dispatch_apply(3)        BSD Library Functions Manual        dispatch_apply(3)

NAME
     dispatch_apply -- schedule blocks for iterative execution

SYNOPSIS
     #include <dispatch/dispatch.h>

     void
     dispatch_apply(size_t iterations, dispatch_queue_t queue,
         void (^block)(size_t));

     void
     dispatch_apply_f(size_t iterations, dispatch_queue_t queue,
         void *context, void (*function)(void *, size_t));

DESCRIPTION
     The dispatch_apply() function provides data-level concurrency through a
     "for (;;)" loop like primitive:

     size_t iterations = 10;

     // 'idx' is zero indexed, just like:
     // for (idx = 0; idx < iterations; idx++)

     dispatch_apply(iterations, DISPATCH_APPLY_AUTO, ^(size_t idx) {
             printf("%zu\n", idx);
     });

     Although any queue can be used, it is strongly recommended to use
     DISPATCH_APPLY_AUTO as the queue argument to both dispatch_apply() and
     dispatch_apply_f(), as shown in the example above, since this allows the
     system to automatically use worker threads that match the configuration
     of the current thread as closely as possible.  No assumptions should be
     made about which global concurrent queue will be used.

     Like a "for (;;)" loop, the dispatch_apply() function is synchronous.  If
     asynchronous behavior is desired, wrap the call to dispatch_apply() with
     a call to dispatch_async() against another queue.

     Sometimes, when the block passed to dispatch_apply() is simple, the use
     of striding can tune performance.  Calculating the optimal stride is best
     left to experimentation.  Start with a stride of one and work upwards
     until the desired performance is achieved (perhaps using a power of two
     search):

     #define STRIDE 3

     dispatch_apply(count / STRIDE, DISPATCH_APPLY_AUTO, ^(size_t idx) {
             size_t j = idx * STRIDE;
             size_t j_stop = j + STRIDE;
             do {
                     printf("%zu\n", j++);
             } while (j < j_stop);
     });

     size_t i;
     for (i = count - (count % STRIDE); i < count; i++) {
             printf("%zu\n", i);
     }

IMPLIED REFERENCES
     Synchronous functions within the dispatch framework hold an implied ref-
     erence on the target queue. In other words, the synchronous function bor-
     rows the reference of the calling function (this is valid because the
     calling function is blocked waiting for the result of the synchronous
     function, and therefore cannot modify the reference count of the target
     queue until after the synchronous function has returned).

     This is in contrast to asynchronous functions which must retain both the
     block and target queue for the duration of the asynchronous operation (as
     the calling function may immediately release its interest in these
     objects).

FUNDAMENTALS
     dispatch_apply() and dispatch_apply_f() attempt to quickly create enough
     worker threads to efficiently iterate work in parallel.  By contrast, a
     loop that passes work items individually to dispatch_async() or
     dispatch_async_f() will incur more overhead and does not express the
     desired parallel execution semantics to the system, so may not create an
     optimal number of worker threads for a parallel workload.  For this rea-
     son, prefer to use dispatch_apply() or dispatch_apply_f() when parallel
     execution is important.

     The dispatch_apply() function is a wrapper around dispatch_apply_f().

CAVEATS
     Unlike dispatch_async(), a block submitted to dispatch_apply() is
     expected to be either independent or dependent only on work already per-
     formed in lower-indexed invocations of the block. If the block's index
     dependency is non-linear, it is recommended to use a for-loop around
     invocations of dispatch_async().

SEE ALSO
     dispatch(3), dispatch_async(3), dispatch_queue_create(3)

Darwin                            May 1, 2009                           Darwin