starpu-max/doc/doxygen/chapters/scheduling_contexts.doxy

/*
 * This file is part of the StarPU Handbook.
 * Copyright (C) 2009--2011  Universit@'e de Bordeaux 1
 * Copyright (C) 2010, 2011, 2012, 2013  Centre National de la Recherche Scientifique
 * Copyright (C) 2011, 2012 Institut National de Recherche en Informatique et Automatique
 * See the file version.doxy for copying conditions.
 */

/*! \page SchedulingContexts Scheduling Contexts

TODO: improve!

\section GeneralIdeas General Ideas

Scheduling contexts represent abstracts sets of workers that allow the
programmers to control the distribution of computational resources
(i.e. CPUs and GPUs) to concurrent parallel kernels. The main goal is
to minimize interferences between the execution of multiple parallel
kernels, by partitioning the underlying pool of workers using
contexts.

\section CreatingAContext Creating A Context

By default, the application submits tasks to an initial context, which
disposes of all the computation ressources available to StarPU (all
the workers). If the application programmer plans to launch several
parallel kernels simultaneusly, by default these kernels will be
executed within this initial context, using a single scheduler
policy(see \ref TaskSchedulingPolicy). Meanwhile, if the application
programmer is aware of the demands of these kernels and of the
specificity of the machine used to execute them, the workers can be
divided between several contexts. These scheduling contexts will
isolate the execution of each kernel and they will permit the use of a
scheduling policy proper to each one of them. In order to create the
contexts, you have to know the indentifiers of the workers running
within StarPU. By passing a set of workers together with the
scheduling policy to the function starpu_sched_ctx_create(), you will
get an identifier of the context created which you will use to
indicate the context you want to submit the tasks to.

\code{.c}
/* the list of ressources the context will manage */
int workerids[3] = {1, 3, 10};

/* indicate the scheduling policy to be used within the context, the list of
   workers assigned to it, the number of workers, the name of the context */
int id_ctx = starpu_sched_ctx_create("dmda", workerids, 3, "my_ctx");

/* let StarPU know that the folowing tasks will be submitted to this context */
starpu_sched_ctx_set_task_context(id);

/* submit the task to StarPU */
starpu_task_submit(task);
\endcode

Note: Parallel greedy and parallel heft scheduling policies do not support the existence of several disjoint contexts on the machine.
Combined workers are constructed depending on the entire topology of the machine, not only the one belonging to a context.

\section ModifyingAContext Modifying A Context

A scheduling context can be modified dynamically. The applications may
change its requirements during the execution and the programmer can
add additional workers to a context or remove if no longer needed. In
the following example we have two scheduling contexts
<c>sched_ctx1</c> and <c>sched_ctx2</c>. After executing a part of the
tasks some of the workers of <c>sched_ctx1</c> will be moved to
context <c>sched_ctx2</c>.

\code{.c}
/* the list of ressources that context 1 will give away */
int workerids[3] = {1, 3, 10};

/* add the workers to context 1 */
starpu_sched_ctx_add_workers(workerids, 3, sched_ctx2);

/* remove the workers from context 2 */
starpu_sched_ctx_remove_workers(workerids, 3, sched_ctx1);
\endcode

\section DeletingAContext Deleting A Context

When a context is no longer needed it must be deleted. The application
can indicate which context should keep the resources of a deleted one.
All the tasks of the context should be executed before doing this. If
the application need to avoid a barrier before moving the resources
from the deleted context to the inheritor one, the application can
just indicate when the last task was submitted. Thus, when this last
task was submitted the resources will be move, but the context should
still be deleted at some point of the application.

\code{.c}
/* when the context 2 will be deleted context 1 will be keep its resources */
starpu_sched_ctx_set_inheritor(sched_ctx2, sched_ctx1);

/* submit tasks to context 2 */
for (i = 0; i < ntasks; i++)
    starpu_task_submit_to_ctx(task[i],sched_ctx2);

/* indicate that context 2 finished submitting and that */
/* as soon as the last task of context 2 finished executing */
/* its workers can be mobed to the inheritor context */
starpu_sched_ctx_finished_submit(sched_ctx1);

/* wait for the tasks of both contexts to finish */
starpu_task_wait_for_all();

/* delete context 2 */
starpu_sched_ctx_delete(sched_ctx2);

/* delete context 1 */
starpu_sched_ctx_delete(sched_ctx1);
\endcode

\section EmptyingAContext Emptying A Context

A context may not have any resources at the begining or at a certain
moment of the execution. Task can still be submitted to these contexts
and they will execute them as soon as they will have resources. A list
of tasks pending to be executed is kept and when workers are added to
the contexts the tasks are submitted. However, if no resources are
allocated the program will not terminate. If these tasks have not much
priority the programmer can forbid the application to submitted them
by calling the function starpu_sched_ctx_stop_task_submission().

\section ContextsSharingWorkers Contexts Sharing Workers

Contexts may share workers when a single context cannot execute
efficiently enough alone on these workers or when the application
decides to express a hierarchy of contexts. The workers apply an
alogrithm of ``Round-Robin'' to chose the context on which they will
``pop'' next. By using the function
starpu_sched_ctx_set_turn_to_other_ctx(), the programmer can impose
the <c>workerid</c> to ``pop'' in the context <c>sched_ctx_id</c>
next.

*/