Is there a standard function to receive messages concurrent, but non-blocking? It seems like all the functions available in std.concurrency are blocking and the closest I found to something that is non-blocking is receiveTimeout however it still waits until the timeout. I would like it to return immediately if there are no messages passed to the thread.
Here is what I came up with using receiveTimeout.
module main;
import std.concurrency;
import std.stdio : writefln, readln;
import core.time;
import core.thread;
void spawnedFunc() {
while (true) {
receiveTimeout(
dur!("nsecs")(1),
(int i) { writefln("Received %s", i); }
);
Thread.sleep(dur!("msecs")(100));
}
}
void main() {
auto tid = spawn(&spawnedFunc);
while (true) {
readln();
send(tid, 42);
}
}
Update:
Is my best bet a function like this?
void receiveNonBlocking(T...)(T ops) {
receiveTimeout(
dur!("nsecs")(1),
ops
);
}
...
receiveNonBlocking(
(int i) { writefln("Received %s", i); }
);
Looking at implementation of receiveTimeout:
if( period.isNegative || !m_putMsg.wait( period ) )
return false;
So, providing negative timeout, e.g. nsecs(-1), is the best choice here. And it wouldn't really impact performance, since implementation of non blocking function wouldn't be much different from current one with timeout. Just one more if check to execute before exit for function with negative timeout.
Related
Along with the main thread, i have one more thread that receives data to write them in a file.
std::queue<std::vector<int>> dataQueue;
std::mutex mutex;
void setData(const std::vector<int>& data) {
std::lock_guard<std::mutex> lock(mutex);
dataQueue.push(data);
}
void write(const std::string& fileName) {
std::unique_ptr<std::ostream> ofs = std::unique_ptr<std::ostream>(new zstr::ofstream(fileName));
while (store) {
std::lock_guard<std::mutex> lock(mutex);
while (!dataQueue.empty()) {
std::vector<int>& data= dataQueue.front();
ofs->write(reinterpret_cast<char*>(data.data()), sizeof(data[0])*data.size());
dataQueue.pop();
}
}
}
}
setData is used by the main thread and write is actually the writing thread. I use std::lock_quard to avoid memory conflict but when locking on the writing thread, it slows down the main thread as it has to wait for the Queue to be unlocked. But i guess i can avoid this as the threads never act on the same element of the queue at the same time.
So i would like to do it lock-free but i don't really understand how i should implement it. I mean, how can i do it without locking anything ? moreover, if the writing thread is faster than the main thread, the queue might be empty most of the time, so it should somehow waits for new data instead of looping infinitly to check for non empty queue.
EDIT: I changed simple std::lock_guard by std::cond_variable so that it could wait when the queue is empty. But the main thread can still be blocked as , when cvQeue.wait(.) is resolved, it reacquire the lock. moreover, what if the main thread does cvQueue.notify_one() but the writing thread is not waiting ?
std::queue<std::vector<int>> dataQueue;
std::mutex mutex;
std::condition_variable cvQueue;
void setData(const std::vector<int>& data) {
std::unique_lock<std::mutex> lock(mutex);
dataQueue.push(data);
cvQueue.notify_one();
}
void write(const std::string& fileName) {
std::unique_ptr<std::ostream> ofs = std::unique_ptr<std::ostream>(new zstr::ofstream(fileName));
while (store) {
std::lock_guard<std::mutex> lock(mutex);
while (!dataQueue.empty()) {
std::unique_lock<std::mutex> lock(mutex);
cvQueue.wait(lock);
ofs->write(reinterpret_cast<char*>(data.data()), sizeof(data[0])*data.size());
dataQueue.pop();
}
}
}
}
If you only have two threads, than you could use a lock-free single-producer-single-consumer (SPSC) queue.
A bounded version can be found here: https://github.com/rigtor/SPSCQueue
Dmitry Vyukov presented an unbounded version here: http://www.1024cores.net/home/lock-free-algorithms/queues/unbounded-spsc-queue (You should note though, that this code should be adapted to use atomics.)
Regarding a blocking pop operation - this is something that lock-free data structures do not provide since such an operation is obviously not lock-free. However, it should be relatively straight forward to adapt the linked implementations in such a way, that a push operation notifies a condition variable if the queue was empty before the push.
i guess i have something that met my needs. I did a LockFreeQueue that uses std::atomic. I can thus manage the state of the head/tail of the queue atomically.
template<typename T>
class LockFreeQueue {
public:
void push(const T& newElement) {
fifo.push(newElement);
tail.fetch_add(1);
cvQueue.notify_one();
}
void pop() {
size_t oldTail = tail.load();
size_t oldHead = head.load();
if (oldTail == oldHead) {
return;
}
fifo.pop();
head.store(++oldHead);
}
bool isEmpty() {
return head.load() == tail.load();
}
T& getFront() {
return fifo.front();
}
void waitForNewElements() {
if (tail.load() == head.load()) {
std::mutex m;
std::unique_lock<std::mutex> lock(m);
cvQueue.wait_for(lock, std::chrono::milliseconds(TIMEOUT_VALUE));
}
}
private:
std::queue<T> fifo;
std::atomic<size_t> head = { 0 };
std::atomic<size_t> tail = { 0 };
std::condition_variable cvQueue;
};
LockFreeQueue<std::vector<int>> dataQueue;
std::atomic<bool> store(true);
void setData(const std::vector<int>& data) {
dataQueue.push(data);
// do other things
}
void write(const std::string& fileName) {
std::unique_ptr<std::ostream> ofs = std::unique_ptr<std::ostream>(new zstr::ofstream(fileName));
while (store.load()) {
dataQueue.waitForNewElements();
while (!dataQueue.isEmpty()) {
std::vector<int>& data= dataQueue.getFront();
ofs->write(reinterpret_cast<char*>(data.data()), sizeof(data[0])*data.size());
dataQueue.pop();
}
}
}
}
I still have one lock in waitForNewElements but it is not locking the whole process as it is waiting for things to do. But the big improvement is that the producer can push while the consumer pop. It is only forbidden when LockFreQueue::tail and LockFreeQueue::head are the same. Meaning that the queue is empty and it enters the waiting state.
The thing that i'm not very satisfied at is cvQueue.wait_for(lock, TIMEOUT_VALUE). I wanted to do a simple cvQueue.wait(lock), but the problem is that when it comes to end the thread, I do store.store(false) in the main thread. So if the writing thread is waiting it will never end without a timeout. So, I set a big enough timeout so that most of the time the condition_variable is resolved by the lock, and when the thread ends it is resolved by the timeout.
If you feel that something must be wrong or must be improved, feel free to comment.
In my attempt to add suspend / resume functionality to my Worker [thread] class, I've happened upon an issue that I cannot explain. (C++1y / VS2015)
The issue looks like a deadlock, however I cannot seem to reproduce it once a debugger is attached and a breakpoint is set before a certain point (see #1) - so it looks like it's a timing issue.
The fix that I could find (#2) doesn't make a lot of sense to me because it requires to hold on to a mutex longer and where client code might attempt to acquire other mutexes, which I understand to actually increase the chance of a deadlock.
But it does fix the issue.
The Worker loop:
Job* job;
while (true)
{
{
std::unique_lock<std::mutex> lock(m_jobsMutex);
m_workSemaphore.Wait(lock);
if (m_jobs.empty() && m_finishing)
{
break;
}
// Take the next job
ASSERT(!m_jobs.empty());
job = m_jobs.front();
m_jobs.pop_front();
}
bool done = false;
bool wasSuspended = false;
do
{
// #2
{ // Removing this extra scoping seemingly fixes the issue BUT
// incurs us holding on to m_suspendMutex while the job is Process()ing,
// which might 1, be lengthy, 2, acquire other locks.
std::unique_lock<std::mutex> lock(m_suspendMutex);
if (m_isSuspended && !wasSuspended)
{
job->Suspend();
}
wasSuspended = m_isSuspended;
m_suspendCv.wait(lock, [this] {
return !m_isSuspended;
});
if (wasSuspended && !m_isSuspended)
{
job->Resume();
}
wasSuspended = m_isSuspended;
}
done = job->Process();
}
while (!done);
}
Suspend / Resume is just:
void Worker::Suspend()
{
std::unique_lock<std::mutex> lock(m_suspendMutex);
ASSERT(!m_isSuspended);
m_isSuspended = true;
}
void Worker::Resume()
{
{
std::unique_lock<std::mutex> lock(m_suspendMutex);
ASSERT(m_isSuspended);
m_isSuspended = false;
}
m_suspendCv.notify_one(); // notify_all() doesn't work either.
}
The (Visual Studio) test:
struct Job: Worker::Job
{
int durationMs = 25;
int chunks = 40;
int executed = 0;
bool Process()
{
auto now = std::chrono::system_clock::now();
auto until = now + std::chrono::milliseconds(durationMs);
while (std::chrono::system_clock::now() < until)
{ /* busy, busy */
}
++executed;
return executed < chunks;
}
void Suspend() { /* nothing here */ }
void Resume() { /* nothing here */ }
};
auto worker = std::make_unique<Worker>();
Job j;
worker->Enqueue(j);
std::this_thread::sleep_for(std::chrono::milliseconds(j.durationMs)); // Wait at least one chunk.
worker->Suspend();
Assert::IsTrue(j.executed < j.chunks); // We've suspended before we finished.
const int testExec = j.executed;
std::this_thread::sleep_for(std::chrono::milliseconds(j.durationMs * 4));
Assert::IsTrue(j.executed == testExec); // We haven't moved on.
// #1
worker->Resume(); // Breaking before this call means that I won't see the issue.
worker->Finalize();
Assert::IsTrue(j.executed == j.chunks); // Now we've finished.
What am I missing / doing wrong? Why does the Process()ing of the job have to be guarded by the suspend mutex?
EDIT: Resume() should not have been holding on to the mutex at the time of notification; that's fixed -- the issue persists.
Of course the Process()ing of the job does not have to be guarded by the suspend mutex.
The access of j.executed - for the asserts as well as for the incrementing - however does need to be synchronized (either by making it an std::atomic<int> or by guarding it with a mutex etc.).
It's still not clear why the issue manifested the way it did (since I'm not writing to the variable on the main thread) -- might be a case of undefined behaviour propagating backwards in time.
I am doing multithreading in C++. This may be something very standard but I can't seem to find it anywhere or know any key terms to search for it online.
I want to do some sort of computation many times but with multiple threads. For each iteration of computation, I want to find the next available thread that has finished its previous computation to do the next iteration. I don't want to cycle through the threads in order since the next thread to be called may not have finished its work yet.
E.g.
Suppose I have a vector of int and I want to sum up the total with 5 threads. I have the to-be-updated total sum stored somewhere and the count for which element I am currently up to. Each thread looks at the count to see the next position and then takes that vector value and adds it to the total sum so far. Then it goes back to look for the count to do the next iteration. So for each iteration, the count increments then looks for the next available thread (maybe one already waiting for count; or maybe they are all busy still working) to do the next iteration. We do not increase the number of threads but I want to be able to somehow search through all the 5 threads for the first one that finish to do the next computation.
How would I go about coding this. Every way I know of involves doing a loop through the threads such that I can't check for the next available one which may be out of order.
Use semafor (or mutex, always mix up those two) on a global variable telling you what is next. The semafor will lock the other threads out as long as you access the variable making that threads access clear.
So, assuming you have an Array of X elements. And a global called nextfree witch is initalized to 0, then a psudo code would look like this:
while (1)
{
<lock semafor INT>
if (nextfree>=X)
{
<release semnafor INT>
<exit and terminate thread>
}
<Get the data based on "nextfree">
nextfree++;
<release semafor INT>
<do your stuff withe the chunk you got>
}
The point here is that each thread will be alone and have exlusive access to the data struct within the semafor lock and therefore can access the next available regardless of what the others doing. (The other threads will have to wait in line if they are done while another thread working on getting next data chunk. When you release only ONE that stands in queue will get access. The rest will have to wait.)
There are some things to be ware of. Semafor's might lock your system if you manage to exit in the wrong position (Withour releasing it) or create a deadlock.
This is a thread pool:
template<class T>
struct threaded_queue {
using lock = std::unique_lock<std::mutex>;
void push_back( T t ) {
{
lock l(m);
data.push_back(std::move(t));
}
cv.notify_one();
}
boost::optional<T> pop_front() {
lock l(m);
cv.wait(l, [this]{ return abort || !data.empty(); } );
if (abort) return {};
auto r = std::move(data.back());
data.pop_back();
return std::move(r);
}
void terminate() {
{
lock l(m);
abort = true;
data.clear();
}
cv.notify_all();
}
~threaded_queue()
{
terminate();
}
private:
std::mutex m;
std::deque<T> data;
std::condition_variable cv;
bool abort = false;
};
struct thread_pool {
thread_pool( std::size_t n = 1 ) { start_thread(n); }
thread_pool( thread_pool&& ) = delete;
thread_pool& operator=( thread_pool&& ) = delete;
~thread_pool() = default; // or `{ terminate(); }` if you want to abandon some tasks
template<class F, class R=std::result_of_t<F&()>>
std::future<R> queue_task( F task ) {
std::packaged_task<R()> p(std::move(task));
auto r = p.get_future();
tasks.push_back( std::move(p) );
return r;
}
template<class F, class R=std::result_of_t<F&()>>
std::future<R> run_task( F task ) {
if (threads_active() >= total_threads()) {
start_thread();
}
return queue_task( std::move(task) );
}
void terminate() {
tasks.terminate();
}
std::size_t threads_active() const {
return active;
}
std::size_t total_threads() const {
return threads.size();
}
void clear_threads() {
terminate();
threads.clear();
}
void start_thread( std::size_t n = 1 ) {
while(n-->0) {
threads.push_back(
std::async( std::launch::async,
[this]{
while(auto task = tasks.pop_front()) {
++active;
try{
(*task)();
} catch(...) {
--active;
throw;
}
--active;
}
}
)
);
}
}
private:
std::vector<std::future<void>> threads;
threaded_queue<std::packaged_task<void()>> tasks;
std::atomic<std::size_t> active;
};
You give it how many threads either at construction or via start_thread.
You then queue_task. This returns a std::future that tells you when the task is completed.
As threads finish a task, they go to the threaded_queue and look for more.
When a threaded_queue is destroyed, it aborts all data in it.
When a thread_pool is destroyed, it aborts all future tasks, then waits for all of the outstanding tasks to finish.
Live example.
I have a data structure :
struct {
mutex m;
condition_variable cv_p2c;
queue<int> qi;
bool finished;
} sdf_inst;
and I have a producer that generate 100 integer and insert them into queue qi after getting lock.
void producer () {
for(int i = 0 ; i < 100 ; i++ ) {
{
unique_lock<mutex> ulck(sdf_inst.m);//LOCK
sdf_inst.qi.push(i);
cout<<"adding "<<i<<endl<<flush;
}
sdf_inst.cv_p2c.notify_one();
}
unique_lock<mutex> ulck(sdf_inst.m);//LOCK
sdf_inst.finished=true;
sdf_inst.cv_p2c.notify_one();
}
After all datas have been inserted, it will acquire the lock and set the finished flag, and exit.
And I have another consumer :
void consumer () {
while(true) {
unique_lock<mutex> ulck(sdf_inst.m);//LOCK
sdf_inst.cv_p2c.wait(ulck,[]{return sdf_inst.qi.empty()==false || sdf_inst.finished==true ; });
print_all();
if(sdf_inst.finished=true) return;
}
}
It just acquire lock, and wait for notify from producer, and print all the data currently in the queue qi with the print_all function below:
void print_all () {
while(sdf_inst.qi.empty()==false) {
int i = sdf_inst.qi.front();
sdf_inst.qi.pop();
cout<<"shared_i "<< i <<endl<<flush;
}
return;
}
I think it should print all the 100 data, but sometimes it print only part of them.
I have studied the code carefully and find no error in synchronization, so may the lost data caused by the queue leakage?
I found the cause of this problem, in consumer():
if(sdf_inst.finished=true) return;
should be
if ( sdf_inst.finished )
{
return;
}
I am trying to write a simple D program with two threads (main and one spawned in main) with a daughter thread receiving message from the parent thread. Here is my piece:
import std.stdio;
import core.thread;
import std.concurrency;
import std.c.stdio;
extern (C) int kbhit();
void readSignal(){
for(;;){
bool stop = receiveOnly!bool();
if(!stop)
writeln("reading signal...");
else
writeln("stop reading signal...");
Thread.getThis.sleep(1.seconds);
}
}
int main()
{
auto reader = spawn(&readSignal);
bool stop = false;
while(true){
if(kbhit()){
stop = true;
}
option 1: reader.send(stop);
option 2: send(reader, stop);
}
}
return 0;
}
Basically I wait for a keyboard hit and the intend to pause the spawned thread.
Neither of options (option 1 by Andrei Alexandrescu and option 2 as in dlang docs) work for me. What do i do wrong?
It looks to me that your main thread is spinning very hard, sending thousands and thousands of messages to the reader. The reader, however, is only reading one per second. The 'true' message is backed up behind a massive number of 'false' messages.
For a situation like this, you may be better off using a shared variable, rather than using messages. Both threads will be able to safely read and write from it.
as mentioned you are flooding the client thread.
Try adding a sleep like this
Thread.sleep(dur!"msecs"(1500));
As mentioned by duselbaer and Anders S, in your example, the number of sends is huge, but the number of receives is limited by the sleep. So, when the thread should actually get a true, it is already preceded by an overwhelming number of falses in the queue.
Generally, if the child thread is able to receive faster than the parent thread sends, it's fine. Here is a working example produced from your code. The sleep calls may be substituted by doing some work, but the relation must generally hold.
import std.stdio;
import core.thread;
import std.concurrency;
import core.stdc.stdio;
extern (C) int kbhit();
void readSignal() {
for(int i = 0; ; i++) {
bool stop = receiveOnly!bool();
if(!stop)
writeln("reading signal... ", i);
else
{
writeln("stop reading signal...");
break;
}
Thread.getThis.sleep(1.msecs); // this should be no more than below
}
}
int main() {
auto reader = spawn(&readSignal);
bool stop = false;
while(!stop) {
if(kbhit()) {
stop = true;
}
send(reader, stop); // option 1
// reader.send(stop); // option 2 does the same thing as above via UFCS
Thread.getThis.sleep(1.msecs); // this should be no less than above
}
return 0;
}