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aggregator.h
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aggregator.h
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/**
* Copyright 2023-2024, XGBoost contributors
*
* Higher level functions built on top the Communicator API, taking care of behavioral differences
* between row-split vs column-split distributed training, and horizontal vs vertical federated
* learning.
*/
#pragma once
#include <limits>
#include <string>
#include <utility>
#include <vector>
#include "communicator-inl.h"
#include "xgboost/collective/result.h" // for Result
#include "xgboost/data.h" // for MetaINfo
#include "../processing/processor.h" // for Processor
namespace xgboost::collective {
/**
* @brief Apply the given function where the labels are.
*
* Normally all the workers have access to the labels, so the function is just applied locally. In
* vertical federated learning, we assume labels are only available on worker 0, so the function is
* applied there, with the results broadcast to other workers.
*
* @tparam Function The function used to calculate the results.
* @param info MetaInfo about the DMatrix.
* @param buffer The buffer storing the results.
* @param size The size of the buffer.
* @param function The function used to calculate the results.
*/
template <typename FN>
void ApplyWithLabels(Context const*, MetaInfo const& info, void* buffer, std::size_t size,
FN&& function) {
if (info.IsVerticalFederated()) {
// We assume labels are only available on worker 0, so the calculation is done there and result
// broadcast to other workers.
std::string message;
if (collective::GetRank() == 0) {
try {
std::forward<FN>(function)();
} catch (dmlc::Error& e) {
message = e.what();
}
}
collective::Broadcast(&message, 0);
if (message.empty()) {
collective::Broadcast(buffer, size, 0);
} else {
LOG(FATAL) << &message[0];
}
} else {
std::forward<FN>(function)();
}
}
/**
* @brief Apply the given function where the labels are.
*
* Normally all the workers have access to the labels, so the function is just applied locally. In
* vertical federated learning, we assume labels are only available on worker 0, so the function is
* applied there, with the results broadcast to other workers.
*
* @tparam T Type of the HostDeviceVector storing the results.
* @tparam Function The function used to calculate the results.
* @param info MetaInfo about the DMatrix.
* @param result The HostDeviceVector storing the results.
* @param function The function used to calculate the results.
*/
template <bool is_gpair, typename T, typename Function>
void ApplyWithLabels(Context const*, MetaInfo const& info, HostDeviceVector<T>* result,
Function&& function) {
if (info.IsVerticalFederated()) {
// We assume labels are only available on worker 0, so the calculation is done there and result
// broadcast to other workers.
std::string message;
if (collective::GetRank() == 0) {
try {
std::forward<Function>(function)();
} catch (dmlc::Error& e) {
message = e.what();
}
}
collective::Broadcast(&message, 0);
if (!message.empty()) {
LOG(FATAL) << &message[0];
return;
}
std::size_t size{};
if (collective::GetRank() == 0) {
size = result->Size();
}
collective::Broadcast(&size, sizeof(std::size_t), 0);
if (info.IsSecure() && is_gpair) {
// Under secure mode, gpairs will be processed to vector and encrypt
// information only available on rank 0
std::size_t buffer_size{};
std::int8_t *buffer;
if (collective::GetRank() == 0) {
std::vector<double> vector_gh;
for (std::size_t i = 0; i < size; i++) {
auto gpair = result->HostVector()[i];
// cast from GradientPair to float pointer
auto gpair_ptr = reinterpret_cast<float*>(&gpair);
// save to vector
vector_gh.push_back(gpair_ptr[0]);
vector_gh.push_back(gpair_ptr[1]);
}
// provide the vectors to the processor interface
size_t size;
auto buf = processor_instance->ProcessGHPairs(&size, vector_gh);
buffer_size = size;
buffer = reinterpret_cast<std::int8_t *>(buf);
}
// broadcast the buffer size for other ranks to prepare
collective::Broadcast(&buffer_size, sizeof(std::size_t), 0);
// prepare buffer on passive parties for satisfying broadcast mpi call
if (collective::GetRank() != 0) {
buffer = reinterpret_cast<std::int8_t *>(malloc(buffer_size));
}
// broadcast the data buffer holding processed gpairs
collective::Broadcast(buffer, buffer_size, 0);
// call HandleGHPairs
size_t size;
processor_instance->HandleGHPairs(&size, buffer, buffer_size);
} else {
// clear text mode, broadcast the data directly
result->Resize(size);
collective::Broadcast(result->HostPointer(), size * sizeof(T), 0);
}
} else {
std::forward<Function>(function)();
}
}
/**
* @brief Find the global max of the given value across all workers.
*
* This only applies when the data is split row-wise (horizontally). When data is split
* column-wise (vertically), the local value is returned.
*
* @tparam T The type of the value.
* @param info MetaInfo about the DMatrix.
* @param value The input for finding the global max.
* @return The global max of the input.
*/
template <typename T>
std::enable_if_t<std::is_trivially_copy_assignable_v<T>, T> GlobalMax(Context const*,
MetaInfo const& info,
T value) {
if (info.IsRowSplit()) {
collective::Allreduce<collective::Operation::kMax>(&value, 1);
}
return value;
}
/**
* @brief Find the global sum of the given values across all workers.
*
* This only applies when the data is split row-wise (horizontally). When data is split
* column-wise (vertically), the original values are returned.
*
* @tparam T The type of the values.
* @param info MetaInfo about the DMatrix.
* @param values Pointer to the inputs to sum.
* @param size Number of values to sum.
*/
template <typename T, std::int32_t kDim>
[[nodiscard]] Result GlobalSum(Context const*, MetaInfo const& info,
linalg::TensorView<T, kDim> values) {
if (info.IsRowSplit()) {
collective::Allreduce<collective::Operation::kSum>(values.Values().data(), values.Size());
}
return Success();
}
template <typename Container>
[[nodiscard]] Result GlobalSum(Context const* ctx, MetaInfo const& info, Container* values) {
return GlobalSum(ctx, info, values->data(), values->size());
}
/**
* @brief Find the global ratio of the given two values across all workers.
*
* This only applies when the data is split row-wise (horizontally). When data is split
* column-wise (vertically), the local ratio is returned.
*
* @tparam T The type of the values.
* @param info MetaInfo about the DMatrix.
* @param dividend The dividend of the ratio.
* @param divisor The divisor of the ratio.
* @return The global ratio of the two inputs.
*/
template <typename T>
T GlobalRatio(Context const* ctx, MetaInfo const& info, T dividend, T divisor) {
std::array<T, 2> results{dividend, divisor};
auto rc = GlobalSum(ctx, info, linalg::MakeVec(results.data(), results.size()));
collective::SafeColl(rc);
std::tie(dividend, divisor) = std::tuple_cat(results);
if (divisor <= 0) {
return std::numeric_limits<T>::quiet_NaN();
} else {
return dividend / divisor;
}
}
} // namespace xgboost::collective