guile/doc/ref/api-scheduling.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2007, 2009, 2010, 2012, 2013
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Scheduling
@section Threads, Mutexes, Asyncs and Dynamic Roots

@menu
* Threads::                     Multiple threads of execution.
* Thread Local Variables::      Some fluids are thread-local.
* Asyncs::                      Asynchronous interrupts.
* Atomics::                     Atomic references.
* Mutexes and Condition Variables:: Synchronization primitives.
* Blocking::                    How to block properly in guile mode.
* Futures::                     Fine-grain parallelism.
* Parallel Forms::              Parallel execution of forms.
@end menu


@node Threads
@subsection Threads
@cindex threads
@cindex Guile threads
@cindex POSIX threads

Guile supports POSIX threads, unless it was configured with
@code{--without-threads} or the host lacks POSIX thread support.  When
thread support is available, the @code{threads} feature is provided
(@pxref{Feature Manipulation, @code{provided?}}).

The procedures below manipulate Guile threads, which are wrappers around
the system's POSIX threads.  For application-level parallelism, using
higher-level constructs, such as futures, is recommended
(@pxref{Futures}).

To use these facilities, load the @code{(ice-9 threads)} module.

@example
(use-modules (ice-9 threads))
@end example

@deffn {Scheme Procedure} all-threads
@deffnx {C Function} scm_all_threads ()
Return a list of all threads.
@end deffn

@deffn {Scheme Procedure} current-thread
@deffnx {C Function} scm_current_thread ()
Return the thread that called this function.
@end deffn

@deffn {Scheme Procedure} call-with-new-thread thunk [handler]
Call @code{thunk} in a new thread and with a new dynamic state,
returning the new thread.  The procedure @var{thunk} is called via
@code{with-continuation-barrier}.

When @var{handler} is specified, then @var{thunk} is called from
within a @code{catch} with tag @code{#t} that has @var{handler} as its
handler.  This catch is established inside the continuation barrier.

Once @var{thunk} or @var{handler} returns, the return value is made
the @emph{exit value} of the thread and the thread is terminated.
@end deffn

@deftypefn {C Function} SCM scm_spawn_thread (scm_t_catch_body body, void *body_data, scm_t_catch_handler handler, void *handler_data)
Call @var{body} in a new thread, passing it @var{body_data}, returning
the new thread.  The function @var{body} is called via
@code{scm_c_with_continuation_barrier}.

When @var{handler} is non-@code{NULL}, @var{body} is called via
@code{scm_internal_catch} with tag @code{SCM_BOOL_T} that has
@var{handler} and @var{handler_data} as the handler and its data.  This
catch is established inside the continuation barrier.

Once @var{body} or @var{handler} returns, the return value is made the
@emph{exit value} of the thread and the thread is terminated.
@end deftypefn

@deffn {Scheme Procedure} thread? obj
@deffnx {C Function} scm_thread_p (obj)
Return @code{#t} ff @var{obj} is a thread; otherwise, return
@code{#f}.
@end deffn

@deffn {Scheme Procedure} join-thread thread [timeout [timeoutval]]
@deffnx {C Function} scm_join_thread (thread)
@deffnx {C Function} scm_join_thread_timed (thread, timeout, timeoutval)
Wait for @var{thread} to terminate and return its exit value.  Only
threads that were created with @code{call-with-new-thread} or
@code{scm_spawn_thread} can be joinable; attempting to join a foreign
thread will raise an error.

When @var{timeout} is given, it specifies a point in time where the
waiting should be aborted.  It can be either an integer as returned by
@code{current-time} or a pair as returned by @code{gettimeofday}.  When
the waiting is aborted, @var{timeoutval} is returned (if it is
specified; @code{#f} is returned otherwise).
@end deffn

@deffn {Scheme Procedure} thread-exited? thread
@deffnx {C Function} scm_thread_exited_p (thread)
Return @code{#t} if @var{thread} has exited, or @code{#f} otherwise.
@end deffn

@deffn {Scheme Procedure} yield
@deffnx {C Function} scm_yield (thread)
If one or more threads are waiting to execute, calling yield forces an
immediate context switch to one of them.  Otherwise, yield has no effect.
@end deffn

@deffn {Scheme Procedure} cancel-thread thread . values
@deffnx {C Function} scm_cancel_thread (thread)
Asynchronously interrupt @var{thread} and ask it to terminate.
@code{dynamic-wind} post thunks will run, but throw handlers will not.
If @var{thread} has already terminated or been signaled to terminate,
this function is a no-op.  Calling @code{join-thread} on the thread will
return the given @var{values}, if the cancel succeeded.

Under the hood, thread cancellation uses @code{system-async-mark} and
@code{abort-to-prompt}.  @xref{Asyncs} for more on asynchronous
interrupts.
@end deffn

@deffn macro make-thread proc arg @dots{}
Apply @var{proc} to @var{arg} @dots{} in a new thread formed by
@code{call-with-new-thread} using a default error handler that displays
the error to the current error port.  The @var{arg} @dots{}
expressions are evaluated in the new thread.
@end deffn

@deffn macro begin-thread expr1 expr2 @dots{}
Evaluate forms @var{expr1} @var{expr2} @dots{} in a new thread formed by
@code{call-with-new-thread} using a default error handler that displays
the error to the current error port.
@end deffn

One often wants to limit the number of threads running to be
proportional to the number of available processors.  These interfaces
are therefore exported by (ice-9 threads) as well.

@deffn {Scheme Procedure} total-processor-count
@deffnx {C Function} scm_total_processor_count ()
Return the total number of processors of the machine, which
is guaranteed to be at least 1.  A ``processor'' here is a
thread execution unit, which can be either:

@itemize
@item an execution core in a (possibly multi-core) chip, in a
  (possibly multi- chip) module, in a single computer, or
@item a thread execution unit inside a core in the case of
  @dfn{hyper-threaded} CPUs.
@end itemize

Which of the two definitions is used, is unspecified.
@end deffn

@deffn {Scheme Procedure} current-processor-count
@deffnx {C Function} scm_current_processor_count ()
Like @code{total-processor-count}, but return the number of
processors available to the current process.  See
@code{setaffinity} and @code{getaffinity} for more
information.
@end deffn


@node Thread Local Variables
@subsection Thread-Local Variables

Sometimes you want to establish a variable binding that is only valid
for a given thread: a ``thread-local variable''.

You would think that fluids or parameters would be Guile's answer for
thread-local variables, since establishing a new fluid binding doesn't
affect bindings in other threads.  @xref{Fluids and Dynamic States}, or
@xref{Parameters}.  However, new threads inherit the fluid bindings that
were in place in their creator threads.  In this way, a binding
established using a fluid (or a parameter) in a thread can escape to
other threads, which might not be what you want.  Or, it might escape
via explicit reification via @code{current-dynamic-state}.

Of course, this dynamic scoping might be exactly what you want; that's
why fluids and parameters work this way, and is what you want for
many common parameters such as the current input and output ports, the
current locale conversion parameters, and the like.  Perhaps this is the
case for most parameters, even.  If your use case for thread-local
bindings comes from a desire to isolate a binding from its setting in
unrelated threads, then fluids and parameters apply nicely.

On the other hand, if your use case is to prevent concurrent access to a
value from multiple threads, then using vanilla fluids or parameters is
not appropriate.  For this purpose, Guile has @dfn{thread-local fluids}.
A fluid created with @code{make-thread-local-fluid} won't be captured by
@code{current-dynamic-state} and won't be propagated to new threads.

@deffn {Scheme Procedure} make-thread-local-fluid [dflt]
@deffnx {C Function} scm_make_thread_local_fluid (dflt)
Return a newly created fluid, whose initial value is @var{dflt}, or
@code{#f} if @var{dflt} is not given.  Unlike fluids made with
@code{make-fluid}, thread local fluids are not captured by
@code{make-dynamic-state}.  Similarly, a newly spawned child thread does
not inherit thread-local fluid values from the parent thread.
@end deffn

@deffn {Scheme Procedure} fluid-thread-local? fluid
@deffnx {C Function} scm_fluid_thread_local_p (fluid)
Return @code{#t} if the fluid @var{fluid} is thread-local, or
@code{#f} otherwise.
@end deffn

For example:

@example
(define %thread-local (make-thread-local-fluid))

(with-fluids ((%thread-local (compute-data)))
  ... (fluid-ref %thread-local) ...)
@end example

You can also make a thread-local parameter out of a thread-local fluid
using the normal @code{fluid->parameter}:

@example
(define param (fluid->parameter (make-thread-local-fluid)))

(parameterize ((param (compute-data)))
  ... (param) ...)
@end example


@node Asyncs
@subsection Asynchronous Interrupts

@cindex asyncs
@cindex asynchronous interrupts
@cindex interrupts

Every Guile thread can be interrupted.  Threads running Guile code will
periodically check if there are pending interrupts and run them if
necessary.  To interrupt a thread, call @code{system-async-mark} on that
thread.

@deffn {Scheme Procedure} system-async-mark proc [thread]
@deffnx {C Function} scm_system_async_mark (proc)
@deffnx {C Function} scm_system_async_mark_for_thread (proc, thread)
Enqueue @var{proc} (a procedure with zero arguments) for future
execution in @var{thread}.  When @var{proc} has already been enqueued
for @var{thread} but has not been executed yet, this call has no effect.
When @var{thread} is omitted, the thread that called
@code{system-async-mark} is used.
@end deffn

Note that @code{scm_system_async_mark_for_thread} is not
``async-signal-safe'' and so cannot be called from a C signal handler.
(Indeed in general, @code{libguile} functions are not safe to call from
C signal handlers.)

Though an interrupt procedure can have any side effect permitted to
Guile code, asynchronous interrupts are generally used either for
profiling or for prematurely canceling a computation.  The former case
is mostly transparent to the program being run, by design, but the
latter case can introduce bugs.  Like finalizers (@pxref{Foreign Object
Memory Management}), asynchronous interrupts introduce concurrency in a
program.  An asynchronous interrupt can run in the middle of some
mutex-protected operation, for example, and potentially corrupt the
program's state.

If some bit of Guile code needs to temporarily inhibit interrupts, it
can use @code{call-with-blocked-asyncs}.  This function works by
temporarily increasing the @emph{async blocking level} of the current
thread while a given procedure is running.  The blocking level starts
out at zero, and whenever a safe point is reached, a blocking level
greater than zero will prevent the execution of queued asyncs.

Analogously, the procedure @code{call-with-unblocked-asyncs} will
temporarily decrease the blocking level of the current thread.  You
can use it when you want to disable asyncs by default and only allow
them temporarily.

In addition to the C versions of @code{call-with-blocked-asyncs} and
@code{call-with-unblocked-asyncs}, C code can use
@code{scm_dynwind_block_asyncs} and @code{scm_dynwind_unblock_asyncs}
inside a @dfn{dynamic context} (@pxref{Dynamic Wind}) to block or
unblock asyncs temporarily.

@deffn {Scheme Procedure} call-with-blocked-asyncs proc
@deffnx {C Function} scm_call_with_blocked_asyncs (proc)
Call @var{proc} and block the execution of asyncs by one level for the
current thread while it is running.  Return the value returned by
@var{proc}.  For the first two variants, call @var{proc} with no
arguments; for the third, call it with @var{data}.
@end deffn

@deftypefn {C Function} {void *} scm_c_call_with_blocked_asyncs (void * (*proc) (void *data), void *data)
The same but with a C function @var{proc} instead of a Scheme thunk.
@end deftypefn

@deffn {Scheme Procedure} call-with-unblocked-asyncs proc
@deffnx {C Function} scm_call_with_unblocked_asyncs (proc)
Call @var{proc} and unblock the execution of asyncs by one level for the
current thread while it is running.  Return the value returned by
@var{proc}.  For the first two variants, call @var{proc} with no
arguments; for the third, call it with @var{data}.
@end deffn

@deftypefn {C Function} {void *} scm_c_call_with_unblocked_asyncs (void *(*proc) (void *data), void *data)
The same but with a C function @var{proc} instead of a Scheme thunk.
@end deftypefn

@deftypefn {C Function} void scm_dynwind_block_asyncs ()
During the current dynwind context, increase the blocking of asyncs by
one level.  This function must be used inside a pair of calls to
@code{scm_dynwind_begin} and @code{scm_dynwind_end} (@pxref{Dynamic
Wind}).
@end deftypefn

@deftypefn {C Function} void scm_dynwind_unblock_asyncs ()
During the current dynwind context, decrease the blocking of asyncs by
one level.  This function must be used inside a pair of calls to
@code{scm_dynwind_begin} and @code{scm_dynwind_end} (@pxref{Dynamic
Wind}).
@end deftypefn

Sometimes you want to interrupt a thread that might be waiting for
something to happen, for example on a file descriptor or a condition
variable.  In that case you can inform Guile of how to interrupt that
wait using the following procedures:

@deftypefn {C Function} int scm_c_prepare_to_wait_on_fd (int fd)
Inform Guile that the current thread is about to sleep, and that if an
asynchronous interrupt is signaled on this thread, Guile should wake up
the thread by writing a zero byte to @var{fd}.  Returns zero if the
prepare succeeded, or nonzero if the thread already has a pending async
and that it should avoid waiting.
@end deftypefn

@deftypefn {C Function} int scm_c_prepare_to_wait_on_cond (scm_i_pthread_mutex_t *mutex, scm_i_pthread_cond_t *cond)
Inform Guile that the current thread is about to sleep, and that if an
asynchronous interrupt is signaled on this thread, Guile should wake up
the thread by acquiring @var{mutex} and signaling @var{cond}.  The
caller must already hold @var{mutex} and only drop it as part of the
@code{pthread_cond_wait} call.  Returns zero if the prepare succeeded,
or nonzero if the thread already has a pending async and that it should
avoid waiting.
@end deftypefn

@deftypefn {C Function} void scm_c_wait_finished (void)
Inform Guile that the current thread has finished waiting, and that
asynchronous interrupts no longer need any special wakeup action; the
current thread will periodically poll its internal queue instead.
@end deftypefn

Guile's own interface to @code{sleep}, @code{wait-condition-variable},
@code{select}, and so on all call the above routines as appropriate.

Finally, note that threads can also be interrupted via POSIX signals.
@xref{Signals}.  As an implementation detail, signal handlers will
effectively call @code{system-async-mark} in a signal-safe way,
eventually running the signal handler using the same async mechanism.
In this way you can temporarily inhibit signal handlers from running
using the above interfaces.


@node Atomics
@subsection Atomics

When accessing data in parallel from multiple threads, updates made by
one thread are not generally guaranteed to be visible by another thread.
It could be that your hardware requires special instructions to be
emitted to propagate a change from one CPU core to another.  Or, it
could be that your hardware updates values with a sequence of
instructions, and a parallel thread could see a value that is in the
process of being updated but not fully updated.

Atomic references solve this problem.  Atomics are a standard, primitive
facility to allow for concurrent access and update of mutable variables
from multiple threads with guaranteed forward-progress and well-defined
intermediate states.

Atomic references serve not only as a hardware memory barrier but also
as a compiler barrier.  Normally a compiler might choose to reorder or
elide certain memory accesses due to optimizations like common
subexpression elimination.  Atomic accesses however will not be
reordered relative to each other, and normal memory accesses will not be
reordered across atomic accesses.

As an implementation detail, currently all atomic accesses and updates
use the sequential consistency memory model from C11.  We may relax this
in the future to the acquire/release semantics, which still issues a
memory barrier so that non-atomic updates are not reordered across
atomic accesses or updates.

To use Guile's atomic operations, load the @code{(ice-9 atomic)} module:

@example
(use-modules (ice-9 atomic))
@end example

@deffn {Scheme Procedure} make-atomic-box init
Return an atomic box initialized to value @var{init}.
@end deffn

@deffn {Scheme Procedure} atomic-box? obj
Return @code{#t} if @var{obj} is an atomic-box object, else
return @code{#f}.
@end deffn

@deffn {Scheme Procedure} atomic-box-ref box
Fetch the value stored in the atomic box @var{box} and return it.
@end deffn

@deffn {Scheme Procedure} atomic-box-set! box  val
Store @var{val} into the atomic box @var{box}.
@end deffn

@deffn {Scheme Procedure} atomic-box-swap! box val
Store @var{val} into the atomic box @var{box}, and return the value that
was previously stored in the box.
@end deffn

@deffn {Scheme Procedure} atomic-box-compare-and-swap! box expected desired
If the value of the atomic box @var{box} is the same as, @var{expected}
(in the sense of @code{eq?}), replace the contents of the box with
@var{desired}.  Otherwise does not update the box.  Returns the previous
value of the box in either case, so you can know if the swap worked by
checking if the return value is @code{eq?} to @var{expected}.
@end deffn


@node Mutexes and Condition Variables
@subsection Mutexes and Condition Variables
@cindex mutex
@cindex condition variable

Mutexes are low-level primitives used to coordinate concurrent access to
mutable data.  Short for ``mutual exclusion'', the name ``mutex''
indicates that only one thread at a time can acquire access to data that
is protected by a mutex -- threads are excluded from accessing data at
the same time.  If one thread has locked a mutex, then another thread
attempting to lock that same mutex will wait until the first thread is
done.

Mutexes can be used to build robust multi-threaded programs that take
advantage of multiple cores.  However, they provide very low-level
functionality and are somewhat dangerous; usually you end up wanting to
acquire multiple mutexes at the same time to perform a multi-object
access, but this can easily lead to deadlocks if the program is not
carefully written.  For example, if objects A and B are protected by
associated mutexes M and N, respectively, then to access both of them
then you need to acquire both mutexes.  But what if one thread acquires
M first and then N, at the same time that another thread acquires N them
M?  You can easily end up in a situation where one is waiting for the
other.

There's no easy way around this problem on the language level.  A
function A that uses mutexes does not necessarily compose nicely with a
function B that uses mutexes.  For this reason we suggest using atomic
variables when you can (@pxref{Atomics}), as they do not have this problem.

Still, if you as a programmer are responsible for a whole system, then
you can use mutexes as a primitive to provide safe concurrent
abstractions to your users.  (For example, given all locks in a system,
if you establish an order such that M is consistently acquired before N,
you can avoid the ``deadly-embrace'' deadlock described above.  The
problem is enumerating all mutexes and establishing this order from a
system perspective.)  Guile gives you the low-level facilities to build
such systems.

In Guile there are additional considerations beyond the usual ones in
other programming languages: non-local control flow and asynchronous
interrupts.  What happens if you hold a mutex, but somehow you cause an
exception to be thrown?  There is no one right answer.  You might want
to keep the mutex locked to prevent any other code from ever entering
that critical section again.  Or, your critical section might be fine if
you unlock the mutex ``on the way out'', via an exception handler or
@code{dynamic-wind}.  @xref{Exceptions}, and @xref{Dynamic Wind}.

But if you arrange to unlock the mutex when leaving a dynamic extent via
@code{dynamic-wind}, what to do if control re-enters that dynamic extent
via a continuation invocation?  Surely re-entering the dynamic extent
without the lock is a bad idea, so there are two options on the table:
either prevent re-entry via @code{with-continuation-barrier} or similar,
or reacquire the lock in the entry thunk of a @code{dynamic-wind}.

You might think that because you don't use continuations, that you don't
have to think about this, and you might be right.  If you control the
whole system, you can reason about continuation use globally.  Or, if
you know all code that can be called in a dynamic extent, and none of
that code can call continuations, then you don't have to worry about
re-entry, and you might not have to worry about early exit either.

However, do consider the possibility of asynchronous interrupts
(@pxref{Asyncs}).  If the user interrupts your code interactively, that
can cause an exception; or your thread might be canceled, which does
the same; or the user could be running your code under some pre-emptive
system that periodically causes lightweight task switching.  (Guile does
not currently include such a system, but it's possible to implement as a
library.)  Probably you also want to defer asynchronous interrupt
processing while you hold the mutex, and probably that also means that
you should not hold the mutex for very long.

All of these additional Guile-specific considerations mean that from a
system perspective, you would do well to avoid these hazards if you can
by not requiring mutexes.  Instead, work with immutable data that can be
shared between threads without hazards, or use persistent data
structures with atomic updates based on the atomic variable library
(@pxref{Atomics}).

There are three types of mutexes in Guile: ``standard'', ``recursive'',
and ``unowned''.

Calling @code{make-mutex} with no arguments makes a standard mutex.  A
standard mutex can only be locked once.  If you try to lock it again
from the thread that locked it to begin with (the "owner" thread), it
throws an error.  It can only be unlocked from the thread that locked it
in the first place.

Calling @code{make-mutex} with the symbol @code{recursive} as the
argument, or calling @code{make-recursive-mutex}, will give you a
recursive mutex.  A recursive mutex can be locked multiple times by its
owner.  It then has to be unlocked the corresponding number of times,
and like standard mutexes can only be unlocked by the owner thread.

Finally, calling @code{make-mutex} with the symbol
@code{allow-external-unlock} creates an unowned mutex.  An unowned mutex
is like a standard mutex, except that it can be unlocked by any thread.
A corollary of this behavior is that a thread's attempt to lock a mutex
that it already owns will block instead of signaling an error, as it
could be that some other thread unlocks the mutex, allowing the owner
thread to proceed.  This kind of mutex is a bit strange and is here for
use by SRFI-18.

The mutex procedures in Guile can operate on all three kinds of mutexes.

To use these facilities, load the @code{(ice-9 threads)} module.

@example
(use-modules (ice-9 threads))
@end example

@sp 1
@deffn {Scheme Procedure} make-mutex [kind]
@deffnx {C Function} scm_make_mutex ()
@deffnx {C Function} scm_make_mutex_with_kind (SCM kind)
Return a new mutex.  It will be a standard non-recursive mutex, unless
the @code{recursive} symbol is passed as the optional @var{kind}
argument, in which case it will be recursive.  It's also possible to
pass @code{unowned} for semantics tailored to SRFI-18's use case; see
above for details.
@end deffn

@deffn {Scheme Procedure} mutex? obj
@deffnx {C Function} scm_mutex_p (obj)
Return @code{#t} if @var{obj} is a mutex; otherwise, return
@code{#f}.
@end deffn

@deffn {Scheme Procedure} make-recursive-mutex
@deffnx {C Function} scm_make_recursive_mutex ()
Create a new recursive mutex.  It is initially unlocked.  Calling this
function is equivalent to calling @code{make-mutex} with the
@code{recursive} kind.
@end deffn

@deffn {Scheme Procedure} lock-mutex mutex [timeout]
@deffnx {C Function} scm_lock_mutex (mutex)
@deffnx {C Function} scm_timed_lock_mutex (mutex, timeout)
Lock @var{mutex} and return @code{#t}.  If the mutex is already locked,
then block and return only when @var{mutex} has been acquired.

When @var{timeout} is given, it specifies a point in time where the
waiting should be aborted.  It can be either an integer as returned
by @code{current-time} or a pair as returned by @code{gettimeofday}.
When the waiting is aborted, @code{#f} is returned.

For standard mutexes (@code{make-mutex}), an error is signaled if the
thread has itself already locked @var{mutex}.

For a recursive mutex (@code{make-recursive-mutex}), if the thread has
itself already locked @var{mutex}, then a further @code{lock-mutex}
call increments the lock count.  An additional @code{unlock-mutex}
will be required to finally release.

When an asynchronous interrupt (@pxref{Asyncs}) is scheduled for a
thread blocked in @code{lock-mutex}, Guile will interrupt the wait, run
the interrupts, and then resume the wait.
@end deffn

@deftypefn {C Function} void scm_dynwind_lock_mutex (SCM mutex)
Arrange for @var{mutex} to be locked whenever the current dynwind
context is entered and to be unlocked when it is exited.
@end deftypefn

@deffn {Scheme Procedure} try-mutex mx
@deffnx {C Function} scm_try_mutex (mx)
Try to lock @var{mutex} and return @code{#t} if successful, or @code{#f}
otherwise.  This is like calling @code{lock-mutex} with an expired
timeout.
@end deffn

@deffn {Scheme Procedure} unlock-mutex mutex
@deffnx {C Function} scm_unlock_mutex (mutex)
Unlock @var{mutex}.  An error is signaled if @var{mutex} is not locked.

``Standard'' and ``recursive'' mutexes can only be unlocked by the
thread that locked them; Guile detects this situation and signals an
error.  ``Unowned'' mutexes can be unlocked by any thread.
@end deffn

@deffn {Scheme Procedure} mutex-owner mutex
@deffnx {C Function} scm_mutex_owner (mutex)
Return the current owner of @var{mutex}, in the form of a thread or
@code{#f} (indicating no owner).  Note that a mutex may be unowned but
still locked.
@end deffn

@deffn {Scheme Procedure} mutex-level mutex
@deffnx {C Function} scm_mutex_level (mutex)
Return the current lock level of @var{mutex}.  If @var{mutex} is
currently unlocked, this value will be 0; otherwise, it will be the
number of times @var{mutex} has been recursively locked by its current
owner.
@end deffn

@deffn {Scheme Procedure} mutex-locked? mutex
@deffnx {C Function} scm_mutex_locked_p (mutex)
Return @code{#t} if @var{mutex} is locked, regardless of ownership;
otherwise, return @code{#f}.
@end deffn

@deffn {Scheme Procedure} make-condition-variable
@deffnx {C Function} scm_make_condition_variable ()
Return a new condition variable.
@end deffn

@deffn {Scheme Procedure} condition-variable? obj
@deffnx {C Function} scm_condition_variable_p (obj)
Return @code{#t} if @var{obj} is a condition variable; otherwise,
return @code{#f}.
@end deffn

@deffn {Scheme Procedure} wait-condition-variable condvar mutex [time]
@deffnx {C Function} scm_wait_condition_variable (condvar, mutex, time)
Wait until @var{condvar} has been signaled.  While waiting,
@var{mutex} is atomically unlocked (as with @code{unlock-mutex}) and
is locked again when this function returns.  When @var{time} is given,
it specifies a point in time where the waiting should be aborted.  It
can be either a integer as returned by @code{current-time} or a pair
as returned by @code{gettimeofday}.  When the waiting is aborted,
@code{#f} is returned.  When the condition variable has in fact been
signaled, @code{#t} is returned.  The mutex is re-locked in any case
before @code{wait-condition-variable} returns.

When an async is activated for a thread that is blocked in a call to
@code{wait-condition-variable}, the waiting is interrupted, the mutex is
locked, and the async is executed.  When the async returns, the mutex is
unlocked again and the waiting is resumed.  When the thread block while
re-acquiring the mutex, execution of asyncs is blocked.
@end deffn

@deffn {Scheme Procedure} signal-condition-variable condvar
@deffnx {C Function} scm_signal_condition_variable (condvar)
Wake up one thread that is waiting for @var{condvar}.
@end deffn

@deffn {Scheme Procedure} broadcast-condition-variable condvar
@deffnx {C Function} scm_broadcast_condition_variable (condvar)
Wake up all threads that are waiting for @var{condvar}.
@end deffn

Guile also includes some higher-level abstractions for working with
mutexes.

@deffn macro with-mutex mutex body1 body2 @dots{}
Lock @var{mutex}, evaluate the body @var{body1} @var{body2} @dots{},
then unlock @var{mutex}.  The return value is that returned by the last
body form.

The lock, body and unlock form the branches of a @code{dynamic-wind}
(@pxref{Dynamic Wind}), so @var{mutex} is automatically unlocked if an
error or new continuation exits the body, and is re-locked if
the body is re-entered by a captured continuation.
@end deffn

@deffn macro monitor body1 body2 @dots{}
Evaluate the body form @var{body1} @var{body2} @dots{} with a mutex
locked so only one thread can execute that code at any one time.  The
return value is the return from the last body form.

Each @code{monitor} form has its own private mutex and the locking and
evaluation is as per @code{with-mutex} above.  A standard mutex
(@code{make-mutex}) is used, which means the body must not
recursively re-enter the @code{monitor} form.

The term ``monitor'' comes from operating system theory, where it
means a particular bit of code managing access to some resource and
which only ever executes on behalf of one process at any one time.
@end deffn


@node Blocking
@subsection Blocking in Guile Mode

Up to Guile version 1.8, a thread blocked in guile mode would prevent
the garbage collector from running.  Thus threads had to explicitly
leave guile mode with @code{scm_without_guile ()} before making a
potentially blocking call such as a mutex lock, a @code{select ()}
system call, etc.  The following functions could be used to temporarily
leave guile mode or to perform some common blocking operations in a
supported way.

Starting from Guile 2.0, blocked threads no longer hinder garbage
collection.  Thus, the functions below are not needed anymore.  They can
still be used to inform the GC that a thread is about to block, giving
it a (small) optimization opportunity for ``stop the world'' garbage
collections, should they occur while the thread is blocked.

@deftypefn {C Function} {void *} scm_without_guile (void *(*func) (void *), void *data)
Leave guile mode, call @var{func} on @var{data}, enter guile mode and
return the result of calling @var{func}.

While a thread has left guile mode, it must not call any libguile
functions except @code{scm_with_guile} or @code{scm_without_guile} and
must not use any libguile macros.  Also, local variables of type
@code{SCM} that are allocated while not in guile mode are not
protected from the garbage collector.

When used from non-guile mode, calling @code{scm_without_guile} is
still allowed: it simply calls @var{func}.  In that way, you can leave
guile mode without having to know whether the current thread is in
guile mode or not.
@end deftypefn

@deftypefn {C Function} int scm_pthread_mutex_lock (pthread_mutex_t *mutex)
Like @code{pthread_mutex_lock}, but leaves guile mode while waiting for
the mutex.
@end deftypefn

@deftypefn  {C Function} int scm_pthread_cond_wait (pthread_cond_t *cond, pthread_mutex_t *mutex)
@deftypefnx {C Function} int scm_pthread_cond_timedwait (pthread_cond_t *cond, pthread_mutex_t *mutex, struct timespec *abstime)
Like @code{pthread_cond_wait} and @code{pthread_cond_timedwait}, but
leaves guile mode while waiting for the condition variable.
@end deftypefn

@deftypefn {C Function} int scm_std_select (int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout)
Like @code{select} but leaves guile mode while waiting.  Also, the
delivery of an async causes this function to be interrupted with error
code @code{EINTR}.
@end deftypefn

@deftypefn {C Function} {unsigned int} scm_std_sleep ({unsigned int} seconds)
Like @code{sleep}, but leaves guile mode while sleeping.  Also, the
delivery of an async causes this function to be interrupted.
@end deftypefn

@deftypefn {C Function} {unsigned long} scm_std_usleep ({unsigned long} usecs)
Like @code{usleep}, but leaves guile mode while sleeping.  Also, the
delivery of an async causes this function to be interrupted.
@end deftypefn


@node Futures
@subsection Futures
@cindex futures
@cindex fine-grain parallelism
@cindex parallelism

The @code{(ice-9 futures)} module provides @dfn{futures}, a construct
for fine-grain parallelism.  A future is a wrapper around an expression
whose computation may occur in parallel with the code of the calling
thread, and possibly in parallel with other futures.  Like promises,
futures are essentially proxies that can be queried to obtain the value
of the enclosed expression:

@lisp
(touch (future (+ 2 3)))
@result{} 5
@end lisp

However, unlike promises, the expression associated with a future may be
evaluated on another CPU core, should one be available.  This supports
@dfn{fine-grain parallelism}, because even relatively small computations
can be embedded in futures.  Consider this sequential code:

@lisp
(define (find-prime lst1 lst2)
  (or (find prime? lst1)
      (find prime? lst2)))
@end lisp

The two arms of @code{or} are potentially computation-intensive.  They
are independent of one another, yet, they are evaluated sequentially
when the first one returns @code{#f}.  Using futures, one could rewrite
it like this:

@lisp
(define (find-prime lst1 lst2)
  (let ((f (future (find prime? lst2))))
    (or (find prime? lst1)
        (touch f))))
@end lisp

This preserves the semantics of @code{find-prime}.  On a multi-core
machine, though, the computation of @code{(find prime? lst2)} may be
done in parallel with that of the other @code{find} call, which can
reduce the execution time of @code{find-prime}.

Futures may be nested: a future can itself spawn and then @code{touch}
other futures, leading to a directed acyclic graph of futures.  Using
this facility, a parallel @code{map} procedure can be defined along
these lines:

@lisp
(use-modules (ice-9 futures) (ice-9 match))

(define (par-map proc lst)
  (match lst
    (()
     '())
    ((head tail ...)
     (let ((tail (future (par-map proc tail)))
           (head (proc head)))
       (cons head (touch tail))))))
@end lisp

Note that futures are intended for the evaluation of purely functional
expressions.  Expressions that have side-effects or rely on I/O may
require additional care, such as explicit synchronization
(@pxref{Mutexes and Condition Variables}).

Guile's futures are implemented on top of POSIX threads
(@pxref{Threads}).  Internally, a fixed-size pool of threads is used to
evaluate futures, such that offloading the evaluation of an expression
to another thread doesn't incur thread creation costs.  By default, the
pool contains one thread per available CPU core, minus one, to account
for the main thread.  The number of available CPU cores is determined
using @code{current-processor-count} (@pxref{Processes}).

When a thread touches a future that has not completed yet, it processes
any pending future while waiting for it to complete, or just waits if
there are no pending futures.  When @code{touch} is called from within a
future, the execution of the calling future is suspended, allowing its
host thread to process other futures, and resumed when the touched
future has completed.  This suspend/resume is achieved by capturing the
calling future's continuation, and later reinstating it (@pxref{Prompts,
delimited continuations}).

@deffn {Scheme Syntax} future exp
Return a future for expression @var{exp}.  This is equivalent to:

@lisp
(make-future (lambda () exp))
@end lisp
@end deffn

@deffn {Scheme Procedure} make-future thunk
Return a future for @var{thunk}, a zero-argument procedure.

This procedure returns immediately.  Execution of @var{thunk} may begin
in parallel with the calling thread's computations, if idle CPU cores
are available, or it may start when @code{touch} is invoked on the
returned future.

If the execution of @var{thunk} throws an exception, that exception will
be re-thrown when @code{touch} is invoked on the returned future.
@end deffn

@deffn {Scheme Procedure} future? obj
Return @code{#t} if @var{obj} is a future.
@end deffn

@deffn {Scheme Procedure} touch f
Return the result of the expression embedded in future @var{f}.

If the result was already computed in parallel, @code{touch} returns
instantaneously.  Otherwise, it waits for the computation to complete,
if it already started, or initiates it.  In the former case, the calling
thread may process other futures in the meantime.
@end deffn


@node Parallel Forms
@subsection Parallel forms
@cindex parallel forms

The functions described in this section are available from

@example
(use-modules (ice-9 threads))
@end example

They provide high-level parallel constructs.  The following functions
are implemented in terms of futures (@pxref{Futures}).  Thus they are
relatively cheap as they re-use existing threads, and portable, since
they automatically use one thread per available CPU core.

@deffn syntax parallel expr @dots{}
Evaluate each @var{expr} expression in parallel, each in its own thread.
Return the results of @var{n} expressions as a set of @var{n} multiple
values (@pxref{Multiple Values}).
@end deffn

@deffn syntax letpar ((var expr) @dots{}) body1 body2 @dots{}
Evaluate each @var{expr} in parallel, each in its own thread, then bind
the results to the corresponding @var{var} variables, and then evaluate
@var{body1} @var{body2} @enddots{}

@code{letpar} is like @code{let} (@pxref{Local Bindings}), but all the
expressions for the bindings are evaluated in parallel.
@end deffn

@deffn {Scheme Procedure} par-map proc lst1 lst2 @dots{}
@deffnx {Scheme Procedure} par-for-each proc lst1 lst2 @dots{}
Call @var{proc} on the elements of the given lists.  @code{par-map}
returns a list comprising the return values from @var{proc}.
@code{par-for-each} returns an unspecified value, but waits for all
calls to complete.

The @var{proc} calls are @code{(@var{proc} @var{elem1} @var{elem2}
@dots{})}, where each @var{elem} is from the corresponding @var{lst} .
Each @var{lst} must be the same length.  The calls are potentially made
in parallel, depending on the number of CPU cores available.

These functions are like @code{map} and @code{for-each} (@pxref{List
Mapping}), but make their @var{proc} calls in parallel.
@end deffn

Unlike those above, the functions described below take a number of
threads as an argument.  This makes them inherently non-portable since
the specified number of threads may differ from the number of available
CPU cores as returned by @code{current-processor-count}
(@pxref{Processes}).  In addition, these functions create the specified
number of threads when they are called and terminate them upon
completion, which makes them quite expensive.

Therefore, they should be avoided.

@deffn {Scheme Procedure} n-par-map n proc lst1 lst2 @dots{}
@deffnx {Scheme Procedure} n-par-for-each n proc lst1 lst2 @dots{}
Call @var{proc} on the elements of the given lists, in the same way as
@code{par-map} and @code{par-for-each} above, but use no more than
@var{n} threads at any one time.  The order in which calls are
initiated within that threads limit is unspecified.

These functions are good for controlling resource consumption if
@var{proc} calls might be costly, or if there are many to be made.  On
a dual-CPU system for instance @math{@var{n}=4} might be enough to
keep the CPUs utilized, and not consume too much memory.
@end deffn

@deffn {Scheme Procedure} n-for-each-par-map n sproc pproc lst1 lst2 @dots{}
Apply @var{pproc} to the elements of the given lists, and apply
@var{sproc} to each result returned by @var{pproc}.  The final return
value is unspecified, but all calls will have been completed before
returning.

The calls made are @code{(@var{sproc} (@var{pproc} @var{elem1} @dots{}
@var{elemN}))}, where each @var{elem} is from the corresponding
@var{lst}.  Each @var{lst} must have the same number of elements.

The @var{pproc} calls are made in parallel, in separate threads.  No more
than @var{n} threads are used at any one time.  The order in which
@var{pproc} calls are initiated within that limit is unspecified.

The @var{sproc} calls are made serially, in list element order, one at
a time.  @var{pproc} calls on later elements may execute in parallel
with the @var{sproc} calls.  Exactly which thread makes each
@var{sproc} call is unspecified.

This function is designed for individual calculations that can be done
in parallel, but with results needing to be handled serially, for
instance to write them to a file.  The @var{n} limit on threads
controls system resource usage when there are many calculations or
when they might be costly.

It will be seen that @code{n-for-each-par-map} is like a combination
of @code{n-par-map} and @code{for-each},

@example
(for-each sproc (n-par-map n pproc lst1 ... lstN))
@end example

@noindent
But the actual implementation is more efficient since each @var{sproc}
call, in turn, can be initiated once the relevant @var{pproc} call has
completed, it doesn't need to wait for all to finish.
@end deffn



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