comp Sci] Chapter 5 Monitor
From owner-learn-outgoing-at-mrbrklyn.com Mon Apr 13 12:02:54 2015
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Date: Mon, 13 Apr 2015 12:02:29 -0400
From: Ruben Safir
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Does anyone at all understand this monitor section in Chapter 5 of the
textbook?





Figure 5.15 Syntax of a monitor.
5.8.1
Monitor Usage
An abstract data type—or ADT —encapsulates data with a set of functions
to operate on that data that are independent of any specific implementation
of the ADT. A monitor type is an ADT that includes a set of programmer-
defined operations that are provided with mutual exclusion within the
monitor.
The monitor type also declares the variables whose values define the state
of an instance of that type, along with the bodies of functions that operate
on those variables. The syntax of a monitor type is shown in Figure 5.15.
The representation of a monitor type cannot be used directly by the various
processes. Thus, a function defined within a monitor can access only those
variables declared locally within the monitor and its formal parameters.
Similarly, the local variables of a monitor can be accessed by only the
local
functions.
The monitor construct ensures that only one process at a time is active
within the monitor. Consequently, the programmer does not need to code
this synchronization constraint explicitly (Figure 5.16). However, the
monitor
construct, as defined so far, is not sufficiently powerful for modeling some
synchronization schemes. For this purpose, we need to define additional syn-
chronization mechanisms. These mechanisms are provided by the condition
construct. A programmer who needs to write a tailor-made synchronization
scheme can define one or more variables of type condition:
condition x, y;
226
Chapter 5ProcessSynchronization
shared data
entry queue
. . .
operations
initialization
code
Figure 5.16 Schematic view of a monitor.
The only operations that can be invoked on a condition variable are wait()
and signal(). The operation
x.wait();
means that the process invoking this operation is suspended until another
process invokes
x.signal();
The x.signal() operation resumes exactly one suspended process. If no
process is suspended, then the signal() operation has no effect; that
is, the
state of x is the same as if the operation had never been executed (Figure
5.17). Contrast this operation with the signal() operation associated with
semaphores, which always affects the state of the semaphore.
Now suppose that, when the x.signal() operation is invoked by a process
P, there exists a suspended process Q associated with condition x.
Clearly, if the
suspended process Q is allowed to resume its execution, the signaling
process
P must wait. Otherwise, both P and Q would be active simultaneously within
the monitor. Note, however, that conceptually both processes can continue
with their execution. Two possibilities exist:
1.2.Signal and wait. P either waits until Q leaves the monitor or waits for
another condition.
Signal and continue. Q either waits until P leaves the monitor or waits
for another condition.
queues associated with
x, y conditions
x
y
shared data
entry5.8 Monitors
queue
227
• • •
operations
initialization
code
Figure 5.17 Monitor with condition variables.
There are reasonable arguments in favor of adopting either option. On
the one hand, since P was already executing in the monitor, the signal-and-
continue method seems more reasonable. On the other, if we allow thread P
to continue, then by the time Q is resumed, the logical condition for
which Q
was waiting may no longer hold. A compromise between these two choices
was adopted in the language Concurrent Pascal. When thread P executes the
signal operation, it immediately leaves the monitor. Hence, Q is immediately
resumed.
Many programming languages have incorporated the idea of the monitor
as described in this section, including Java and C# (pronounced “C-sharp”).
Other languages—such as Erlang—provide some type of concurrency support
using a similar mechanism.

  1. 2015-04-01 Ruben <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] external paths
  2. 2015-04-01 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] omp pthread madness
  3. 2015-04-02 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: Fw: Perl Developer / Linux SysAdmin Opportunities in South Florida
  4. 2015-04-02 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: omp pthread madness
  5. 2015-04-02 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: omp pthread madness
  6. 2015-04-02 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: [New discussion] which technologies/skills are good to have
  7. 2015-04-02 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Re: omp pthread madness
  8. 2015-04-05 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Re: Depths and heights
  9. 2015-04-09 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Re: Kernel thread scheduling
  10. 2015-04-09 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Re: Kernel thread scheduling
  11. 2015-04-13 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] BASh Shell Scripting
  12. 2015-04-13 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Chapter 5 Monitor
  13. 2015-04-13 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: [Perlweekly] #194 - The CPAN PR Challenge Now Has a Mini-Me!
  14. 2015-04-13 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Kernel Hacking Resources
  15. 2015-04-14 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: Invitation: Vintage Computer Festival
  16. 2015-04-14 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] hashing slots sizes
  17. 2015-04-15 Ruben <mrbrklyn-at-panix.com> Re: [LIU Comp Sci] BASh Shell Scripting
  18. 2015-04-15 Maneesh Kongara <maneeshkongara-at-gmail.com> Re: [LIU Comp Sci] BASh Shell Scripting
  19. 2015-04-15 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: Monday: Join 121 HackerNestologists at "HackerNest NYC April
  20. 2015-04-15 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: You're confirmed for: NYLUG Open hacker hours
  21. 2015-04-15 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Hash Tables Examples
  22. 2015-04-15 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Re: hashing quadradic probes
  23. 2015-04-15 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] [ruben-at-mrbrklyn.com: Re: Heap Management Problem]
  24. 2015-04-16 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: Re: virtualbox
  25. 2015-04-16 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] linsched
  26. 2015-04-16 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] scheduler project thus far
  27. 2015-04-17 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Come Sunday Morning
  28. 2015-04-17 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Kernel Scheduling Project
  29. 2015-04-17 Ruben <ruben.safir-at-my.liu.edu> Subject: [LIU Comp Sci] more advanced coding musings ...
  30. 2015-04-19 Tony Genao <tony.genao-at-my.liu.edu> Re: [LIU Comp Sci] Fwd: [NYLXS - HANGOUT] Summers Here
  31. 2015-04-19 Ruben Safir <mrbrklyn-at-panix.com> Re: [LIU Comp Sci] Fwd: [NYLXS - HANGOUT] Summers Here
  32. 2015-04-19 Tony Genao <tony.genao-at-my.liu.edu> Re: [LIU Comp Sci] Fwd: [NYLXS - HANGOUT] Summers Here
  33. 2015-04-19 Ruben <mrbrklyn-at-panix.com> Re: [LIU Comp Sci] Fwd: [NYLXS - HANGOUT] Summers Here
  34. 2015-04-19 Tony Genao <tony.genao-at-my.liu.edu> Re: [LIU Comp Sci] Fwd: [NYLXS - HANGOUT] Summers Here
  35. 2015-04-19 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: [NYLXS - HANGOUT] Summers Here
  36. 2015-04-19 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Re: The more I learn the less I know
  37. 2015-04-22 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: Re: wait queues semiphores kernel implementations
  38. 2015-04-22 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] linux locks and mutex
  39. 2015-04-23 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: Re: Kernel Scheduler and wiat queues
  40. 2015-04-25 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: Kernel Scheduling and wait queues
  41. 2015-04-27 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: OS Cpt 9 27 question
  42. 2015-04-27 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: Tomorrow: You and 30 other Flatirons are going to "Flatiron
  43. 2015-04-28 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Re: DMCA exemption commenting process broken beyond repair
  44. 2015-04-29 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Do you have notes for this question
  45. 2015-04-29 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Professional Troll job available
  46. 2015-04-29 Ruben <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Re: DMCA exemption commenting process broken beyond repair

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