StackOverflow2013

Note that there are some explanatory texts on larger screens.

plurals

PO
text
Body
copied!<p>Given that AsParallel transparently parallelizes LINQ queries, the question arises, “Why didn’t Microsoft simply parallelize the standard query operators and make PLINQ the default?”</p> <p>There are a number of reasons for the opt-in approach. First, for PLINQ to be useful there has to be a reasonable amount of computationally intensive work for it to farm out to worker threads. Most LINQ to Objects queries execute very quickly, and not only would parallelization be unnecessary, but the overhead of partitioning, collating, and coordinating the extra threads may actually slow things down.</p> <p>Additionally:</p> <p>The output of a PLINQ query (by default) may differ from a LINQ query with respect to element ordering.</p> <p>The following query operators prevent a query from being parallelized, unless the source elements are in their original indexing position:</p> <p>Take, TakeWhile, Skip, and SkipWhile The indexed versions of Select, SelectMany, and ElementAt Most query operators change the indexing position of elements (including those that remove elements, such as Where). This means that if you want to use the preceding operators, they’ll usually need to be at the start of the query.</p> <p>The following query operators are parallelizable, but use an expensive partitioning strategy that can sometimes be slower than sequential processing:</p> <p>Join, GroupBy, GroupJoin, Distinct, Union, Intersect, and Except The Aggregate operator’s seeded overloads in their standard incarnations are not parallelizable — PLINQ provides special overloads to deal with this.</p> <p><strong>When to Use PLINQ</strong> It’s tempting to search your existing applications for LINQ queries and experiment with parallelizing them. This is usually unproductive, because most problems for which LINQ is obviously the best solution tend to execute very quickly and so don’t benefit from parallelization. A better approach is to find a CPU-intensive bottleneck and then consider, “Can this be expressed as a LINQ query?” (A welcome side effect of such restructuring is that LINQ typically makes code smaller and more readable.)</p> <p>PLINQ is well suited to embarrassingly parallel problems. It also works well for structured blocking tasks, such as calling several web services at once (see Calling Blocking or I/O-Intensive Functions).</p> <p>PLINQ can be a poor choice for imaging, because collating millions of pixels into an output sequence creates a bottleneck. Instead, it’s better to write pixels directly to an array or unmanaged memory block and use the Parallel class or task parallelism to manage the multithreading. (It is possible, however, to defeat result collation using ForAll. Doing so makes sense if the image processing algorithm naturally lends itself to LINQ.)</p>

Querying!

Guidance

An individual column

Larger individual text columns get their own page to allow for proper reading.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload