Module parallel

Run computations in parallel, or not based the parallel feature toggle.

in_parallel(…)

The in_parallel(…) is the typical fan-out-fan-in mode of parallelism, with thread local storage made available to a consume(…) function to process input. The result is sent to the Reduce running in the calling thread to aggregate the results into a single output, which is returned by in_parallel().

Interruptions can be achieved by letting the reducers feed(…) method fail.

It gets a boost in usability as it allows threads to borrow variables from the stack, most commonly the repository itself or the data to work on.

This mode of operation doesn’t lend itself perfectly to being wrapped for async as it appears like a single long-running operation which runs as fast as possible, which is cancellable only by merit of stopping the input or stopping the output aggregation.

reduce::Stepwise

The Stepwise iterator works exactly as in_parallel() except that the processing of the output produced by consume(I, &mut State) -> O is made accessible by the Iterator trait’s next() method. As produced work is not buffered, the owner of the iterator controls the progress made.

Getting the final output of the Reduce is achieved through the consuming Stepwise::finalize() method, which is functionally equivalent to calling in_parallel().

In an async context this means that progress is only made each time next() is called on the iterator, while merely dropping the iterator will wind down the computation without any result.

Maintaining Safety

In order to assure that threads don’t outlive the data they borrow because their handles are leaked, we enforce the 'static lifetime for its inputs, making it less intuitive to use. It is, however, possible to produce suitable input iterators as long as they can hold something on the heap.

Re-exports

• pub use reduce::Reduce;

Modules

• reduce

Structs

• EagerIter
  Evaluate any iterator in their own thread.
• InOrderIter
  An iterator which olds iterated items with a sequential ID starting at 0 long enough to dispense them in order.

Enums

• EagerIterIf
  An conditional EagerIter, which may become a just-in-time iterator running in the main thread depending on a condition.

Functions

• build_threadparallel
  Create a builder for threads which allows them to be spawned into a scope and configured prior to spawning.
• in_parallelparallel
  Read items from input and consume them in multiple threads, whose output output is collected by a reducer. Its task is to aggregate these outputs into the final result returned by this function with the benefit of not having to be thread-safe.
• in_parallel_ifparallel
  Run in_parallel() only if the given condition() returns true when eagerly evaluated.
• in_parallel_with_finalizeparallel
  Read items from input and consume them in multiple threads, whose output output is collected by a reducer. Its task is to aggregate these outputs into the final result returned by this function with the benefit of not having to be thread-safe. Caall finalize to finish the computation, once per thread, if there was no error sending results earlier.
• in_parallel_with_sliceparallel
  An experiment to have fine-grained per-item parallelization with built-in aggregation via thread state. This is only good for operations where near-random access isn’t detrimental, so it’s not usually great for file-io as it won’t make use of sorted inputs well. Note that periodic is not guaranteed to be called in case other threads come up first and finish too fast. consume(&mut item, &mut stat, &Scope, &threads_available, &should_interrupt) is called for performing the actual computation. Note that threads_available should be decremented to start a thread that can steal your own work (as stored in item), which allows callees to implement their own work-stealing in case the work is distributed unevenly. Work stealing should only start after having processed at least one item to give all threads naturally operating on the slice some time to start. Starting threads while slice-workers are still starting up would lead to over-allocation of threads, which is why the number of threads left may turn negative. Once threads are started and stopped, be sure to adjust the thread-count accordingly.
• joinparallel
  Runs left and right in parallel, returning their output when both are done.
• num_threadsparallel
  Returns the amount of threads the system can effectively use as the amount of its logical cores.
• optimize_chunk_size_and_thread_limitparallel
  Return the ‘optimal’ (size of chunks, amount of threads as Option, amount of threads) to use in in_parallel() for the given desired_chunk_size, num_items, thread_limit and available_threads.
• threadsparallel
  Runs f with a scope to be used for spawning threads that will not outlive the function call. That way it’s possible to handle threads without needing the ’static lifetime for data they interact with.

Type Aliases

• Scopeparallel
  A scope to start threads within.
• SequenceId
  A counter for items that are in sequence, to be able to put them back into original order later.