starpu-max/doc/doxygen/chapters/330_scheduling_contexts.doxy

/*
 * This file is part of the StarPU Handbook.
//  * Copyright (C) 2009--2011  Universit@'e de Bordeaux
 * Copyright (C) 2010, 2011, 2012, 2013, 2016  CNRS
 * Copyright (C) 2011, 2012 INRIA
 * See the file version.doxy for copying conditions.
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/*! \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 resources available to StarPU (all
the workers). If the application programmer plans to launch several
parallel kernels simultaneously, 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. 

Scheduling Contexts may be created in two ways: either the programmers indicates
the set of workers corresponding to each context (providing he knows the 
identifiers of the workers running within StarPU), or the programmer
does not provide any worker list and leaves the Hypervisor assign
workers to each context according to their needs (\ref SchedulingContextHypervisor)

Both cases require a call to the function
starpu_sched_ctx_create(), which requires as input the worker
list (the exact list or a NULL pointer) and a list of optional
parameters such as the scheduling policy, terminated by a 0. The
scheduling policy can be a character list corresponding to the name of
a StarPU predefined policy or the pointer to a custom policy. The
function returns an identifier of the context created which you will
use to indicate the context you want to submit the tasks to.

Please note that if no scheduling policy is specified, the context
will be used as another type of resource: a cluster, which we consider
contexts without scheduler (eventually delegated to another
runtime). For more information see \ref ClusteringAMachine. It is
therefore <b>mandatory</b> to stipulate the context's scheduler to use
it in this traditional way.

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

/* indicate the list of workers assigned to it, the number of workers,
the name of the context and the scheduling policy to be used within
the context */
int id_ctx = starpu_sched_ctx_create(workerids, 3, "my_ctx", STARPU_SCHED_CTX_POLICY_NAME, "dmda", 0);

/* let StarPU know that the following 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 SubmittingTasksToAContext Submitting Tasks To A Context
The application may submit tasks to several contexts either 
simultaneously or sequnetially. If several threads of submission
are used the function starpu_sched_ctx_set_context() may be called just
before starpu_task_submit(). Thus StarPU considers that 
the current thread will submit tasks to the coresponding context.
 
When the application may not assign a thread of submission to each
context, the id of the context must be indicated by using the
function starpu_task_submit_to_ctx() or the field \ref STARPU_SCHED_CTX 
for starpu_task_insert().

\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. 
Thus, the programmer may use either a barrier and then delete the context 
directly, or just indicate
that other tasks will not be submitted later on to the context (such that when 
the last task is executed its workers will be moved to the inheritor)
and delete the context at the end of the execution (when a barrier will
be used eventually).

\code{.c}
/* when the context 2 is deleted context 1 inherits 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 moved 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 have no resources at the begining or at a certain
moment of the execution. Task can still be submitted to these contexts
and they will be executed as soon as the contexts will have resources. A list
of tasks pending to be executed is kept and when workers are added to
the contexts these tasks start being submitted. However, if resources 
are never allocated to the context the program will not terminate. 
If these tasks have low
priority the programmer can forbid the application to submit 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.

*/