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first element once the full output was known anyway, which includes the O r d e r By operator and APIs like ToA r r a y
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Order Preservation Because PLINQ runs in parallel, the elements fed into a query may become scrambled during execution The symptom of this is that order among ele ments in the output may not directly correspond to the elements in the input As a simple example of this, there is no guarantee that a and b will be equal after the following snippet is run
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new i nt [ ] { a, 1, 2, 3 , 4, 5 } j ( f rom x in a As P a r a l l e l ( ) s e l e c t x ) ToArraY ( ) j
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On one hand, this seems absurd The query maps the identity function against all elements in the array But if you stop to think about all of the par titioning and merging going on in order to do that mapping in parallel, it would require PLINQ to expend a considerable amount of effort in order to preserve the input ordering For many problems this is acceptable In fact, because of LINQ' s set oriented and 5QL-like nature, many people don' t expect order to be preserved by LINQ itself But if this does matter to your problem, you can force PLINQ to preserve the ordering in its output with the A s O r d e r e d API As noted above, this comes at some expense, which is why it is opt in
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p u b l i c s t a t i c I P a r a l l e l E n u me r a b l e < T > AsOrd e red < T > ( t h i s I P a r a l l e l E n u me r a b l e < T > s o u r c e )j
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The only legal position for AsOrdered is when immediately preceded by an A S P a r a l l e l The API will throw an exception otherwise 50 if we wanted to force order preservation on our example above, it would look like this:
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new i nt [ ] { a, 1, 2 , 3, 4 , 5 } j ( from x in a A s P a r a l l e l ( ) AsOrdered ( ) s e l e c t x ) ToArraY ( ) j
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There is also an AsUnordered API that can be used in the middle of a query to turn off ordering for a particular set of operators This can be used with operators like Take that have a deeply ingrained notion of order For instance, if your query contains T ake ( leee ) , you presumably care about it taking the
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Syn c h ro n i z a t i o n P ri m i tives
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first 1 ,000 elements That requires use of ASO rdered But perhaps once you've taken those 1 ,000 elements, you don't want to pay the cost of order preser vation for all subsequent operators; this is particularly true of the merge step, whose performance order preservation can impact dramatically
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Synchronization Primitives
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Parallel Extensions provides several useful synchronization primitives to support common data and control synchronization needs Several of these will be familiar to you if you've read the whole book up to this point
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ISupportsCancelation The System T h r e a d i n g I S u p p o rt s C a n c e l a t i o n interface indicates that some class supports object level cancellation Canceling such an object will immediately wake up all threads that are blocked on it This is useful when some thread participating in an operation fails to reach a synchronization point or in support of responsive GUls that need to be able to tear down potentially lengthy parallel computations at the request of the end user The interface itself is very straightforward
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p u b l i c interface I S u pport s C a n c e l at ion { void C a n c e 1 ( ) ; bool I s C a n c e l e d { get ; }
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You'll notice that TPL's Ta s k class implements this interface, as do many of the types we're about to see Though simple, this interface allows general purpose cancellation frameworks to be built that operate on a number of different kinds of cancellable things
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CountdownEvent An extremely common pattern in parallel programming is fork/j oin, where a thread may spawn a certain number of activities and must later wait for them to complete That's the purpose of System T h r e a d i n g C o u n t down E v e n t type We saw this in 1 3, Data and Task Parallelism, and wrote a few code samples that relied on such a primitive (eg, to implement parallel for loops and the like)
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