Difference between revisions of "Nondetermism"

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= Nondeterministic execution in BPJ model checking =
 
In common processing event selection in BP is deterministic.
 
In common processing event selection in BP is deterministic.
However, in some cases it is desired to specify nondetermistic choices - e.g., when simulating environment behavior using b-threads as part of model checking. The BPmc model checker explores all possible execution paths defined by this nondeterminism.
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However, in some cases it is desired to specify nondetermistic choices - e.g., when simulating environment behavior using b-threads as part of model checking. The BPJ model checking capability explores all possible execution paths defined by this nondeterminism.
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In fact, often, the only non-determinism in execution is in b-threads that simulate the environment.
  
Setting up nondeterminism:
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== Specifiying nondeterminism ==
* In BP, event selection is deteremined by the unique priority of each b-thread, and by the order of events in the requested event set.  
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* In BP, event selection is deteremined by the unique priority of each b-thread, and by the order of events in the requested event set.
* When the difference in priority of two b-threads is less than a given "epsilon" (a real number provided by the user), the two b-threads  are considered of same priority.  For example, to create 48 bthreads of the same priority, choose epsilon of 1.00 and  create the set of these b-threads so that their priorities have the same whole part and differ only by fractional part.  (e.g. priority values of 7.001 – 7.048).
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* When the difference in priority of two b-threads is less than a given "epsilon" (a real number provided by the user), the two b-threads  are considered of same priority.  For example, to create 48 bthreads of the same priority, choose epsilon of 1.00 and  create the set of these b-threads so that their priorities have the same whole part and differ only by fractional part.  (e.g. priority values of 7.001 – 7.048). The value of "epsilon" is determined by the method call setBthreadEpsilon().
* In a given requested-events set, events that are in the same second level event set are considered of same priority . E.g. in the requested event set R={a, {b,c,d},{e,f,g}}. b,c,d are considered of the same priority, and e,f,g are considered of the same priority.
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* Additionally, during model checking if an event in a requested event set of any b-thread is selected (requested and not blocked, and non-deterministic execution actually selected it), then if the event was in a nested second-level set, then all other events in the same nested second-level set are considered to be of the same priority. For example, if the requested even set is R={{e1,e2},e3,{e4,e5,e6,e7},e8} - then if e5 was selected (say, all preceding events were blocked), then e6 and e7, and only them are considered of the same priority and will be explored in non-deterministic execution. This priority processing is relevant only to the first event selection in b-thread. Once the selected second-level set is exhausted, (or if the event was a first level in the first place), non-deterministic execution continues with the next b-thread of the "same" priority as the previous b-thread.
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See execution below for more details.
  
How BPJ determines non-detereministic selection :
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== Nondeterministic execution ==
Scan b-threads in order of priority
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Scan the requested event set of each b-thread in event order (the set is enumerable.
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When executing in model-checking mode, nondeterministic event selection is performed according to the following algorithm :
When the first event that is requested and not blocked is found  it is one alternative.
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If it  was in a  second level event set (a set within a set),  all its siblings in the same set are considered having the same priority, and are added as alternatives (if not blocked).   If it is in a first level of the requested event set, this event is the only one from this b-thread.
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* Scan b-threads in order of priority
When done with the alternatives from one b-thread (first requested and not blocked, and possibly its sibling in the second level set) –go to the next b-thread of “same” priority as defined with epsilon, and repeat the above . At most one second-level set will be selected from the requested events of that b-thread, but not necessarily the first one).
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* Scan the requested event set of each b-thread in event order (the set is enumerable).
Example
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* When the first event that is requested and not blocked is found  it is remembered as one alternative.
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* If the selected event was in a  second level event set (a set within a set),  all its siblings in the same second level set that are not blocked are considered having the same priority, and are added as alternatives.
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* If the event is in a first level of the requested event set, this event is the only one from this b-thread.
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* After exploring all execution subtrees anchored at the alternatives from one b-thread go to the next b-thread of “same” priority as defined with epsilon, and repeat the above . At most one second-level set will be selected from the requested events of that b-thread, but not necessarily the first one.
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* When all alternatives in a given execution path are explored the execution returns to the next higher level in the nondeterministic execution path to select the next alternative at that level.
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* BPJ model checking uses the Java package javaflow to restore the stack of all b-threads in order to resume execution of another subtree at a previously visited synchronization point.
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== Programming Constraints ==
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* While in regular runs a b-thread can terminate,  in a MC run A b-thread should not terminate, to allow BPJ model checking to backtrack. Precede/replace the return with bSync(none,none,none); In a regular run this will just terminate. In MC run it will allow the b-thread to backtrack from the end.
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== Example ==

Latest revision as of 09:40, 19 March 2014

Nondeterministic execution in BPJ model checking

In common processing event selection in BP is deterministic. However, in some cases it is desired to specify nondetermistic choices - e.g., when simulating environment behavior using b-threads as part of model checking. The BPJ model checking capability explores all possible execution paths defined by this nondeterminism. In fact, often, the only non-determinism in execution is in b-threads that simulate the environment.

Specifiying nondeterminism

  • In BP, event selection is deteremined by the unique priority of each b-thread, and by the order of events in the requested event set.
  • When the difference in priority of two b-threads is less than a given "epsilon" (a real number provided by the user), the two b-threads are considered of same priority. For example, to create 48 bthreads of the same priority, choose epsilon of 1.00 and create the set of these b-threads so that their priorities have the same whole part and differ only by fractional part. (e.g. priority values of 7.001 – 7.048). The value of "epsilon" is determined by the method call setBthreadEpsilon().
  • Additionally, during model checking if an event in a requested event set of any b-thread is selected (requested and not blocked, and non-deterministic execution actually selected it), then if the event was in a nested second-level set, then all other events in the same nested second-level set are considered to be of the same priority. For example, if the requested even set is R={{e1,e2},e3,{e4,e5,e6,e7},e8} - then if e5 was selected (say, all preceding events were blocked), then e6 and e7, and only them are considered of the same priority and will be explored in non-deterministic execution. This priority processing is relevant only to the first event selection in b-thread. Once the selected second-level set is exhausted, (or if the event was a first level in the first place), non-deterministic execution continues with the next b-thread of the "same" priority as the previous b-thread.

See execution below for more details.

Nondeterministic execution

When executing in model-checking mode, nondeterministic event selection is performed according to the following algorithm :

  • Scan b-threads in order of priority
  • Scan the requested event set of each b-thread in event order (the set is enumerable).
  • When the first event that is requested and not blocked is found it is remembered as one alternative.
  • If the selected event was in a second level event set (a set within a set), all its siblings in the same second level set that are not blocked are considered having the same priority, and are added as alternatives.
  • If the event is in a first level of the requested event set, this event is the only one from this b-thread.
  • After exploring all execution subtrees anchored at the alternatives from one b-thread go to the next b-thread of “same” priority as defined with epsilon, and repeat the above . At most one second-level set will be selected from the requested events of that b-thread, but not necessarily the first one.
  • When all alternatives in a given execution path are explored the execution returns to the next higher level in the nondeterministic execution path to select the next alternative at that level.
  • BPJ model checking uses the Java package javaflow to restore the stack of all b-threads in order to resume execution of another subtree at a previously visited synchronization point.

Programming Constraints

  • While in regular runs a b-thread can terminate, in a MC run A b-thread should not terminate, to allow BPJ model checking to backtrack. Precede/replace the return with bSync(none,none,none); In a regular run this will just terminate. In MC run it will allow the b-thread to backtrack from the end.

Example