Chris@42: Previous: How Many Threads to Use?, Up: Multi-threaded FFTW [Contents][Index]
Chris@42:Users writing multi-threaded programs (including OpenMP) must concern Chris@42: themselves with the thread safety of the libraries they Chris@42: use—that is, whether it is safe to call routines in parallel from Chris@42: multiple threads. FFTW can be used in such an environment, but some Chris@42: care must be taken because the planner routines share data Chris@42: (e.g. wisdom and trigonometric tables) between calls and plans. Chris@42:
Chris@42:The upshot is that the only thread-safe (re-entrant) routine in FFTW is
Chris@42: fftw_execute (and the new-array variants thereof). All other routines
Chris@42: (e.g. the planner) should only be called from one thread at a time. So,
Chris@42: for example, you can wrap a semaphore lock around any calls to the
Chris@42: planner; even more simply, you can just create all of your plans from
Chris@42: one thread. We do not think this should be an important restriction
Chris@42: (FFTW is designed for the situation where the only performance-sensitive
Chris@42: code is the actual execution of the transform), and the benefits of
Chris@42: shared data between plans are great.
Chris@42:
Note also that, since the plan is not modified by fftw_execute,
Chris@42: it is safe to execute the same plan in parallel by multiple
Chris@42: threads. However, since a given plan operates by default on a fixed
Chris@42: array, you need to use one of the new-array execute functions (see New-array Execute Functions) so that different threads compute the transform of different data.
Chris@42:
(Users should note that these comments only apply to programs using Chris@42: shared-memory threads or OpenMP. Parallelism using MPI or forked processes Chris@42: involves a separate address-space and global variables for each process, Chris@42: and is not susceptible to problems of this sort.) Chris@42:
Chris@42:If you are configured FFTW with the --enable-debug or
Chris@42: --enable-debug-malloc flags (see Installation on Unix),
Chris@42: then fftw_execute is not thread-safe. These flags are not
Chris@42: documented because they are intended only for developing
Chris@42: and debugging FFTW, but if you must use --enable-debug then you
Chris@42: should also specifically pass --disable-debug-malloc for
Chris@42: fftw_execute to be thread-safe.
Chris@42:
Starting from FFTW-3.3.5, FFTW supports a new API to make the Chris@42: planner thread-safe: Chris@42:
void fftw_make_planner_thread_safe(void); Chris@42:
This call installs a hook that wraps a lock around all planner calls.
Chris@42: This API is meant for “applications” that use “plugins” in multiple
Chris@42: threads, where each “plugin” calls single-threaded FFTW but is unaware
Chris@42: of the other “plugins” doing the same thing at the same time. In this
Chris@42: case, the “application” calls fftw_make_planner_thread_safe()
Chris@42: at the beginning to protect “plugins” from each other.
Chris@42:
Chris@42: Previous: How Many Threads to Use?, Up: Multi-threaded FFTW [Contents][Index]
Chris@42: