CUB Developer Overview

This living document serves as a guide to the design of the internal structure of CUB.

CUB provides layered algorithms that correspond to the thread/warp/block/device hierarchy of threads in CUDA. There are distinct algorithms for each layer and higher-level layers build on top of those below.

For example, CUB has four flavors of reduce, one for each layer: ThreadReduce, WarpReduce, BlockReduce, and DeviceReduce. Each is unique in how it is invoked, how many threads participate, and on which thread(s) the result is valid.

These layers naturally build on each other. For example, :cpp:struct:`WarpReduce <cub::WarpReduce>` uses :cpp:func:`ThreadReduce <cub::internal::ThreadReduce>`, :cpp:struct:`BlockReduce <cub::BlockReduce>` uses :cpp:struct:`WarpReduce <cub::WarpReduce>`, etc.

:cpp:func:`ThreadReduce <cub::internal::ThreadReduce>`

A normal function invoked and executed sequentially by a single thread that returns a valid result on that thread

Single thread functions are usually an implementation detail and not exposed in CUB's public API

:cpp:struct:`WarpReduce <cub::WarpReduce>` and :cpp:struct:`BlockReduce <cub::BlockReduce>`

A "cooperative" function where threads concurrently invoke the same function to execute parallel work

The function's return value is well-defined only on the "first" thread (lowest thread index)

:cpp:struct:`DeviceReduce <cub::DeviceReduce>`

A normal function invoked by a single thread that spawns additional threads to execute parallel work

Result is stored in the pointer provided to the function

Function returns a cudaError_t error code

Function does not synchronize the host with the device

The table below provides a summary of these functions:

layer	coop invocation	parallel execution	max threads	valid result in
:cpp:func:`ThreadReduce <cub::internal::ThreadReduce>`	`-`	`-`	`1`	invoking thread
:cpp:struct:`WarpReduce <cub::WarpReduce>`	`+`	`+`	`32`	main thread
:cpp:struct:`BlockReduce <cub::BlockReduce>`	`+`	`+`	`1024`	main thread
:cpp:struct:`DeviceReduce <cub::DeviceReduce>`	`-`	`+`	`\infty`	global memory

The details of how each of these layers are implemented is described below.

Common Patterns

While CUB's algorithms are unique at each layer, there are commonalities among all of them:

Algorithm interfaces are provided as types (classes)[1]

Algorithms need temporary storage

Algorithms dispatch to specialized implementations depending on compile-time and runtime information

Cooperative algorithms require the number of threads at compile time (template parameter)

Invoking any CUB algorithm follows the same general pattern:

Select the class for the desired algorithm

Query the temporary storage requirements

Allocate the temporary storage

Pass the temporary storage to the algorithm

Invoke it via the appropriate member function

An example of :cpp:struct:`cub::BlockReduce` demonstrates these patterns in practice:

__global__ void kernel(int* per_block_results)
{
  // (1) Select the desired class
  // `cub::BlockReduce` is a class template that must be instantiated for the
  // input data type and the number of threads. Internally the class is
  // specialized depending on the data type, number of threads, and hardware
  // architecture. Type aliases are often used for convenience:
  using BlockReduce = cub::BlockReduce<int, 128>;
  // (2) Query the temporary storage
  // The type and amount of temporary storage depends on the selected instantiation
  using TempStorage = typename BlockReduce::TempStorage;
  // (3) Allocate the temporary storage
  __shared__ TempStorage temp_storage;
  // (4) Pass the temporary storage
  // Temporary storage is passed to the constructor of the `BlockReduce` class
  BlockReduce block_reduce{temp_storage};
  // (5) Invoke the algorithm
  // The `Sum()` member function performs the sum reduction of `thread_data` across all 128 threads
  int thread_data[4] = {1, 2, 3, 4};
  int block_result = block_reduce.Sum(thread_data);

  per_block_results[blockIdx.x] = block_result;
}

[1]	Algorithm interfaces are provided as classes because it provides encapsulation for things like temporary storage requirements and enables partial template specialization for customizing an algorithm for specific data types or number of threads.

Thread-level

In contrast to algorithms at the warp/block/device layer, single threaded functionality like cub::ThreadReduce is typically implemented as a sequential function and rarely exposed to the user.

template <
    int         LENGTH,
    typename    T,
    typename    ReductionOp,
    typename    PrefixT,
    typename    AccumT = detail::accumulator_t<ReductionOp, PrefixT, T>>
__device__ __forceinline__ AccumT ThreadReduce(
    T           (&input)[LENGTH],
    ReductionOp reduction_op,
    PrefixT     prefix)
{
    return ...;
}

Warp-level

CUB warp-level algorithms are specialized for execution by threads in the same CUDA warp. These algorithms may only be invoked by 1 <= n <= 32 consecutive threads in the same warp.

Overview

Warp-level functionality is provided by types (classes) to provide encapsulation and enable partial template specialization.

For example, :cpp:struct:`cub::WarpReduce` is a class template:

template <typename T,
          int LOGICAL_WARP_THREADS = 32,
          int LEGACY_PTX_ARCH = 0>
class WarpReduce {
  // ...
  // (1)   define `_TempStorage` type
  // ...
  _TempStorage &temp_storage;
public:

  // (2)   wrap `_TempStorage` in uninitialized memory
  struct TempStorage : Uninitialized<_TempStorage> {};

  __device__ __forceinline__ WarpReduce(TempStorage &temp_storage)
  // (3)   reinterpret cast
    : temp_storage(temp_storage.Alias())
  {}

  // (4)   actual algorithms
  __device__ __forceinline__ T Sum(T input);
};

In CUDA, the hardware warp size is 32 threads. However, CUB enables warp-level algorithms on "logical" warps of 1 <= n <= 32 threads. The size of the logical warp is required at compile time via the LOGICAL_WARP_THREADS non-type template parameter. This value is defaulted to the hardware warp size of 32. There is a vital difference in the behavior of warp-level algorithms that depends on the value of LOGICAL_WARP_THREADS:

If LOGICAL_WARP_THREADS is a power of two - warp is partitioned into sub-warps, each reducing its data independently from other sub-warps. The terminology used in CUB: 32 threads are called hardware warp. Groups with less than 32 threads are called logical or virtual warp since it doesn't correspond directly to any hardware unit.
If LOGICAL_WARP_THREADS is not a power of two - there's no partitioning. That is, only the first logical warp executes algorithm.

It's important to note that LEGACY_PTX_ARCH has been recently deprecated. This parameter used to affect specialization selection (see below). It was conflicting with the PTX dispatch refactoring and limited NVHPC support.

Temporary storage usage

Warp-level algorithms require temporary storage for scratch space and inter-thread communication. The temporary storage needed for a given instantiation of an algorithm is known at compile time and is exposed through the TempStorage member type definition. It is the caller's responsibility to create this temporary storage and provide it to the constructor of the algorithm type. It is possible to reuse the same temporary storage for different algorithm invocations, but it is unsafe to do so without first synchronizing to ensure the first invocation is complete.

using WarpReduce = cub::WarpReduce<int>;

// Allocate WarpReduce shared memory for four warps
__shared__ WarpReduce::TempStorage temp_storage[4];

// Get this thread's warp id
int warp_id = threadIdx.x / 32;
int aggregate_1 = WarpReduce(temp_storage[warp_id]).Sum(thread_data_1);
// illegal, has to add `__syncwarp()` between the two
int aggregate_2 = WarpReduce(temp_storage[warp_id]).Sum(thread_data_2);
// illegal, has to add `__syncwarp()` between the two
foo(temp_storage[warp_id]);

Specialization

The goal of CUB is to provide users with algorithms that abstract the complexities of achieving speed-of-light performance across a variety of use cases and hardware. It is a CUB developer's job to abstract this complexity from the user by providing a uniform interface that statically dispatches to the optimal code path. This is usually accomplished via customizing the implementation based on compile time information like the logical warp size, the data type, and the target architecture. For example, :cpp:struct:`cub::WarpReduce` dispatches to two different implementations based on if the logical warp size is a power of two (described above):

using InternalWarpReduce = cub::detail::conditional_t<
  IS_POW_OF_TWO,
  WarpReduceShfl<T, LOGICAL_WARP_THREADS>,  // shuffle-based implementation
  WarpReduceSmem<T, LOGICAL_WARP_THREADS>>; // smem-based implementation

Specializations provide different shared memory requirements, so the actual _TempStorage type is defined as:

typedef typename InternalWarpReduce::TempStorage _TempStorage;

and algorithm implementation look like:

__device__ __forceinline__ T Sum(T input, int valid_items) {
  return InternalWarpReduce(temp_storage)
      .Reduce(input, valid_items, cub::Sum());
}

Due to LEGACY_PTX_ARCH issues described above, we can't specialize on the PTX version. NV_IF_TARGET shall be used by specializations instead:

template <typename T, int LOGICAL_WARP_THREADS, int LEGACY_PTX_ARCH = 0>
struct WarpReduceShfl
{


template <typename ReductionOp>
__device__ __forceinline__ T ReduceImpl(T input, int valid_items,
                                        ReductionOp reduction_op)
{
  // ... base case (SM < 80) ...
}

template <class U = T>
__device__ __forceinline__
  typename std::enable_if<std::is_same<int, U>::value ||
                          std::is_same<unsigned int, U>::value,
                          T>::type
    ReduceImpl(T input,
              int,      // valid_items
              cub::Sum) // reduction_op
{
  T output = input;

  NV_IF_TARGET(NV_PROVIDES_SM_80,
              (output = __reduce_add_sync(member_mask, input);),
              (output = ReduceImpl<cub::Sum>(
                    input, LOGICAL_WARP_THREADS, cub::Sum{});));

  return output;
}


};

Specializations are stored in the cub/warp/specializations directory.

Block-scope

Overview

Block-scope algorithms are provided by structures as well:

template <typename T,
          int BLOCK_DIM_X,
          BlockReduceAlgorithm ALGORITHM = BLOCK_REDUCE_WARP_REDUCTIONS,
          int BLOCK_DIM_Y = 1,
          int BLOCK_DIM_Z = 1,
          int LEGACY_PTX_ARCH = 0>
class BlockReduce {
public:
  struct TempStorage : Uninitialized<_TempStorage> {};

  // (1) new constructor
  __device__ __forceinline__ BlockReduce()
      : temp_storage(PrivateStorage()),
        linear_tid(RowMajorTid(BLOCK_DIM_X, BLOCK_DIM_Y, BLOCK_DIM_Z)) {}

  __device__ __forceinline__ BlockReduce(TempStorage &temp_storage)
      : temp_storage(temp_storage.Alias()),
        linear_tid(RowMajorTid(BLOCK_DIM_X, BLOCK_DIM_Y, BLOCK_DIM_Z)) {}
};

While warp-scope algorithms only provide a single constructor that requires the user to provide temporary storage, block-scope algorithms provide two constructors:

The default constructor that allocates the required shared memory internally.

The constructor that requires the user to provide temporary storage as argument.

In the case of the default constructor, the block-level algorithm uses the PrivateStorage() member function to allocate the required shared memory. This ensures that shared memory required by the algorithm is only allocated when the default constructor is actually called in user code. If the default constructor is never called, then the algorithm will not allocate superfluous shared memory.

__device__ __forceinline__ _TempStorage& PrivateStorage()
{
  __shared__ _TempStorage private_storage;
  return private_storage;
}

The __shared__ memory has static semantic, so it's safe to return a reference here.

Specialization

Block-scope facilities usually expose algorithm selection to the user. The algorithm is represented by the enumeration part of the API. For the reduction case, BlockReduceAlgorithm is provided. Specializations are stored in the cub/block/specializations directory.

Temporary storage usage

For block-scope algorithms, it's unsafe to use temporary storage without synchronization:

using BlockReduce = cub::BlockReduce<int, 128> ;

__shared__ BlockReduce::TempStorage temp_storage;

int aggregate_1 = BlockReduce(temp_storage).Sum(thread_data_1);
// illegal, has to add `__syncthreads` between the two
int aggregate_2 = BlockReduce(temp_storage).Sum(thread_data_2);
// illegal, has to add `__syncthreads` between the two
foo(temp_storage);

Device-scope

Overview

Device-scope functionality is provided by static member functions:

struct DeviceReduce
{


template <typename InputIteratorT, typename OutputIteratorT>
CUB_RUNTIME_FUNCTION static
cudaError_t Sum(
    void *d_temp_storage, size_t &temp_storage_bytes, InputIteratorT d_in,
    OutputIteratorT d_out, int num_items, cudaStream_t stream = 0)
{
  using OffsetT = int;
  using OutputT =
      cub::detail::non_void_value_t<OutputIteratorT,
                                    cub::detail::value_t<InputIteratorT>>;

  return DispatchReduce<InputIteratorT, OutputIteratorT, OffsetT,
                        cub::Sum>::Dispatch(d_temp_storage, temp_storage_bytes,
                                            d_in, d_out, num_items, cub::Sum(),
                                            OutputT(), stream);
}


};

Device-scope facilities always return cudaError_t and accept stream as the last parameter (main stream by default). The first two parameters are always void *d_temp_storage, size_t &temp_storage_bytes. The algorithm invocation consists of two phases. During the first phase, temporary storage size is calculated and returned in size_t &temp_storage_bytes. During the second phase, temp_storage_bytes of memory is expected to be allocated and d_temp_storage is expected to be the pointer to this memory.

// Determine temporary device storage requirements
void *d_temp_storage {};
std::size_t temp_storage_bytes {};

cub::DeviceReduce::Sum(d_temp_storage, temp_storage_bytes, d_in, d_out, num_items);
// Allocate temporary storage
cudaMalloc(&d_temp_storage, temp_storage_bytes);
// Run sum-reduction
cub::DeviceReduce::Sum(d_temp_storage, temp_storage_bytes, d_in, d_out, num_items);

Warning

Even if the algorithm doesn't need temporary storage as a scratch space, we still require one byte of memory to be allocated.

Dispatch layer

The dispatch layer is specific to the device scope (DispatchReduce) and located in cub/device/dispatch. High-level control flow can be represented by the code below. A more precise description is given later.

// Device-scope API
cudaError_t cub::Device::Algorithm(args) {
  return DispatchAlgorithm::Dispatch(args); // (1)
}

// Dispatch entry point
static cudaError_t DispatchAlgorithm::Dispatch(args) { // (1)
  DispatchAlgorithm invokable(args);
  // MaxPolicy - tail of linked list contaning architecture-specific tunings
  return MaxPolicy::Invoke(get_runtime_ptx_version(), invokable); // (2)
}

// Chained policy - linked list of tunings
cudaError_t ChainedPolicy::Invoke(ptx_version, invokable) { // (2)
  if (ptx_version < ChainedPolicy::PTX_VERSION) {
    ChainedPolicy::PrevPolicy::Invoke(ptx_version, invokable); // (2)
  }
  invokable.Invoke<ChainedPolicy::PTX_VERSION>(); // (3)
}

// Dispatch object - parameters closure
template <class Policy>
cudaError_t DispatchAlgorithm::Invoke() { // (3)
    kernel<MaxPolicy><<<grid_size, Policy::BLOCK_THREADS>>>(args); // (4)
}

template <class ChainedPolicy>
void __global__ __launch_bounds__(ChainedPolicy::ActivePolicy::BLOCK_THREADS)
kernel(args) { // (4)
    using policy = ChainedPolicy::ActivePolicy; // (5)
    using agent = AgentAlgorithm<policy>; // (6)
    agent a(args);
    a.Process();
}

template <int PTX_VERSION, typename PolicyT, typename PrevPolicyT>
struct ChainedPolicy {
  using ActivePolicy = conditional_t<(CUB_PTX_ARCH < PTX_VERSION), // (5)
                                    PrevPolicyT::ActivePolicy,
                                    PolicyT>;
};

template <class Policy>
struct AlgorithmAgent { // (6)
  void Process();
};

The code above represents control flow. Let's look at each of the building blocks closer.

The dispatch entry point is typically represented by a static member function that constructs an object of DispatchReduce and passes it to ChainedPolicy Invoke method:

template <typename InputIteratorT,
          // ...
          typename SelectedPolicy>
struct DispatchReduce : SelectedPolicy
{

  //
  CUB_RUNTIME_FUNCTION __forceinline__ static
  cudaError_t Dispatch(
      void *d_temp_storage, size_t &temp_storage_bytes,
      InputIteratorT d_in,
      /* ... */)
  {
    typedef typename DispatchSegmentedReduce::MaxPolicy MaxPolicyT;

    if (num_segments <= 0) return cudaSuccess;

    cudaError error = cudaSuccess;
    do {
      // Get PTX version
      int ptx_version = 0;
      error = CubDebug(PtxVersion(ptx_version));
      if (cudaSuccess != error)
      {
        break;
      }

      // Create dispatch functor
      DispatchSegmentedReduce dispatch(
          d_temp_storage, temp_storage_bytes, d_in, /* ... */);

      // Dispatch to chained policy
      MaxPolicyT::Invoke(ptx_version, dispatch);
    } while (0);

    return error;
  }

};

For many algorithms, the dispatch layer is part of the API. The first reason for this to be the case is size_t support. Our API uses int as a type for num_items. Users rely on the dispatch layer directly to workaround this. Exposing the dispatch layer also allows users to tune algorithms for their use cases. In the newly added functionality, the dispatch layer should not be exposed.

The ChainedPolicy converts the runtime PTX version to the closest compile-time one:

template <int PTX_VERSION,
          typename PolicyT,
          typename PrevPolicyT>
struct ChainedPolicy
{
  using ActivePolicy =
    cub::detail::conditional_t<(CUB_PTX_ARCH < PTX_VERSION),
                              typename PrevPolicyT::ActivePolicy,
                              PolicyT>;

  template <typename FunctorT>
  CUB_RUNTIME_FUNCTION __forceinline__
  static cudaError_t Invoke(int ptx_version, FunctorT& op)
  {
      if (ptx_version < PTX_VERSION) {
          return PrevPolicyT::Invoke(ptx_version, op);
      }
      return op.template Invoke<PolicyT>();
  }
};

The dispatch object's Invoke method is then called with proper policy:

template <typename InputIteratorT,
          // ...
          typename SelectedPolicy = DefaultTuning>
struct DispatchReduce : SelectedPolicy
{

template <typename ActivePolicyT>
CUB_RUNTIME_FUNCTION __forceinline__
cudaError_t Invoke()
{
  using MaxPolicyT = typename DispatchSegmentedReduce::MaxPolicy;

  return InvokePasses<ActivePolicyT /* (1) */>(
    DeviceReduceKernel<MaxPolicyT /* (2) */, InputIteratorT /* ... */>);
}

};

This is where the actual work happens. Note how the kernel is used against MaxPolicyT (2) while the kernel invocation part uses ActivePolicyT (1). This is an important part:

template <typename ChainedPolicyT /* ... */ >
__launch_bounds__(int(ChainedPolicyT::ActivePolicy::ReducePolicy::BLOCK_THREADS))
__global__ void DeviceReduceKernel(InputIteratorT d_in /* ... */)
{
  // Thread block type for reducing input tiles
  using AgentReduceT =
      AgentReduce<typename ChainedPolicyT::ActivePolicy::ReducePolicy,
                  InputIteratorT, OutputIteratorT, OffsetT, ReductionOpT>;

  // Shared memory storage
  __shared__ typename AgentReduceT::TempStorage temp_storage;

  // Consume input tiles
  OutputT block_aggregate =
      AgentReduceT(temp_storage, d_in, reduction_op).ConsumeTiles(even_share);

  // Output result
  if (threadIdx.x == 0) {
    d_out[blockIdx.x] = block_aggregate;
  }
}

The kernel gets compiled for each PTX version that was provided to the compiler. During the device pass, ChainedPolicy compares CUDA_ARCH against the template parameter to select ActivePolicy type alias. During the host pass, Invoke is compiled for each architecture in the tuning list. If we use ActivePolicy instead of MaxPolicy as a kernel template parameter, we will compile O(N^2) kernels instead of O(N).

Finally, the tuning looks like:

template <typename InputT /* ... */>
struct DeviceReducePolicy
{
  /// SM35
  struct Policy350 : ChainedPolicy<350, Policy350, Policy300> {
    typedef AgentReducePolicy<256, 20, InputT, 4, BLOCK_REDUCE_WARP_REDUCTIONS,
                              LOAD_LDG>
        ReducePolicy;
  };

  /// SM60
  struct Policy600 : ChainedPolicy<600, Policy600, Policy350> {
    typedef AgentReducePolicy<256, 16, InputT, 4, BLOCK_REDUCE_WARP_REDUCTIONS,
                              LOAD_LDG>
        ReducePolicy;
  };

  /// MaxPolicy
  typedef Policy600 MaxPolicy;
};

The kernels in the dispatch layer shouldn't contain a lot of code. Usually, the functionality is extracted into the agent layer. All the kernel does is derive the proper policy type, unwrap the policy to initialize the agent and call one of its Consume / Process methods. Agents are frequently reused by unrelated device-scope algorithms.

Temporary storage usage

It's safe to reuse storage in the stream order:

cub::DeviceReduce::Sum(nullptr, storage_bytes, d_in, d_out, num_items, stream_1);
// allocate temp storage
cub::DeviceReduce::Sum(d_storage, storage_bytes, d_in, d_out, num_items, stream_1);
// fine not to synchronize stream
cub::DeviceReduce::Sum(d_storage, storage_bytes, d_in, d_out, num_items, stream_1);
// illegal, should call cudaStreamSynchronize(stream)
cub::DeviceReduce::Sum(d_storage, storage_bytes, d_in, d_out, num_items, stream_2);

Temporary storage management

Often times temporary storage for device-scope algorithms has a complex structure. To simplify temporary storage management and make it safer, we introduced cub::detail::temporary_storage::layout:

cub::detail::temporary_storage::layout<2> storage_layout;

auto slot_1 = storage_layout.get_slot(0);
auto slot_2 = storage_layout.get_slot(1);

auto allocation_1 = slot_1->create_alias<int>();
auto allocation_2 = slot_1->create_alias<double>(42);
auto allocation_3 = slot_2->create_alias<char>(12);

if (condition)
{
  allocation_1.grow(num_items);
}

if (d_temp_storage == nullptr)
{
  temp_storage_bytes = storage_layout.get_size();
  return;
}

storage_layout.map_to_buffer(d_temp_storage, temp_storage_bytes);

// different slots, safe to use simultaneously
use(allocation_1.get(), allocation_3.get(), stream);
// `allocation_2` alias `allocation_1`, safe to use in stream order
use(allocation_2.get(), stream);

Symbols visibility

Using CUB/Thrust in shared libraries is a known source of issues. For a while, the solution to these issues consisted of wrapping CUB/Thrust namespaces with the THRUST_CUB_WRAPPED_NAMESPACE macro so that different shared libraries have different symbols. This solution has poor discoverability, since issues present themselves in forms of segmentation faults, hangs, wrong results, etc. To eliminate the symbol visibility issues on our end, we follow the following rules:

Hiding kernel launchers: it's important that kernel launchers like Thrust triple_chevron always reside in the same library as the API using these kernel launchers.

Hiding all kernels: it's important that kernels always reside in the same library as the API using these kernels.

Incorporating GPU architectures into symbol names: it's important that kernels compiled for a given GPU architecture are always used by the host API compiled for that architecture.

To satisfy (1), thrust::cuda_cub::launcher::triple_chevron visibility is hidden.

To satisfy (2), instead of annotating kernels as __global__ we annotate them as CUB_DETAIL_KERNEL_ATTRIBUTES. Apart from annotating a kernel as global function, the macro contains hidden visibility attribute.

To satisfy (3), CUB symbols are placed inside an inline namespace containing the set of GPU architectures for which the TU is being compiled.

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CUB Developer Overview

Common Patterns

Thread-level

Warp-level

Overview

Temporary storage usage

Specialization

Block-scope

Overview

Specialization

Temporary storage usage

Device-scope

Overview

Dispatch layer

Temporary storage usage

Temporary storage management

Symbols visibility

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CUB Developer Overview

Common Patterns

Thread-level

Warp-level

Overview

Temporary storage usage

Specialization

Block-scope

Overview

Specialization

Temporary storage usage

Device-scope

Overview

Dispatch layer

Temporary storage usage

Temporary storage management

Symbols visibility