Getting Started {#getting_started}

This guide walks through the core features of dispenso with working examples. Each section includes a complete, compilable example that you can build and run.

Coming from another library? Jump directly to the migration guide: Migrating from TBB | Migrating from OpenMP

Installation

See the README for installation instructions. Dispenso requires C++14 and CMake 3.12+.

To build the examples:

mkdir build && cd build
cmake .. -DDISPENSO_BUILD_EXAMPLES=ON
make

Basic Concepts

Thread Pools

At the heart of dispenso is the ThreadPool. A thread pool manages a set of worker threads that execute tasks. You can use the global thread pool or create your own:

#include <dispenso/thread_pool.h>

// Use the global thread pool (recommended for most cases)
dispenso::ThreadPool& pool = dispenso::globalThreadPool();

// Or create a custom pool with a specific number of threads
dispenso::ThreadPool myPool(4);  // 4 worker threads

Note: globalThreadPool() defaults to std::thread::hardware_concurrency() - 1 worker threads, since the calling thread typically participates in computation. Use dispenso::resizeGlobalThreadPool(n) to change it.

Task Sets

A TaskSet groups related tasks and provides a way to wait for their completion:

#include <dispenso/task_set.h>

dispenso::TaskSet taskSet(dispenso::globalThreadPool());

taskSet.schedule([]() { /* task 1 */ });
taskSet.schedule([]() { /* task 2 */ });

taskSet.wait();  // Block until all tasks complete

---

Your First Parallel Loop

The simplest way to parallelize work is with parallel_for. It distributes loop iterations across available threads.

Simple per-element parallel loop:

#include <dispenso/parallel_for.h>

// Process each element independently in parallel
dispenso::parallel_for(0, kArraySize, [&](size_t i) { output[i] = std::sqrt(input[i]); });

Reduction with per-thread state:

std::vector<double> partialSums;
dispenso::parallel_for(
    partialSums,
    []() { return 0.0; }, // State initializer
    size_t{0},
    kArraySize,
    [&](double& localSum, size_t start, size_t end) {
      for (size_t i = start; i < end; ++i) {
        localSum += input[i];
      }
    });

// Combine partial sums
double totalSum = 0.0;
for (double partial : partialSums) {
  totalSum += partial;
}

Adaptive chunking for uneven work:

The default chunking (kStatic) splits the range into equal chunks — ideal when every iteration costs about the same. When some iterations are far more expensive than others, pass kAdaptive so idle workers can steal the remaining work instead of waiting on a slow chunk:

dispenso::ParForOptions options;
options.defaultChunking = dispenso::ParForChunking::kAdaptive;
dispenso::parallel_for(0, kArraySize, [&](size_t i) { expensiveVariableWork(i); }, options);

See full example.

Key points:

• Use the simple form for independent per-element work
• Use chunked ranges when you want to control work distribution
• Per-thread state enables efficient reductions
• Options let you control parallelism and chunking strategy (kStatic vs kAdaptive)

---

Parallel Iteration with for_each

When you have a container rather than an index range, use for_each:

Parallel for_each on a vector:

#include <dispenso/for_each.h>

std::vector<double> values = {1.0, 4.0, 9.0, 16.0, 25.0, 36.0, 49.0, 64.0};

// Apply square root to each element in parallel
dispenso::for_each(values.begin(), values.end(), [](double& val) { val = std::sqrt(val); });

for_each_n with explicit count:

std::vector<int> partial = {10, 20, 30, 40, 50, 60, 70, 80, 90, 100};

// Only process first 5 elements
dispenso::for_each_n(partial.begin(), 5, [](int& n) { n += 100; });

See full example.

Key points:

• Works with any iterator type (including non-random-access iterators)
• for_each_n takes an explicit count
• Pass a TaskSet for external synchronization control

---

Working with Tasks

For more complex task patterns, use TaskSet and ConcurrentTaskSet directly:

Basic TaskSet:

#include <dispenso/task_set.h>

dispenso::TaskSet taskSet(dispenso::globalThreadPool());
std::atomic<int> counter(0);

for (int i = 0; i < 10; ++i) {
  taskSet.schedule([&counter, i]() { counter.fetch_add(i, std::memory_order_relaxed); });
}

taskSet.wait();

ConcurrentTaskSet with nested scheduling:

dispenso::ConcurrentTaskSet taskSet(dispenso::globalThreadPool());
std::atomic<int> total(0);

for (int i = 0; i < 5; ++i) {
  taskSet.schedule([&taskSet, &total, i]() {
    // Each task schedules two sub-tasks
    for (int j = 0; j < 2; ++j) {
      taskSet.schedule(
          [&total, i, j]() { total.fetch_add(i * 10 + j, std::memory_order_relaxed); });
    }
  });
}

taskSet.wait();

See full example.

Key points:

• TaskSet is for single-threaded scheduling
• ConcurrentTaskSet allows scheduling from multiple threads
• Both support cancellation for cooperative early termination
• The destructor waits for all tasks to complete

---

Futures for Async Results

When you need return values from async operations, use Future:

Basic async and get:

#include <dispenso/future.h>

dispenso::Future<int> future = dispenso::async([]() {
  int result = 0;
  for (int i = 1; i <= 100; ++i) {
    result += i;
  }
  return result;
});

int result = future.get();  // blocks until ready

Chaining with then():

dispenso::Future<double> chainedFuture = dispenso::async([]() {
                                           return 16.0;
                                         })
                                             .then([](dispenso::Future<double>&& prev) {
                                               return std::sqrt(prev.get());
                                             })
                                             .then([](dispenso::Future<double>&& prev) {
                                               return prev.get() * 2.0;
                                             });

when_all for multiple futures:

dispenso::Future<int> f1 = dispenso::async([]() { return 10; });
dispenso::Future<int> f2 = dispenso::async([]() { return 20; });
dispenso::Future<int> f3 = dispenso::async([]() { return 30; });

auto allFutures = dispenso::when_all(std::move(f1), std::move(f2), std::move(f3));
auto tuple = allFutures.get();
int sum = std::get<0>(tuple).get() + std::get<1>(tuple).get() + std::get<2>(tuple).get();

when_any for the first ready future:

dispenso::Future<int> f1 = dispenso::async([]() { return 10; });
dispenso::Future<int> f2 = dispenso::async([]() { return 20; });

// Result holds the argument index (0-based) of the first future to become ready.
dispenso::Future<size_t> which = dispenso::when_any(std::move(f1), std::move(f2));
size_t firstReady = which.get();

See full example.

Key points:

• async() launches work and returns a Future
• then() chains dependent computations
• when_all() waits for multiple futures; when_any() returns the index of the first ready
• make_ready_future() creates an already-completed future

---

Task Graphs

For complex dependency patterns, build a task graph:

Diamond dependency pattern:

#include <dispenso/graph.h>
#include <dispenso/graph_executor.h>

//         A
//        / \
//       B   C
//        \ /
//         D
dispenso::Graph graph;

dispenso::Node& A = graph.addNode([&]() { r[0] = 1.0f; });
dispenso::Node& B = graph.addNode([&]() { r[1] = r[0] * 2.0f; });
dispenso::Node& C = graph.addNode([&]() { r[2] = r[0] + 5.0f; });
dispenso::Node& D = graph.addNode([&]() { r[3] = r[1] + r[2]; });

B.dependsOn(A);
C.dependsOn(A);
D.dependsOn(B, C);

setAllNodesIncomplete(graph);
dispenso::ConcurrentTaskSet taskSet(dispenso::globalThreadPool());
dispenso::ConcurrentTaskSetExecutor executor;
executor(taskSet, graph);

See full example.

Key points:

• Use dependsOn() to specify prerequisites
• Multiple executors available: single-thread, parallel_for, ConcurrentTaskSet
• Graphs can be re-executed after calling setAllNodesIncomplete()
• Subgraphs help organize large graphs

---

Pipelines

For streaming data through stages, use pipelines:

3-stage pipeline (generator -> transform -> sink):

#include <dispenso/pipeline.h>

std::vector<int> results;
int counter = 0;

dispenso::pipeline(
    // Stage 1: Generator - produces values
    [&counter]() -> dispenso::OpResult<int> {
      if (counter >= 10) {
        return {}; // Empty result signals end of input
      }
      return counter++;
    },
    // Stage 2: Transform - squares the value
    [](int value) { return value * value; },
    // Stage 3: Sink - collects results
    [&results](int value) { results.push_back(value); });

See full example.

Key points:

• Generator stage produces values (returns OpResult<T> or std::optional<T>)
• Transform stages process values (can filter by returning empty result)
• Sink stage consumes final values
• Use stage() with a limit for parallel stages

---

Parallel Invoke

For launching a fixed set of heterogeneous tasks in parallel (not a loop), use parallel_invoke. It's the classic fork-join idiom: submit siblings to the pool, run the last one inline on the calling thread.

Fork-join with parallel_invoke:

#include <dispenso/parallel_invoke.h>

dispenso::ConcurrentTaskSet tasks(dispenso::globalThreadPool());

double a = 0.0, b = 0.0, c = 0.0;
dispenso::parallel_invoke(
    tasks,
    [&]() { a = expensiveComputation1(); },
    [&]() { b = expensiveComputation2(); },
    [&]() { c = expensiveComputation3(); }  // runs inline
);

tasks.wait();
double result = a + b + c;

See full example.

Key points:

• The last functor runs inline on the calling thread
• Does not call wait() — the caller drives synchronization
• Composes naturally with recursive divide-and-conquer algorithms
• No type erasure or allocation overhead

---

CPU Affinity and Topology

CpuSet provides portable CPU affinity and NUMA topology detection:

Query topology and pin threads:

#include <dispenso/cpu_set.h>

// Query available CPUs
int32_t available = dispenso::CpuSet::availableCount();
printf("Available CPUs: %d\n", available);

// Get cache-sharing groups for cache-aware scheduling
const auto& l3Groups = dispenso::CpuSet::l3CacheGroups();
for (const auto& group : l3Groups) {
  printf("L3 cache group (id %d): %zu CPUs\n", group.cacheId, group.cpus.size());
}

// Pin the current thread to a specific CPU
dispenso::CpuSet pinSet;
pinSet.add(0);
pinSet.bindCurrentThread();

See full example.

Key points:

• Works on Linux (full support), Windows (full support), FreeBSD (full support), macOS (topology only)
• CpuSet::l2CacheGroups() / l3CacheGroups() return CPU groups that share cache levels
• Use for NUMA-aware thread placement and cache-friendly scheduling

---

Thread-Safe Containers

ConcurrentVector

A vector that supports concurrent push_back and growth:

Concurrent push_back from multiple threads:

#include <dispenso/concurrent_vector.h>
#include <dispenso/parallel_for.h>

dispenso::ConcurrentVector<int> vec;

dispenso::parallel_for(0, 1000, [&vec](size_t i) { vec.push_back(static_cast<int>(i)); });

Iterator stability during concurrent modification:

dispenso::ConcurrentVector<int> vec;
vec.push_back(1);
vec.push_back(2);
vec.push_back(3);

auto it = vec.begin();
int& firstElement = *it;

// Push more elements concurrently
dispenso::parallel_for(0, 100, [&vec](size_t i) { vec.push_back(static_cast<int>(i + 100)); });

// Original iterator and reference are still valid
assert(*it == 1);
assert(firstElement == 1);

See full example.

Key points:

• Iterators and references remain stable during growth
• Use grow_by() for efficient batch insertion
• Reserve capacity upfront when size is known
• Not all operations are concurrent-safe (see docs)

SmallVector

A vector-like container with inline storage for small sizes, avoiding heap allocation when element count stays below a compile-time threshold:

#include <dispenso/small_vector.h>

// Store up to 8 elements inline (on the stack)
dispenso::SmallVector<int, 8> vec;

for (int i = 0; i < 6; ++i) {
  vec.push_back(i);  // No heap allocation — fits inline
}

vec.push_back(100);  // Still inline
vec.push_back(200);  // Still inline (8 elements)
vec.push_back(300);  // Transitions to heap — transparent to the caller

See full example.

Key points:

• Template parameter N controls inline capacity (default 4)
• Transparent fallback to heap when size exceeds N
• Once on heap, stays on heap until clear() — preserves reserve() guarantees
• Same API as std::vector (iterators, push_back, operator[], etc.)

ChaseLevDeque

A lock-free single-producer, multi-consumer work-stealing deque. The owner pushes and pops from the bottom; thieves steal from the top:

#include <dispenso/chase_lev_deque.h>

dispenso::ChaseLevDeque<int, 256> deque;

// Owner pushes work
for (int i = 0; i < 100; ++i) {
  deque.try_push(i);
}

// Owner pops (LIFO — most recent first)
int val = 0;
bool ok = deque.try_pop(val);

// Thieves steal (FIFO — oldest first, from other threads)
int stolen = 0;
bool got = deque.try_steal(stolen);

See full example.

Key points:

• Lock-free and wait-free for the owner (push/pop)
• steal() is lock-free for thieves
• Dynamically resizable — grows as needed
• Classic data structure for work-stealing schedulers

MpmcRingBuffer

A bounded, lock-free multi-producer multi-consumer ring buffer. try_push and try_pop are fail-fast — they return false rather than block when the buffer is full or empty:

#include <dispenso/mpmc_ring_buffer.h>

dispenso::MpmcRingBuffer<int, 1024> ring;

// Any thread may push...
if (!ring.try_push(42)) {
  // buffer full — apply backpressure or retry later
}

// ...and any thread may pop
int value = 0;
if (ring.try_pop(value)) {
  // consumed `value`
}

// Batch push reserves N slots with a single reservation
std::vector<int> items = {1, 2, 3};
size_t pushed = ring.try_push_batch(items.data(), items.size());

See full example.

Key points:

• Capacity is fixed at compile time (rounded up to a power of two by default)
• Fail-fast: no blocking and no retry loops inside the buffer
• try_push_batch enqueues K items with a single reservation

SPSCRingBuffer

When there is exactly one producer thread and one consumer thread, prefer SPSCRingBuffer — it avoids the CAS overhead of the MPMC variant:

#include <dispenso/spsc_ring_buffer.h>

dispenso::SPSCRingBuffer<int, 1024> ring;

// Producer thread
while (!ring.try_push(item)) {
  // full — wait for the consumer to drain a slot
}

// Consumer thread
int value = 0;
if (ring.try_pop(value)) {
  // consumed `value`
}

See full example.

Key points:

• Designed for exactly one producer and one consumer thread
• Lower overhead than MpmcRingBuffer (no CAS on the hot path)

---

Synchronization Primitives

Latch

A one-shot barrier for thread synchronization:

count_down + wait pattern:

#include <dispenso/latch.h>

constexpr int kNumWorkers = 3;
dispenso::Latch workComplete(kNumWorkers);
std::vector<int> results(kNumWorkers, 0);

dispenso::ConcurrentTaskSet taskSet(dispenso::globalThreadPool());

for (int i = 0; i < kNumWorkers; ++i) {
  taskSet.schedule([&workComplete, &results, i]() {
    results[static_cast<size_t>(i)] = (i + 1) * 10;
    workComplete.count_down();  // Signal work is done (non-blocking)
  });
}

workComplete.wait();  // Main thread waits for all workers

See full example.

Key points:

• arrive_and_wait() decrements and blocks
• count_down() decrements without blocking
• wait() blocks without decrementing
• Cannot be reset (one-shot)

---

Resource Pooling

Manage expensive-to-create resources with ResourcePool:

Basic buffer pool with RAII:

#include <dispenso/resource_pool.h>

// Create a pool of 4 buffers
dispenso::ResourcePool<Buffer> bufferPool(4, []() { return Buffer(); });

dispenso::parallel_for(0, 100, [&bufferPool](size_t i) {
  // Acquire a resource from the pool (blocks if none available)
  auto resource = bufferPool.acquire();

  // Use the resource
  resource.get().process(static_cast<int>(i));

  // Resource automatically returned to pool when 'resource' goes out of scope
});

See full example.

Key points:

• Resources automatically return to pool when RAII wrapper destructs
• acquire() blocks if no resources available
• Good for database connections, buffers, etc.
• Can be used to limit concurrency

---

Next Steps

• Browse the API Reference for complete documentation
• Check out the tests for more usage examples
• See the benchmarks for performance testing patterns