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

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Text Processing Pipeline

We study a text processing pipeline that finds the most frequent character of each string from an input source. Parallelism exhibits in the form of a three-stage pipeline that transforms the input string to a final pair type.

# Formulate the Text Processing Pipeline Problem

Given an input vector of strings, we want to compute the most frequent character for each string using a series of transform operations. For example:

# input strings
abade
ddddf
eefge
xyzzd
ijjjj
jiiii
kkijk
# output
a:2
d:4
e:3
z:2
j:4
i:4
k:3

We decompose the algorithm into three stages:

1. read a `std::string` from the input vector
2. generate a `std::unorder_map<char, size_t>` frequency map from the string
3. reduce the most frequent character to a `std::pair<char, size_t>` from the map

The first and the third stages process inputs and generate results in serial, and the second stage can run in parallel. The algorithm is a perfect fit to pipeline parallelism, as different stages can overlap with each other in time across parallel lines.

# Create a Text Processing Pipeline

We create a pipeline of three pipes (stages) and two parallel lines to solve the problem. The number of parallel lines is a tunable parameter. In most cases, we can just use `std::thread::hardware_concurrency` as the line count. The first pipe reads an input string from the vector in order, the second pipe transforms the input string from the first pipe to a frequency map in parallel, and the third pipe reduces the frequency map to find the most frequent character. The overall implementation is shown below:

#include <taskflow/taskflow.hpp>
#include <taskflow/algorithm/pipeline.hpp>
// Function: format the map
std::string format_map(const std::unordered_map<char, size_t>& map) {
std::ostringstream oss;
for(const auto& [i, j] : map) {
oss << i << ':' << j << ' ';
}
return oss.str();
}
int main() {
tf::Taskflow taskflow("text-filter pipeline");
tf::Executor executor;
const size_t num_lines = 2;
// input data
std::vector<std::string> input = {
"abade",
"ddddf",
"eefge",
"xyzzd",
"ijjjj",
"jiiii",
"kkijk"
};
// custom data storage
using data_type = std::variant<
std::string, std::unordered_map<char, size_t>, std::pair<char, size_t>
>;
std::array<data_type, num_lines> mybuffer;
// the pipeline consists of three pipes (serial-parallel-serial)
// and up to two concurrent scheduling tokens
tf::Pipeline pl(num_lines,
// first pipe processes the input data
tf::Pipe{tf::PipeType::SERIAL, [&](tf::Pipeflow& pf) {
if(pf.token() == input.size()) {
pf.stop();
}
else {
printf("stage 1: input token = %s\n", input[pf.token()].c_str());
mybuffer[pf.line()] = input[pf.token()];
}
}},
// second pipe counts the frequency of each character
tf::Pipe{tf::PipeType::PARALLEL, [&](tf::Pipeflow& pf) {
std::unordered_map<char, size_t> map;
for(auto c : std::get<std::string>(mybuffer[pf.line()])) {
map[c]++;
}
printf("stage 2: map = %s\n", format_map(map).c_str());
mybuffer[pf.line()] = map;
}},
// third pipe reduces the most frequent character
tf::Pipe{tf::PipeType::SERIAL, [&mybuffer](tf::Pipeflow& pf) {
auto& map = std::get<std::unordered_map<char, size_t>>(mybuffer[pf.line()]);
auto sol = std::max_element(map.begin(), map.end(), [](auto& a, auto& b){
return a.second < b.second;
});
printf("stage 3: %c:%zu\n", sol->first, sol->second);
// not necessary to store the last-stage data, just for demo purpose
mybuffer[pf.line()] = *sol;
}}
);
// build the pipeline graph using composition
tf::Task init = taskflow.emplace([](){ std::cout << "ready\n"; })
.name("starting pipeline");
tf::Task task = taskflow.composed_of(pl)
.name("pipeline");
tf::Task stop = taskflow.emplace([](){ std::cout << "stopped\n"; })
.name("pipeline stopped");
// create task dependency
init.precede(task);
task.precede(stop);
// dump the pipeline graph structure (with composition)
taskflow.dump(std::cout);
// run the pipeline
executor.run(taskflow).wait();
return 0;
}
tf::Executor
class to create an executor
Definition executor.hpp:62
tf::Executor::run
tf::Future< void > run(Taskflow &taskflow)
runs a taskflow once
tf::Task::dump
void dump(std::ostream &ostream) const
dumps the task through an output stream
Definition task.hpp:1171
tf::Task::precede
Task & precede(Ts &&... tasks)
adds precedence links from this to other tasks
Definition task.hpp:947
tf::Taskflow
class to create a taskflow object
Definition taskflow.hpp:64
tf::PipeType::SERIAL
@ SERIAL
serial type
Definition pipeline.hpp:117
tf::PipeType::PARALLEL
@ PARALLEL
parallel type
Definition pipeline.hpp:115

## Define the Data Buffer

Taskflow does not provide any data abstraction to perform pipeline scheduling, but give users full control over data management in their applications. In this example, we create an one-dimensional buffer of a std::variant data type to store the output of each pipe in a uniform storage:

using data_type = std::variant<
std::string, std::unordered_map<char, size_t>, std::pair<char, size_t>
>;
std::array<std::array<data_type, num_pipes>, num_lines> mybuffer;

Attention
  One-dimensional buffer is sufficient because Taskflow enables only one scheduling token per line at a time.

## Define the Pipes

The first pipe reads one string and puts it in the corresponding entry at the buffer, `mybuffer[pf.line()]`. Since we read in each string in order, we declare the pipe as a serial type:

tf::Pipe{tf::PipeType::SERIAL, [&](tf::Pipeflow& pf) {
if(pf.token() == input.size()) {
pf.stop();
}
else {
mybuffer[pf.line()] = input[pf.token()];
printf("stage 1: input token = %s\n", input[pf.token()].c_str());
}
}},
tf::Pipe
class to create a pipe object for a pipeline stage
Definition pipeline.hpp:144
tf::Pipeflow
class to create a pipeflow object used by the pipe callable
Definition pipeline.hpp:43

The second pipe needs to get the input string from the previous pipe and then transforms that input string into a frequency map that records the occurrence of each character in the string. As multiple transforms can operate simultaneously, we declare the pipe as a parallel type:

tf::Pipe{tf::PipeType::PARALLEL, [&](tf::Pipeflow& pf) {
std::unordered_map<char, size_t> map;
for(auto c : std::get<std::string>(mybuffer[pf.line()])) {
map[c]++;
}
mybuffer[pf.line()] = map;
printf("stage 2: map = %s\n", format_map(map).c_str());
}}

Similarly, the third pipe needs to get the input frequency map from the previous pipe and then reduces the result to find the most frequent character. We may not need to store the result in the buffer but other places defined by the application (e.g., an output file). As we want to output the result in the same order as the input, we declare the pipe as a serial type:

tf::Pipe{tf::PipeType::SERIAL, [&mybuffer](tf::Pipeflow& pf) {
auto& map = std::get<std::unordered_map<char, size_t>>(mybuffer[pf.line()]);
auto sol = std::max_element(map.begin(), map.end(), [](auto& a, auto& b){
return a.second < b.second;
});
printf("stage 3: %c:%zu\n", sol->first, sol->second);
}}

## Define the Task Graph

To build up the taskflow graph for the pipeline, we create a module task out of the pipeline structure and connect it with two tasks that outputs messages before and after the pipeline:

tf::Task init = taskflow.emplace([](){ std::cout << "ready\n"; })
.name("starting pipeline");
tf::Task task = taskflow.composed_of(pl)
.name("pipeline");
tf::Task stop = taskflow.emplace([](){ std::cout << "stopped\n"; })
.name("pipeline stopped");
init.precede(task);
task.precede(stop);
tf::Task
class to create a task handle over a taskflow node
Definition task.hpp:263
tf::Task::name
const std::string & name() const
queries the name of the task
Definition task.hpp:1077
tf::Task::composed_of
Task & composed_of(T &object)
creates a module task from a taskflow
Definition task.hpp:979

## Submit the Task Graph

Finally, we submit the taskflow to the execution and run it once:

executor.run(taskflow).wait();

As the second stage is a parallel pipe, the output may interleave. One possible result is shown below:

ready
stage 1: input token = abade
stage 1: input token = ddddf
stage 2: map = f:1 d:4
stage 2: map = e:1 d:1 a:2 b:1
stage 3: a:2
stage 1: input token = eefge
stage 2: map = g:1 e:3 f:1
stage 3: d:4
stage 1: input token = xyzzd
stage 3: e:3
stage 1: input token = ijjjj
stage 2: map = z:2 x:1 d:1 y:1
stage 3: z:2
stage 1: input token = jiiii
stage 2: map = j:4 i:1
stage 3: j:4
stage 2: map = i:4 j:1
stage 1: input token = kkijk
stage 3: i:4
stage 2: map = j:1 k:3 i:1
stage 3: k:3
stopped

We can see seven outputs at the third stage that show the most frequent character for each of the seven strings in order (`a:2`, `d:4`, `e:3`, `z:2`, `j:4`, `i:4`, `k:3`). The taskflow graph of this pipeline workload is shown below: