| Taskflow: A General-purpose Task-parallel Programming System

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Release 3.6.0 (2023/05/07)

Taskflow 3.6.0 is the 7th release in the 3.x line! This release includes several new changes, such as dynamic task graph parallelism, improved parallel algorithms, modified GPU tasking interface, documentation, examples, and unit tests.

# Download

Taskflow 3.6.0 can be downloaded from here.

# System Requirements

To use Taskflow v3.6.0, you need a compiler that supports C++17:

- GNU C++ Compiler at least v8.4 with -std=c++17
- Clang C++ Compiler at least v6.0 with -std=c++17
- Microsoft Visual Studio at least v19.27 with /std:c++17
- AppleClang Xcode Version at least v12.0 with -std=c++17
- Nvidia CUDA Toolkit and Compiler (nvcc) at least v11.1 with -std=c++17
- Intel C++ Compiler at least v19.0.1 with -std=c++17
- Intel DPC++ Clang Compiler at least v13.0.0 with -std=c++17 and SYCL20

Taskflow works on Linux, Windows, and Mac OS X.

# Release Summary

This release contains several changes to largely enhance the programmability of GPU tasking and standard parallel algorithms. More importantly, we have introduced a new dependent asynchronous tasking model that offers great flexibility for expressing dynamic task graph parallelism.

# New Features

## Taskflow Core

- Added new async methods to support dynamic task graph creation
  - tf::Executor::dependent_async(F&& func, Tasks&&... tasks)
  - tf::Executor::dependent_async(F&& func, I first, I last)
  - tf::Executor::silent_dependent_async(F&& func, Tasks&&... tasks)
  - tf::Executor::silent_dependent_async(F&& func, I first, I last)
- Added new async and join methods to tf::Runtime
  - tf::Runtime::async
  - tf::Runtime::silent_async
  - tf::Runtime::corun_all
- Added a new partitioner interface to optimize parallel algorithms
  - tf::GuidedPartitioner
  - tf::StaticPartitioner
  - tf::DynamicPartitioner
  - tf::RandomPartitioner
- Added parallel-scan algorithms to Taskflow
  - tf::Taskflow::inclusive_scan(B first, E last, D d_first, BOP bop)
  - tf::Taskflow::inclusive_scan(B first, E last, D d_first, BOP bop, T init)
  - tf::Taskflow::transform_inclusive_scan(B first, E last, D d_first, BOP bop, UOP uop)
  - tf::Taskflow::transform_inclusive_scan(B first, E last, D d_first, BOP bop, UOP uop, T init)
  - tf::Taskflow::exclusive_scan(B first, E last, D d_first, T init, BOP bop)
  - tf::Taskflow::transform_exclusive_scan(B first, E last, D d_first, T init, BOP bop, UOP uop)
- Added parallel-find algorithms to Taskflow
  - tf::Taskflow::find_if(B first, E last, T& result, UOP predicate, P&& part)
  - tf::Taskflow::find_if_not(B first, E last, T& result, UOP predicate, P&& part)
  - tf::Taskflow::min_element(B first, E last, T& result, C comp, P&& part)
  - tf::Taskflow::max_element(B first, E last, T& result, C comp, P&& part)
- Modified tf::Subflow as a derived class from tf::Runtime
- Extended parallel algorithms to support different partitioning algorithms
  - tf::Taskflow::for_each_index(B first, E last, S step, C callable, P&& part)
  - tf::Taskflow::for_each(B first, E last, C callable, P&& part)
  - tf::Taskflow::transform(B first1, E last1, O d_first, C c, P&& part)
  - tf::Taskflow::transform(B1 first1, E1 last1, B2 first2, O d_first, C c, P&& part)
  - tf::Taskflow::reduce(B first, E last, T& result, O bop, P&& part)
  - tf::Taskflow::transform_reduce(B first, E last, T& result, BOP bop, UOP uop, P&& part)
- Improved the performance of tf::Taskflow::sort for plain-old-data (POD) type

## cudaFlow

- removed algorithms that require buffer from tf::cudaFlow due to update limitation
- removed support for a dedicated cudaFlow task in Taskflow
  - all usage of tf::cudaFlow and tf::cudaFlowCapturer are standalone now

## Utilities

- Added all_same templates to check if a parameter pack has the same type

## Taskflow Profiler (TFProf)

- Removed cudaFlow and syclFlow tasks

# Bug Fixes

- Fixed the compilation error caused by clashing `MAX_PRIORITY` wtih `winspool.h` (#459)
- Fixed the compilation error caused by tf::TaskView::for_each_successor and tf::TaskView::for_each_dependent
- Fixed the infinite-loop bug when corunning a module task from tf::Runtime

If you encounter any potential bugs, please submit an issue at issue tracker.

# Breaking Changes

- Dropped support for cancelling asynchronous tasks

// previous - no longer supported
tf::Future<int> fu = executor.async([](){
return 1;
});
fu.cancel();
std::optional<int> res = fu.get(); // res may be std::nullopt or 1
// now - use std::future instead
std::future<int> fu = executor.async([](){
return 1;
});
int res = fu.get();
tf::Future
class to access the result of an execution
Definition taskflow.hpp:630
tf::Future::cancel
bool cancel()
cancels the execution of the running taskflow associated with this future object
Definition taskflow.hpp:721

- Dropped in-place support for running tf::cudaFlow from a dedicated task

// previous - no longer supported
taskflow.emplace([](tf::cudaFlow& cf){
cf.offload();
});
// now - user to fully control tf::cudaFlow for maximum flexibility
taskflow.emplace([](){
tf::cudaFlow cf;
// offload the cudaflow asynchronously through a stream
tf::cudaStream stream;
cf.run(stream);
// wait for the cudaflow completes
stream.synchronize();
});
tf::cudaStreamBase
class to create a CUDA stream with unique ownership
Definition cuda_stream.hpp:189
tf::cudaStreamBase::synchronize
cudaStreamBase & synchronize()
synchronizes the associated stream
Definition cuda_stream.hpp:232
tf::cudaStreamBase::run
cudaStreamBase & run(const cudaGraphExecBase< C, D > &exec)
runs the given executable CUDA graph

- Dropped in-place support for running tf::cudaFlowCapturer from a dedicated task

// previous - now longer supported
taskflow.emplace([](tf::cudaFlowCapturer& cf){
cf.offload();
});
// now - user to fully control tf::cudaFlowCapturer for maximum flexibility
taskflow.emplace([](){
tf::cudaFlowCapturer cf;
// offload the cudaflow asynchronously through a stream
tf::cudaStream stream;
cf.run(stream);
// wait for the cudaflow completes
stream.synchronize();
});

- Dropped in-place support for running tf::syclFlow from a dedicated task
  - SYCL can just be used out of box together with Taskflow
- Move all buffer query methods of CUDA standard algorithms inside execution policy
  - tf::cudaExecutionPolicy<NT, VT>::reduce_bufsz
  - tf::cudaExecutionPolicy<NT, VT>::scan_bufsz
  - tf::cudaExecutionPolicy<NT, VT>::merge_bufsz
  - tf::cudaExecutionPolicy<NT, VT>::min_element_bufsz
  - tf::cudaExecutionPolicy<NT, VT>::max_element_bufsz

// previous - no longer supported
tf::cuda_reduce_buffer_size<tf::cudaDefaultExecutionPolicy, int>(N);
// now (and similarly for other parallel algorithms)
tf::cudaDefaultExecutionPolicy policy(stream);
policy.reduce_bufsz<int>(N);

- Renamed tf::Executor::run_and_wait to tf::Executor::corun for expressiveness
- Renamed tf::Executor::loop_until to tf::Executor::corun_until for expressiveness
- Renamed tf::Runtime::run_and_wait to tf::Runtime::corun for expressiveness
- Disabled argument support for all asynchronous tasking features
  - users are responsible for creating their own wrapper to make the callable

// previous - async allows passing arguments to the callable
executor.async([](int i){ std::cout << i << std::endl; }, 4);
// now - users are responsible of wrapping the arumgnets into a callable
executor.async([i=4]( std::cout << i << std::endl; ){});

- Replaced `named_async` with an overload that takes the name string on the first argument

// previous - explicitly calling named_async to assign a name to an async task
executor.named_async("name", [](){});
// now - overlaod
executor.async("name", [](){});

# Documentation

- Revised Request Cancellation to remove support of cancelling async tasks
- Revised Asynchronous Tasking to include asynchronous tasking from tf::Runtime
  - Launch Async Tasks from a Runtime
- Revised Taskflow algorithms to include execution policy
  - Partitioning Algorithm
  - Parallel Iterations
  - Parallel Transforms
  - Parallel Reduction
- Added Parallel Scan
- Added Asynchronous Tasking with Dependencies

# Miscellaneous Items

We have published Taskflow in the following venues:

- Dian-Lun Lin, Yanqing Zhang, Haoxing Ren, Shih-Hsin Wang, Brucek Khailany and Tsung-Wei Huang, "GenFuzz: GPU-accelerated Hardware Fuzzing using Genetic Algorithm with Multiple Inputs," ACM/IEEE Design Automation Conference (DAC), San Francisco, CA, 2023
- Tsung-Wei Huang, "qTask: Task-parallel Quantum Circuit Simulation with Incrementality," IEEE International Parallel and Distributed Processing Symposium (IPDPS), St. Petersburg, Florida, 2023
- Elmir Dzaka, Dian-Lun Lin, and Tsung-Wei Huang, "Parallel And-Inverter Graph Simulation Using a Task-graph Computing System," IEEE International Parallel and Distributed Processing Symposium Workshop (IPDPSW), St. Petersburg, Florida, 2023

Please do not hesitate to contact Dr. Tsung-Wei Huang if you intend to collaborate with us on using Taskflow in your scientific computing projects.