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// Ceres Solver - A fast non-linear least squares minimizer
// Copyright 2018 Google Inc. All rights reserved.
// http://ceres-solver.org/
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are met:
//
// * Redistributions of source code must retain the above copyright notice,
// this list of conditions and the following disclaimer.
// * Redistributions in binary form must reproduce the above copyright notice,
// this list of conditions and the following disclaimer in the documentation
// and/or other materials provided with the distribution.
// * Neither the name of Google Inc. nor the names of its contributors may be
// used to endorse or promote products derived from this software without
// specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
// POSSIBILITY OF SUCH DAMAGE.
//
// Author: vitus@google.com (Michael Vitus)
// This include must come before any #ifndef check on Ceres compile options.
#include "ceres/internal/config.h"
#ifdef CERES_USE_CXX_THREADS
#include <cmath>
#include <condition_variable>
#include <memory>
#include <mutex>
#include "ceres/concurrent_queue.h"
#include "ceres/parallel_for.h"
#include "ceres/scoped_thread_token.h"
#include "ceres/thread_token_provider.h"
#include "glog/logging.h"
namespace ceres::internal {
namespace {
// This class creates a thread safe barrier which will block until a
// pre-specified number of threads call Finished. This allows us to block the
// main thread until all the parallel threads are finished processing all the
// work.
class BlockUntilFinished {
public:
explicit BlockUntilFinished(int num_total)
: num_finished_(0), num_total_(num_total) {}
// Increment the number of jobs that have finished and signal the blocking
// thread if all jobs have finished.
void Finished() {
std::lock_guard<std::mutex> lock(mutex_);
++num_finished_;
CHECK_LE(num_finished_, num_total_);
if (num_finished_ == num_total_) {
condition_.notify_one();
}
}
// Block until all threads have signaled they are finished.
void Block() {
std::unique_lock<std::mutex> lock(mutex_);
condition_.wait(lock, [&]() { return num_finished_ == num_total_; });
}
private:
std::mutex mutex_;
std::condition_variable condition_;
// The current number of jobs finished.
int num_finished_;
// The total number of jobs.
int num_total_;
};
// Shared state between the parallel tasks. Each thread will use this
// information to get the next block of work to be performed.
struct SharedState {
SharedState(int start, int end, int num_work_items)
: start(start),
end(end),
num_work_items(num_work_items),
i(0),
thread_token_provider(num_work_items),
block_until_finished(num_work_items) {}
// The start and end index of the for loop.
const int start;
const int end;
// The number of blocks that need to be processed.
const int num_work_items;
// The next block of work to be assigned to a worker. The parallel for loop
// range is split into num_work_items blocks of work, i.e. a single block of
// work is:
// for (int j = start + i; j < end; j += num_work_items) { ... }.
int i;
std::mutex mutex_i;
// Provides a unique thread ID among all active threads working on the same
// group of tasks. Thread-safe.
ThreadTokenProvider thread_token_provider;
// Used to signal when all the work has been completed. Thread safe.
BlockUntilFinished block_until_finished;
};
} // namespace
int MaxNumThreadsAvailable() { return ThreadPool::MaxNumThreadsAvailable(); }
// See ParallelFor (below) for more details.
void ParallelFor(ContextImpl* context,
int start,
int end,
int num_threads,
const std::function<void(int)>& function) {
CHECK_GT(num_threads, 0);
CHECK(context != nullptr);
if (end <= start) {
return;
}
// Fast path for when it is single threaded.
if (num_threads == 1) {
for (int i = start; i < end; ++i) {
function(i);
}
return;
}
ParallelFor(
context, start, end, num_threads, [&function](int /*thread_id*/, int i) {
function(i);
});
}
// This implementation uses a fixed size max worker pool with a shared task
// queue. The problem of executing the function for the interval of [start, end)
// is broken up into at most num_threads blocks and added to the thread pool. To
// avoid deadlocks, the calling thread is allowed to steal work from the worker
// pool. This is implemented via a shared state between the tasks. In order for
// the calling thread or thread pool to get a block of work, it will query the
// shared state for the next block of work to be done. If there is nothing left,
// it will return. We will exit the ParallelFor call when all of the work has
// been done, not when all of the tasks have been popped off the task queue.
//
// A unique thread ID among all active tasks will be acquired once for each
// block of work. This avoids the significant performance penalty for acquiring
// it on every iteration of the for loop. The thread ID is guaranteed to be in
// [0, num_threads).
//
// A performance analysis has shown this implementation is on par with OpenMP
// and TBB.
void ParallelFor(ContextImpl* context,
int start,
int end,
int num_threads,
const std::function<void(int thread_id, int i)>& function) {
CHECK_GT(num_threads, 0);
CHECK(context != nullptr);
if (end <= start) {
return;
}
// Fast path for when it is single threaded.
if (num_threads == 1) {
// Even though we only have one thread, use the thread token provider to
// guarantee the exact same behavior when running with multiple threads.
ThreadTokenProvider thread_token_provider(num_threads);
const ScopedThreadToken scoped_thread_token(&thread_token_provider);
const int thread_id = scoped_thread_token.token();
for (int i = start; i < end; ++i) {
function(thread_id, i);
}
return;
}
// We use a std::shared_ptr because the main thread can finish all
// the work before the tasks have been popped off the queue. So the
// shared state needs to exist for the duration of all the tasks.
const int num_work_items = std::min((end - start), num_threads);
std::shared_ptr<SharedState> shared_state(
new SharedState(start, end, num_work_items));
// A function which tries to perform a chunk of work. This returns false if
// there is no work to be done.
auto task_function = [shared_state, &function]() {
int i = 0;
{
// Get the next available chunk of work to be performed. If there is no
// work, return false.
std::lock_guard<std::mutex> lock(shared_state->mutex_i);
if (shared_state->i >= shared_state->num_work_items) {
return false;
}
i = shared_state->i;
++shared_state->i;
}
const ScopedThreadToken scoped_thread_token(
&shared_state->thread_token_provider);
const int thread_id = scoped_thread_token.token();
// Perform each task.
for (int j = shared_state->start + i; j < shared_state->end;
j += shared_state->num_work_items) {
function(thread_id, j);
}
shared_state->block_until_finished.Finished();
return true;
};
// Add all the tasks to the thread pool.
for (int i = 0; i < num_work_items; ++i) {
// Note we are taking the task_function as value so the shared_state
// shared pointer is copied and the ref count is increased. This is to
// prevent it from being deleted when the main thread finishes all the
// work and exits before the threads finish.
context->thread_pool.AddTask([task_function]() { task_function(); });
}
// Try to do any available work on the main thread. This may steal work from
// the thread pool, but when there is no work left the thread pool tasks
// will be no-ops.
while (task_function()) {
}
// Wait until all tasks have finished.
shared_state->block_until_finished.Block();
}
} // namespace ceres::internal
#endif // CERES_USE_CXX_THREADS