starpu-max/doc/doxygen/chapters/320_scheduling.doxy

/* StarPU --- Runtime system for heterogeneous multicore architectures.
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/*! \page Scheduling Scheduling

\section TaskSchedulingPolicy Task Scheduling Policies

The basics of the scheduling policy are the following:

<ul>
<li>The scheduler gets to schedule tasks (<c>push</c> operation) when they become
ready to be executed, i.e. they are not waiting for some tags, data dependencies
or task dependencies.</li>
<li>Workers pull tasks (<c>pop</c> operation) one by one from the scheduler.
</ul>

This means scheduling policies usually contain at least one queue of tasks to
store them between the time when they become available, and the time when a
worker gets to grab them.

By default, StarPU uses the work-stealing scheduler <c>lws</c>. This is
because it provides correct load balance and locality even if the application codelets do
not have performance models. Other non-modelling scheduling policies can be
selected among the list below, thanks to the environment variable \ref
STARPU_SCHED. For instance <c>export STARPU_SCHED=dmda</c> . Use <c>help</c> to
get the list of available schedulers.


\subsection NonPerformanceModelingPolicies Non Performance Modelling Policies

- The <b>eager</b> scheduler uses a central task queue, from which all workers draw tasks
to work on concurrently. This however does not permit to prefetch data since the scheduling
decision is taken late. If a task has a non-0 priority, it is put at the front of the queue.

- The <b>random</b> scheduler uses a queue per worker, and distributes tasks randomly according to assumed worker
overall performance.

- The <b>ws</b> (work stealing) scheduler uses a queue per worker, and schedules
a task on the worker which released it by
default. When a worker becomes idle, it steals a task from the most loaded
worker.

- The <b>lws</b> (locality work stealing) scheduler uses a queue per worker, and schedules
a task on the worker which released it by
default. When a worker becomes idle, it steals a task from neighbour workers. It
also takes into account priorities.

- The <b>prio</b> scheduler also uses a central task queue, but sorts tasks by
priority specified by the programmer (between -5 and 5).

- The <b>heteroprio</b> scheduler uses different priorities for the different processing units.
This scheduler must be configured to work correclty and to expect high-performance
as described in the corresponding section.

\subsection DMTaskSchedulingPolicy Performance Model-Based Task Scheduling Policies

If (<b>and only if</b>) your application <b>codelets have performance models</b> (\ref
PerformanceModelExample), you should change the scheduler thanks to the
environment variable \ref STARPU_SCHED, to select one of the policies below, in
order to take advantage of StarPU's performance modelling. For instance
<c>export STARPU_SCHED=dmda</c> . Use <c>help</c> to get the list of available
schedulers.

<b>Note:</B> Depending on the performance model type chosen, some preliminary
calibration runs may be needed for the model to converge. If the calibration
has not been done, or is insufficient yet, or if no performance model is
specified for a codelet, every task built from this codelet will be scheduled
using an <b>eager</b> fallback policy.

<b>Troubleshooting:</b> Configuring and recompiling StarPU using the
\ref enable-verbose "--enable-verbose" \c configure option displays some statistics at the end of
execution about the percentage of tasks which have been scheduled by a DM*
family policy using performance model hints. A low or zero percentage may be
the sign that performance models are not converging or that codelets do not
have performance models enabled.

- The <b>dm</b> (deque model) scheduler takes task execution performance models into account to
perform a HEFT-similar scheduling strategy: it schedules tasks where their
termination time will be minimal. The difference with HEFT is that <b>dm</b>
schedules tasks as soon as they become available, and thus in the order they
become available, without taking priorities into account.

- The <b>dmda</b> (deque model data aware) scheduler is similar to dm, but it also takes
into account data transfer time.

- The <b>dmdap</b> (deque model data aware prio) scheduler is similar to dmda,
except that it sorts tasks by priority order, which allows to become even closer
to HEFT by respecting priorities after having made the scheduling decision (but
it still schedules tasks in the order they become available).

- The <b>dmdar</b> (deque model data aware ready) scheduler is similar to dmda,
but it also privileges tasks whose data buffers are already available
on the target device.

- The <b>dmdas</b> combines dmdap and dmdas: it sorts tasks by priority order,
but for a given priority it will privilege tasks whose data buffers are already
available on the target device.

- The <b>dmdasd</b> (deque model data aware sorted decision) scheduler is similar
to dmdas, except that when scheduling a task, it takes into account its priority
when computing the minimum completion time, since this task may get executed
before others, and thus the latter should be ignored.

- The <b>heft</b> (heterogeneous earliest finish time) scheduler is a deprecated
alias for <b>dmda</b>.

- The <b>pheft</b> (parallel HEFT) scheduler is similar to dmda, it also supports
parallel tasks (still experimental). Should not be used when several contexts using
it are being executed simultaneously.

- The <b>peager</b> (parallel eager) scheduler is similar to eager, it also
supports parallel tasks (still experimental). Should not be used when several 
contexts using it are being executed simultaneously.

\subsection ExistingModularizedSchedulers Modularized Schedulers

StarPU provides a powerful way to implement schedulers, as documented in \ref
DefiningANewModularSchedulingPolicy . It is currently shipped with the following
pre-defined Modularized Schedulers :


- <b>modular-eager</b> , <b>modular-eager-prefetching</b> are eager-based Schedulers (without and with prefetching)), they are \n
naive schedulers, which try to map a task on the first available resource
they find. The prefetching variant queues several tasks in advance to be able to
do data prefetching. This may however degrade load balancing a bit.

- <b>modular-prio</b>, <b>modular-prio-prefetching</b>, <b>modular-eager-prio</b> are prio-based Schedulers (without / with prefetching):,
similar to Eager-Based Schedulers. Can handle tasks which have a defined
priority and schedule them accordingly.
The <b>modular-eager-prio</b> variant integrates the eager and priority queue in a
single component. This allows it to do a better job at pushing tasks.

- <b>modular-random</b>, <b>modular-random-prio</b>, <b>modular-random-prefetching</b>, <b>modular-random-prio-prefetching</b> are random-based Schedulers (without/with prefetching) : \n
Select randomly a resource to be mapped on for each task.

- <b>modular-ws</b>) implements Work Stealing:
Maps tasks to workers in round robin, but allows workers to steal work from other workers.

- <b>modular-heft</b>, <b>modular-heft2</b>, and <b>modular-heft-prio</b> are
HEFT Schedulers : \n
Maps tasks to workers using a heuristic very close to
Heterogeneous Earliest Finish Time.
It needs that every task submitted to StarPU have a
defined performance model (\ref PerformanceModelCalibration)
to work efficiently, but can handle tasks without a performance
model. <b>modular-heft</b> just takes tasks by priority order. <b>modular-heft2</b> takes
at most 5 tasks of the same priority and checks which one fits best.
<b>modular-heft-prio</b> is similar to <b>modular-heft</b>, but only decides the memory
node, not the exact worker, just pushing tasks to one central queue per memory
node.

- <b>modular-heteroprio</b> is a Heteroprio Scheduler: \n
Maps tasks to worker similarly to HEFT, but first attribute accelerated tasks to
GPUs, then not-so-accelerated tasks to CPUs.

\section TaskDistributionVsDataTransfer Task Distribution Vs Data Transfer

Distributing tasks to balance the load induces data transfer penalty. StarPU
thus needs to find a balance between both. The target function that the
scheduler <c>dmda</c> of StarPU
tries to minimize is <c>alpha * T_execution + beta * T_data_transfer</c>, where
<c>T_execution</c> is the estimated execution time of the codelet (usually
accurate), and <c>T_data_transfer</c> is the estimated data transfer time. The
latter is estimated based on bus calibration before execution start,
i.e. with an idle machine, thus without contention. You can force bus
re-calibration by running the tool <c>starpu_calibrate_bus</c>. The
beta parameter defaults to <c>1</c>, but it can be worth trying to tweak it
by using <c>export STARPU_SCHED_BETA=2</c> (\ref STARPU_SCHED_BETA) for instance, since during
real application execution, contention makes transfer times bigger.
This is of course imprecise, but in practice, a rough estimation
already gives the good results that a precise estimation would give.

\section Energy-basedScheduling Energy-based Scheduling

If the application can provide some energy consumption performance model (through
the field starpu_codelet::energy_model), StarPU will
take it into account when distributing tasks. The target function that
the scheduler <c>dmda</c> minimizes becomes <c>alpha * T_execution +
beta * T_data_transfer + gamma * Consumption</c> , where <c>Consumption</c>
is the estimated task consumption in Joules. To tune this parameter, use
<c>export STARPU_SCHED_GAMMA=3000</c> (\ref STARPU_SCHED_GAMMA) for instance, to express that each Joule
(i.e kW during 1000us) is worth 3000us execution time penalty. Setting
<c>alpha</c> and <c>beta</c> to zero permits to only take into account energy consumption.

This is however not sufficient to correctly optimize energy: the scheduler would
simply tend to run all computations on the most energy-conservative processing
unit. To account for the consumption of the whole machine (including idle
processing units), the idle power of the machine should be given by setting
<c>export STARPU_IDLE_POWER=200</c> (\ref STARPU_IDLE_POWER) for 200W, for instance. This value can often
be obtained from the machine power supplier.

The energy actually consumed by the total execution can be displayed by setting
<c>export STARPU_PROFILING=1 STARPU_WORKER_STATS=1</c> (\ref STARPU_PROFILING and \ref STARPU_WORKER_STATS).

For OpenCL devices, on-line task consumption measurement is currently supported through the
<c>CL_PROFILING_POWER_CONSUMED</c> OpenCL extension, implemented in the MoviSim
simulator.

For CUDA devices, on-line task consumption measurement is supported on V100
cards and beyond. This however only works for quite long tasks, since the
measurement granularity is about 10ms.

Applications can however provide explicit measurements by using the function
starpu_perfmodel_update_history() (examplified in \ref PerformanceModelExample
with the <c>energy_model</c> performance model). Fine-grain measurement
is often not feasible with the feedback provided by the hardware, so the
user can for instance run a given task a thousand times, measure the global
consumption for that series of tasks, divide it by a thousand, repeat for
varying kinds of tasks and task sizes, and eventually feed StarPU with these
manual measurements through starpu_perfmodel_update_history().  For instance,
for CUDA devices, <c>nvidia-smi -q -d POWER</c> can be used to get the current
consumption in Watt. Multiplying this value by the average duration of a
single task gives the consumption of the task in Joules, which can be given to
starpu_perfmodel_update_history().

Another way to provide the energy performance is to define a
perfmodel with starpu_perfmodel::type ::STARPU_PER_ARCH or
::STARPU_PER_WORKER , and set the starpu_perfmodel::arch_cost_function or
starpu_perfmodel::worker_cost_function field to a function which shall return
the estimated consumption of the task in Joules. Such a function can for instance
use starpu_task_expected_length() on the task (in µs), multiplied by the
typical power consumption of the device, e.g. in W, and divided by 1000000. to
get Joules.

\section StaticScheduling Static Scheduling

In some cases, one may want to force some scheduling, for instance force a given
set of tasks to GPU0, another set to GPU1, etc. while letting some other tasks
be scheduled on any other device. This can indeed be useful to guide StarPU into
some work distribution, while still letting some degree of dynamism. For
instance, to force execution of a task on CUDA0:

\code{.c}
task->execute_on_a_specific_worker = 1;
task->workerid = starpu_worker_get_by_type(STARPU_CUDA_WORKER, 0);
\endcode

or equivalently

\code{.c}
starpu_task_insert(&cl, ..., STARPU_EXECUTE_ON_WORKER, starpu_worker_get_by_type(STARPU_CUDA_WORKER, 0), ...);
\endcode

One can also specify a set worker(s) which are allowed to take the task, as an
array of bit, for instance to allow workers 2 and 42:

\code{.c}
task->workerids = calloc(2,sizeof(uint32_t));
task->workerids[2/32] |= (1 << (2%32));
task->workerids[42/32] |= (1 << (42%32));
task->workerids_len = 2;
\endcode

One can also specify the order in which tasks must be executed by setting the
starpu_task::workerorder field. If this field is set to a non-zero value, it
provides the per-worker consecutive order in which tasks will be executed,
starting from 1. For a given of such task, the worker will thus not execute
it before all the tasks with smaller order value have been executed, notably
in case those tasks are not available yet due to some dependencies. This
eventually gives total control of task scheduling, and StarPU will only serve as
a "self-timed" task runtime. Of course, the provided order has to be runnable,
i.e. a task should should not depend on another task bound to the same worker
with a bigger order.

Note however that using scheduling contexts while statically scheduling tasks on workers
could be tricky. Be careful to schedule the tasks exactly on the workers of the corresponding
contexts, otherwise the workers' corresponding scheduling structures may not be allocated or
the execution of the application may deadlock. Moreover, the hypervisor should not be used when
statically scheduling tasks.

\section Configuring Heteroprio

Within Heteroprio, one priority per processing unit type is assigned  to each task, such that a task has several
priorities. Each worker pops the task that has the highest priority for the hardware type it uses, which
could be CPU or CUDA for example. Therefore, the priorities has to be used to manage the critical path,
but also to promote the consumption of tasks by the more appropriate workers.

The tasks are stored inside buckets, where each bucket corresponds to a priority set. Then each
worker uses an indirect access array to know the order in which it should access the buckets. Moreover,
all the tasks inside a bucket must be compatible with all the processing units that may access it (at least).

As an example, see the following code where we have 5 types of tasks.
CPU workers can compute all of them, but CUDA workers can only execute
tasks of types 0 and 1, and is expected to go 20 and 30 time
faster than the CPU, respectively.
\code{.c}
// In the file that init StarPU
#include <starpu_heteroprio.h>

////////////////////////////////////////////////////

// Before calling starpu_init
struct starpu_conf conf;
starpu_conf_init(&conf);
// Inform StarPU to use Heteroprio
conf.sched_policy_name = "heteroprio";
// Inform StarPU about the function that will init the priorities in Heteroprio
// where init_heteroprio is a function to implement
conf.sched_policy_init = &init_heteroprio;
// Do other things with conf if needed, then init StarPU
starpu_init(&conf);

////////////////////////////////////////////////////

void init_heteroprio(unsigned sched_ctx) {
  // CPU uses 5 buckets and visits them in the natural order
  starpu_heteroprio_set_nb_prios(ctx, STARPU_CPU_IDX, 5);
  // It uses direct mapping idx => idx
  for(unsigned idx = 0; idx < 5; ++idx){
    starpu_heteroprio_set_mapping(ctx, STARPU_CPU_IDX, idx, idx);
    // If there is no CUDA worker we must tell that CPU is faster
    starpu_heteroprio_set_faster_arch(ctx, STARPU_CPU_IDX, idx);
  }
  
  if(starpu_cuda_worker_get_count()){
    // CUDA is enabled and uses 2 buckets
    starpu_heteroprio_set_nb_prios(ctx, STARPU_CUDA_IDX, 2);
    // CUDA will first look at bucket 1
    starpu_heteroprio_set_mapping(ctx, STARPU_CUDA_IDX, 0, 1);
    // CUDA will then look at bucket 2
    starpu_heteroprio_set_mapping(ctx, STARPU_CUDA_IDX, 1, 2);

    // For bucket 1 CUDA is the fastest
    starpu_heteroprio_set_faster_arch(ctx, STARPU_CUDA_IDX, 1);
    // And CPU is 30 times slower
    starpu_heteroprio_set_arch_slow_factor(ctx, STARPU_CPU_IDX, 1, 30.0f);
    
    // For bucket 0 CUDA is the fastest
    starpu_heteroprio_set_faster_arch(ctx, STARPU_CUDA_IDX, 0);
    // And CPU is 20 times slower
    starpu_heteroprio_set_arch_slow_factor(ctx, STARPU_CPU_IDX, 0, 20.0f);
  }
}
\endcode

Then, when a task is inserted <b>the priority of the task will be used to 
select in which bucket is has to be stored</b>.
So, in the given example, the priority of a task will be between 0 and 4 included.
However, tasks of priorities 0-1 must provide CPU and CUDA kernels, and
tasks of priorities 2-4 must provide CPU kernels (at least).

*/