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4. Advanced Scheduling

# 4.1 Energy-based Scheduling

Note: by default, StarPU does not let CPU workers sleep, to let them react to task release as quickly as possible. For idle time to really let CPU cores save energy, one needs to use the `configure` option --enable-blocking-drivers.

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 dmda minimizes becomes `alpha * T_execution + beta * T_data_transfer + gamma * Consumption` , where `Consumption` is the estimated task consumption in Joules. To tune this parameter, use `export STARPU_SCHED_GAMMA=3000` (STARPU_SCHED_GAMMA) for instance, to express that each Joule (i.e. kW during 1000us) is worth 3000us execution time penalty. Setting `alpha` and `beta` 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 `export STARPU_IDLE_POWER=200` (STARPU_IDLE_POWER) for 200W, for instance. This value can often be obtained from the machine power supplier, e.g. by running

    ipmitool -I lanplus -H mymachine-ipmi -U myuser -P mypasswd sdr type Current

The energy actually consumed by the total execution can be displayed by setting `export STARPU_PROFILING=1 STARPU_WORKER_STATS=1` (STARPU_PROFILING and STARPU_WORKER_STATS).

For OpenCL devices, on-line task consumption measurement is currently supported through the OpenCL extension `CL_PROFILING_POWER_CONSUMED`, 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 feeding the energy performance model by hand. Fine-grain measurement is often not feasible with the feedback provided by the hardware, so users 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. For CUDA devices starting with V100, the starpu_energy_start() and starpu_energy_stop() helpers, described in Measuring energy and power with StarPU below, make it easy.

For older models, one can use `nvidia-smi -q -d POWER` 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(). (examplified in PerformanceModelExample with the performance model `energy_model`).

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 field starpu_perfmodel::arch_cost_function or starpu_perfmodel::worker_cost_function 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. An example is in the file `tests/perfmodels/regression_based_energy.c`.

There are other functions in StarPU that are used to measure the energy consumed by the system during execution. The starpu_energy_use() function declares that there are the energy consumptions of the task, while the starpu_energy_used() function returns the total energy consumed since the start of measurement.

## 4.1.1 Measuring energy and power with StarPU

We have extended the performance model of StarPU to measure energy and power values of CPUs. These values are measured using the existing Performance API (PAPI) analysis library. PAPI provides the tool designer and application engineer with a consistent interface and methodology for use of the performance counter hardware found in most major microprocessors. PAPI enables software engineers to see, in near real time, the relation between software performance and processor events.

- To measure energy consumption of CPUs, we use the `RAPL` events, which are available on CPU architecture: `RAPL_ENERGY_PKG` that represents the whole CPU socket power consumption, and `RAPL_ENERGY_DRAM` that represents the RAM power consumption.

PAPI provides a generic, portable interface for the hardware performance counters available on all modern CPUs and some other components of interest that are scattered across the chip and system.

In order to use the right `rapl events` for energy measurement, user should check the `rapl events` available on the machine, using this command:

    $ papi_native_avail

Depending on the system configuration, users may have to run this as root to get the performance counter values.

Since the measurement is for all the CPUs and the memory, the approach taken here is to run a series of tasks on all of them and to take the overall measurement.

- The example below illustrates the energy and power measurements, using the functions starpu_energy_start() and starpu_energy_stop().

In this example, we launch several tasks of the same type in parallel. To perform the energy requirement measurement of a program, we call starpu_energy_start(), which initializes energy measurement counters and starpu_energy_stop(struct starpu_perfmodel *model, struct starpu_task *task, unsigned nimpl, unsigned ntasks, int workerid, enum starpu_worker_archtype archi) to stop counting and update the performance model. This ends up yielding the average energy requirement of a single task. The example below illustrates this for a given task type.

unsigned N = starpu_cpu_worker_get_count() * 40;
starpu_energy_start(-1, STARPU_CPU_WORKER);
for (i = 0; i < N; i++)
starpu_task_insert(&cl, STARPU_EXECUTE_WHERE, STARPU_CPU, STARPU_R, arg1, STARPU_RW, arg2, 0);
starpu_task_t *specimen = starpu_task_build(&cl, STARPU_R, arg1, STARPU_RW, arg2, 0);
starpu_energy_stop(&codelet.energy_model, specimen, 0, N, -1, STARPU_CPU_WORKER);
. . .
STARPU_CPU
#define STARPU_CPU
Definition starpu_task.h:58
STARPU_RW
@ STARPU_RW
Definition starpu_data.h:60
STARPU_R
@ STARPU_R
Definition starpu_data.h:58
starpu_task_build
struct starpu_task * starpu_task_build(struct starpu_codelet *cl,...)
STARPU_EXECUTE_WHERE
#define STARPU_EXECUTE_WHERE
Definition starpu_task_util.h:166
starpu_task_insert
int starpu_task_insert(struct starpu_codelet *cl,...)
starpu_energy_start
int starpu_energy_start(int workerid, enum starpu_worker_archtype archi)
starpu_energy_stop
int starpu_energy_stop(struct starpu_perfmodel *model, struct starpu_task *task, unsigned nimpl, unsigned ntasks, int workerid, enum starpu_worker_archtype archi)
starpu_cpu_worker_get_count
unsigned starpu_cpu_worker_get_count(void)
STARPU_CPU_WORKER
@ STARPU_CPU_WORKER
Definition starpu_worker.h:67

The example starts 40 times more tasks of the same type than there are CPU execution units. Once the tasks are distributed over all CPUs, the latter are all executing the same type of tasks (with the same data size and parameters); each CPU will in the end execute 40 tasks. A specimen task is then constructed and passed to starpu_energy_stop(), which will fold into the performance model the energy requirement measurement for that type and size of task.

For the energy and power measurements, depending on the system configuration, users may have to run applications as root to use PAPI library.

The function starpu_energy_stop() uses `PAPI_stop()` to stop counting and store the values into the array. We calculate both energy in `Joules` and power consumption in `Watt`. We call the function starpu_perfmodel_update_history() in the performance model to provide explicit measurements.

- In the CUDA case, nvml provides per-GPU energy measurement. We can thus calibrate the performance models per GPU:

unsigned N = 40;
for (i = 0; i < starpu_cuda_worker_get_count(); i++) {
int workerid = starpu_worker_get_by_type(STARPU_CUDA_WORKER, i);
starpu_energy_start(workerid, STARPU_CUDA_WORKER);
for (i = 0; i < N; i++)
starpu_task_insert(&cl, STARPU_EXECUTE_ON_WORKER, workerid, STARPU_R, arg1, STARPU_RW, arg2, 0);
starpu_task_t *specimen = starpu_task_build(&cl, STARPU_R, arg1, STARPU_RW, arg2, 0);
starpu_energy_stop(&codelet.energy_model, specimen, 0, N, workerid, STARPU_CUDA_WORKER);
}
STARPU_EXECUTE_ON_WORKER
#define STARPU_EXECUTE_ON_WORKER
Definition starpu_task_util.h:159
starpu_worker_get_by_type
int starpu_worker_get_by_type(enum starpu_worker_archtype type, int num)
starpu_cuda_worker_get_count
unsigned starpu_cuda_worker_get_count(void)
STARPU_CUDA_WORKER
@ STARPU_CUDA_WORKER
Definition starpu_worker.h:68

- A complete example is available in `tests/perfmodels/regression_based_memset.c`

# 4.2 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:

task->execute_on_a_specific_worker = 1;
task->workerid = starpu_worker_get_by_type(STARPU_CUDA_WORKER, 0);

An example is in the file `tests/errorcheck/invalid_tasks.c`.

or equivalently

starpu_task_insert(&cl, ..., STARPU_EXECUTE_ON_WORKER, starpu_worker_get_by_type(STARPU_CUDA_WORKER, 0), ...);

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

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;

One can also specify the order in which tasks must be executed by setting the field starpu_task::workerorder. An example is available in the file `tests/main/execute_schedule.c`. 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 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.

# 4.3 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).

These priorities are now automatically assigned by Heteroprio in auto calibration mode using heuristics. If you want to set these priorities manually, you can change STARPU_HETEROPRIO_USE_AUTO_CALIBRATION and follow the example below.

In this example code, 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 are expected to go 20 and 30 time faster than the CPU, respectively.

#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_callback = &init_heteroprio;
// Do other things with conf if needed, then init StarPU
starpu_init(&conf);
starpu_init
int starpu_init(struct starpu_conf *conf)
starpu_conf_init
int starpu_conf_init(struct starpu_conf *conf)
starpu_conf
Definition starpu.h:110
starpu_heteroprio.h
void init_heteroprio(unsigned sched_ctx) {
// CPU uses 5 buckets and visits them in the natural order
starpu_heteroprio_set_nb_prios(sched_ctx, STARPU_CPU_WORKER, 5);
// It uses direct mapping idx => idx
for(unsigned idx = 0; idx < 5; ++idx){
starpu_heteroprio_set_mapping(sched_ctx, STARPU_CPU_WORKER, idx, idx);
// If there is no CUDA worker we must tell that CPU is faster
starpu_heteroprio_set_faster_arch(sched_ctx, STARPU_CPU_WORKER, idx);
}
if(starpu_cuda_worker_get_count()){
// CUDA is enabled and uses 2 buckets
starpu_heteroprio_set_nb_prios(sched_ctx, STARPU_CUDA_WORKER, 2);
// CUDA will first look at bucket 1
starpu_heteroprio_set_mapping(sched_ctx, STARPU_CUDA_WORKER, 0, 1);
// CUDA will then look at bucket 2
starpu_heteroprio_set_mapping(sched_ctx, STARPU_CUDA_WORKER, 1, 2);
// For bucket 1 CUDA is the fastest
starpu_heteroprio_set_faster_arch(sched_ctx, STARPU_CUDA_WORKER, 1);
// And CPU is 30 times slower
starpu_heteroprio_set_arch_slow_factor(sched_ctx, STARPU_CPU_WORKER, 1, 30.0f);
// For bucket 0 CUDA is the fastest
starpu_heteroprio_set_faster_arch(sched_ctx, STARPU_CUDA_WORKER, 0);
// And CPU is 20 times slower
starpu_heteroprio_set_arch_slow_factor(sched_ctx, STARPU_CPU_WORKER, 0, 20.0f);
}
}
starpu_heteroprio_set_faster_arch
void starpu_heteroprio_set_faster_arch(unsigned sched_ctx_id, enum starpu_worker_archtype arch, unsigned bucket_id)
starpu_heteroprio_set_arch_slow_factor
void starpu_heteroprio_set_arch_slow_factor(unsigned sched_ctx_id, enum starpu_worker_archtype arch, unsigned bucket_id, float slow_factor)
starpu_heteroprio_set_mapping
void starpu_heteroprio_set_mapping(unsigned sched_ctx_id, enum starpu_worker_archtype arch, unsigned source_prio, unsigned dest_bucket_id)
starpu_heteroprio_set_nb_prios
void starpu_heteroprio_set_nb_prios(unsigned sched_ctx_id, enum starpu_worker_archtype arch, unsigned max_prio)

Then, when a task is inserted, the priority of the task will be used to select in which bucket is has to be stored. 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). The full source code of this example is available in the file `examples/scheduler/heteroprio_test.c`

## 4.3.1 Using locality aware Heteroprio

Heteroprio supports a mode where locality is evaluated to guide the distribution of the tasks (see https://peerj.com/articles/cs-190.pdf). Currently, this mode is available using the dedicated function or an environment variable STARPU_HETEROPRIO_USE_LA, and can be configured using environment variables.

void starpu_heteroprio_set_use_locality(unsigned sched_ctx_id, unsigned use_locality);
starpu_heteroprio_set_use_locality
void starpu_heteroprio_set_use_locality(unsigned sched_ctx_id, unsigned use_locality)

In this mode, multiple strategies are available to determine which memory node's workers are the most qualified for executing a specific task. This strategy can be set with STARPU_LAHETEROPRIO_PUSH and available strategies are:

- WORKER: the worker which pushed the task is preferred for the execution.
- LcS: the node with the shortest data transfer time (estimated by StarPU) is the most qualified
- LS_SDH: the node with the smallest data amount to be transferred will be preferred.
- LS_SDH2: similar to LS_SDH, but data in write access is counted in a quadratic manner to give them more importance.
- LS_SDHB: similar to LS_SDH, but data in write access is balanced with a coefficient (its value is set to 1000) and for the same amount of data, the one with fewer pieces of data to be transferred will be preferred.
- LC_SMWB: similar to LS_SDH, but the amount of data in write access gets multiplied by a coefficient which gets closer to 2 as the amount of data in read access gets larger than the data in write access.
- AUTO: strategy by default, this one selects the best strategy and changes it in runtime to improve performance

Other environment variables to configure LaHeteteroprio are documented in ConfiguringLaHeteroprio

## 4.3.2 Using Heteroprio in auto-calibration mode

In this mode, Heteroprio saves data about each program execution, in order to improve future ones. By default, these files are stored in the folder used by perfmodel, but this can be changed using the STARPU_HETEROPRIO_DATA_DIR environment variable. You can also specify the data filename directly using STARPU_HETEROPRIO_DATA_FILE.

Additionally, to assign priorities to tasks, Heteroprio needs a way to detect that some tasks are similar. By default, Heteroprio looks for tasks with the same perfmodel, or with the same codelet's name if no perfmodel was assigned. This behavior can be changed to only consider the codelet's name by setting STARPU_HETEROPRIO_CODELET_GROUPING_STRATEGY to `1`

Other environment variables to configure AutoHeteteroprio are documented in ConfiguringAutoHeteroprio