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A task size describes how long a function runs, so only functions carry one. In #7848, an executor, a launch policy or a scheduler could also carry a size in their thread_schedule_hint, and that size won over the size of the function. An executor runs many different functions (for instance all the loop bodies of the parallel algorithms), so a size attached to it says nothing about any of them, and it hid the sizes the functions did have. This PR makes the function the only source of a task size:
set_task_size and the continuation spawner always store the size of the function a thread is created for. A size in the hint of an executor, a launch policy or a scheduler has no effect. The public constructors of thread_schedule_hint stay, as they are used in other contexts.
parallel_executor no longer replaces the size of the function with its own.
Bulk operations don't pass sizes on, as their tasks run the function many times. Hierarchical spawning is the exception: it creates a task for each element, which gets the size of the function, and the tasks that only spawn other tasks are marked small.
The iteration wrappers of for_each, for_loop, transform and the partitioners forward the size of the loop body, the same way they forward its annotation.
The executors manual gets a section that describes how task sizes are controlled, and task_size_hint checks the new rules on every executor path.
The manual also says which run times the sizes stand for: a small task runs for about a microsecond or less, a medium task for tens to hundreds of microseconds, and a large task for a millisecond or more. This is needed for the scheduler to make use of the sizes. A size that only orders tasks relative to each other can't be compared with anything the scheduler knows, while a known run time can be compared with its own costs, for instance the cost of stealing a task (on the order of a microsecond) or of waking a worker. That allows decisions like not stealing small tasks, waking a worker only for a large task or a backlog of smaller ones, or releasing a fork-join worker early only when the work left behind outweighs the cost.
Known gap: the threads created by scheduler_executor and explicit_scheduler_executor get no size, as the scheduler creates the thread before it sees the function. Annotations have the same gap.
This should go into 2.0.0: #7848 is part of the release, and the executor sizes would otherwise be released as API.
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Added bulk task-size classification, scaling small tasks to medium or large based on the number of calls.
Documentation
Added guidance on task-size labels, approximate duration ranges, propagation through supported operations, and exceptions for scheduler executors and bulk operations.
Explained how parallel algorithm bodies and supported wrappers carry task sizes, and how custom wrappers can forward them.
Behavior Changes
Function-provided task sizes now replace other hints, including when unknown. Executor and launch-policy hints do not determine thread sizes in the covered operations; scheduler-based executor threads remain unknown.
Function-provided task sizes now replace executor and launch-policy hints, including when the function size is unknown. Selected algorithm and hierarchical-spawning wrappers forward task-size metadata. Documentation and tests describe and check these behaviors.
Changes: Task-size behavior
Layer / File(s)
Summary
Task-size calculation and precedence libs/core/functional/include/hpx/functional/traits/get_task_size.hpp, libs/core/threading_base/include/hpx/threading_base/sized_function.hpp, libs/core/execution/include/hpx/execution/detail/future_exec.hpp, libs/core/executors/include/hpx/executors/parallel_executor.hpp
The new bulk_task_size API scales a task size by the number of calls. Task-size assignment uses the callable’s size, including unknown. Continuation spawning uses the continuation’s size, and parallel executor paths dispatch the original callable.
Task-size forwarding through wrappers libs/core/algorithms/include/hpx/parallel/algorithms/for_each.hpp, libs/core/algorithms/include/hpx/parallel/algorithms/for_loop.hpp, libs/core/algorithms/include/hpx/parallel/algorithms/transform.hpp, libs/core/algorithms/include/hpx/parallel/util/detail/partitioner_iteration.hpp, libs/core/executors/include/hpx/executors/detail/hierarchical_spawning.hpp, docs/sphinx/manual/executors.rst
Algorithm wrappers forward task-size lookup to their wrapped callable. Hierarchical spawning marks launcher tasks as small and applies the callable’s size to posted per-item work. The documentation describes annotations, propagation rules, and custom wrapper specialization.
Tests check bulk task-size thresholds and task-size results across launch policies, parallel and hierarchical executors, bulk operations, and scheduler executors.
sequenceDiagram
participant ParallelExecutor
participant HierarchicalSpawning
participant PostedTask
ParallelExecutor->>HierarchicalSpawning: bulk execution with callable
HierarchicalSpawning->>PostedTask: post per-item call with callable task size
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Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documenta…
Write docstrings for the functions missing them to satisfy the coverage threshold.
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The title clearly and concisely identifies the main change: task sizes now come from functions only.
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The description directly explains the task-size changes, affected execution paths, documentation updates, tests, and known gaps.
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Full details: Docstring Coverage
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Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 2 functions across 10 files. (1 skipped: 1 unsupported.)
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Write docstrings for the functions missing them to satisfy the coverage threshold.
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Actionable comments posted: 1
🪄 Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @libs/core/threading_base/tests/unit/task_size_hint.cpp:
- Around line 382-401: Update the parallel_executor_spawn_hierarchically cases
in test_executor_paths so their small-task expectations do not assume a fixed
worker count; preserve the stable large-task expectations of 16 and 15.
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A task size describes how long a function runs, so only functions carry one.
A size given to an executor, a launch policy or a scheduler through their
thread_schedule_hint used to win over the function's size; it now has no
effect: set_task_size and the continuation spawner always store the size of
the function a thread is created for, and parallel_executor no longer hides
the function's size when it has one of its own.
Bulk operations don't pass sizes on, as their tasks run the function many
times. Hierarchical spawning is the exception: it creates a task for each
element, which gets the function's size, and the tasks that only spawn them
are small. The iteration wrappers of for_each, for_loop, transform and the
partitioners forward the size of the loop body, like they forward its
annotation.
The executors manual gets a section on task sizes, and task_size_hint checks
the new rules on every executor path.
Signed-off-by: Hartmut Kaiser <[email protected]>
A scheduler can base decisions on task sizes only if they stand for known
run times: it has to compare them with its own costs, for instance the cost
of stealing a task or of waking a worker. A size that only orders tasks
relative to each other doesn't allow that. The executors manual now gives
the run times small, medium and large stand for, in a note that also says
why they matter.
Signed-off-by: Hartmut Kaiser <[email protected]>
The numbers of tasks the executor paths create depend on the number of
workers: hierarchical spawning splits the elements among them, and
parallel_executor runs a single chunk inline. CTest runs the test with one
worker, but run directly it used all cores and failed. The test now sets
hpx.os_threads=1 itself.
Signed-off-by: Hartmut Kaiser <[email protected]>
Both are available in namespace hpx, so the task sizes section of the
executors manual uses the shorter names.
Signed-off-by: Hartmut Kaiser <[email protected]>
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Actionable comments posted: 1
🪄 Fix comments: Fix CodeRabbit comments on this PR
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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
Review comments at @docs/sphinx/manual/executors.rst:
- Around line 643-644: Qualify the thread-size claim in the executor
documentation: note that threads created by scheduler_executor and
explicit_scheduler_executor have unknown size because they are created before
the function is known, even when it uses hpx::large_task.
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hpx::bulk_task_size(size, calls) gives the size of a task that calls a
function of the given size calls times. Each size stands for a hundred
times the run time of the one below it (small up to about 100 ns,
medium about 10 us, large about 1 ms or more), named by the constants
small_tasks_per_small_task, small_tasks_per_medium_task and
small_tasks_per_large_task: a hundred small calls make a medium task, a
hundred medium calls a large one. The docs note on task sizes gives
these run times.
Signed-off-by: Hartmut Kaiser <[email protected]>
Loop bodies such as comparing or adding builtin types run for about a
nanosecond, far below what small_ stands for. Add task_size::nano below
small_ (the sizes are renumbered, task_size_bits grows to 3 bits, the
hint stays 4 bytes) and hpx::nano_task(f).
Replace the small_tasks_per_* constants by task_size_duration(size),
which returns the run time a size stands for as a std::chrono duration
(nano 1 ns, small 100 ns, medium 10 us, large 1 ms). bulk_task_size
uses only chrono arithmetic on it, so a hundred nano calls make a small
task. Callers must not rely on the unit of the returned duration, which
leaves room for tasks that carry real run times later.
Signed-off-by: Hartmut Kaiser <[email protected]>
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Follow-up to #7848 (step 1 of #7837).
A task size describes how long a function runs, so only functions carry one. In #7848, an executor, a launch policy or a scheduler could also carry a size in their thread_schedule_hint, and that size won over the size of the function. An executor runs many different functions (for instance all the loop bodies of the parallel algorithms), so a size attached to it says nothing about any of them, and it hid the sizes the functions did have. This PR makes the function the only source of a task size:
The executors manual gets a section that describes how task sizes are controlled, and task_size_hint checks the new rules on every executor path.
The manual also says which run times the sizes stand for: a small task runs for about a microsecond or less, a medium task for tens to hundreds of microseconds, and a large task for a millisecond or more. This is needed for the scheduler to make use of the sizes. A size that only orders tasks relative to each other can't be compared with anything the scheduler knows, while a known run time can be compared with its own costs, for instance the cost of stealing a task (on the order of a microsecond) or of waking a worker. That allows decisions like not stealing small tasks, waking a worker only for a large task or a backlog of smaller ones, or releasing a fork-join worker early only when the work left behind outweighs the cost.
Known gap: the threads created by scheduler_executor and explicit_scheduler_executor get no size, as the scheduler creates the thread before it sees the function. Annotations have the same gap.
This should go into 2.0.0: #7848 is part of the release, and the executor sizes would otherwise be released as API.