Function std.parallelism.TaskPool.map.map
A semi-lazy parallel map that can be used for pipelining. The map
functions are evaluated for the first bufSize elements and stored in a
buffer and made available to popFront. Meanwhile, in the
background a second buffer of the same size is filled. When the first
buffer is exhausted, it is swapped with the second buffer and filled while
the values from what was originally the second buffer are read. This
implementation allows for elements to be written to the buffer without
the need for atomic operations or synchronization for each write, and
enables the mapping function to be evaluated efficiently in parallel.
auto map(S)
(
S source,
size_t bufSize = 100,
size_t workUnitSize = size_t
)
if (isInputRange!S);
map has more overhead than the simpler procedure used by amap
but avoids the need to keep all results in memory simultaneously and works
with non-random access ranges.
Parameters
| Name | Description |
|---|---|
| source | The input range
to be mapped. If source is not random
access it will be lazily buffered to an array of size bufSize before
the map function is evaluated. (For an exception to this rule, see Notes.)
|
| bufSize | The size of the buffer to store the evaluated elements. |
| workUnitSize | The number of elements to evaluate in a single
Task. Must be less than or equal to bufSize, and
should be a fraction of bufSize such that all worker threads can be
used. If the default of size_t.max is used, workUnitSize will be set to
the pool-wide default. |
Returns
An input range representing the results of the map. This range
has a length iff source has a length.
Notes
If a range returned by map or asyncBuf is used as an input to
map, then as an optimization the copying from the output buffer
of the first range to the input buffer of the second range is elided, even
though the ranges returned by map and asyncBuf are non-random
access ranges. This means that the bufSize parameter passed to the
current call to map will be ignored and the size of the buffer
will be the buffer size of source.
Example
// Pipeline reading a file, converting each line
// to a number, taking the logarithms of the numbers,
// and performing the additions necessary to find
// the sum of the logarithms.
auto lineRange = File("numberList.txt")Exception Handling:
Any exceptions thrown while iterating over source
or computing the map function are re-thrown on a call to popFront or,
if thrown during construction, are simply allowed to propagate to the
caller. In the case of exceptions thrown while computing the map function,
the exceptions are chained as in TaskPool.