I want to wait on both a file descriptor and a mutex, what's the recommended way to do this?

Question

I would like to spawn off threads to perform certain tasks, and use a thread-safe queue to communicate with them. I would also like to be doing IO to a variety of file descriptors while I'm waiting.

What's the recommended way to accomplish this? Do I have to created an inter-thread pipe and write to it when the queue goes from no elements to some elements? Isn't there a better way?

And if I have to create the inter-thread pipe, why don't more libraries that implement shared queues allow you to create the shared queue and inter-thread pipe as a single entity?

Does the fact I want to do this at all imply a fundamental design flaw?

I'm asking this about both C++ and Python. And I'm mildly interested in a cross-platform solution, but primarily interested in Linux.

For a more concrete example...

I have some code which will be searching for stuff in a filesystem tree. I have several communications channels open to the outside world through sockets. Requests that may (or may not) result in a need to search for stuff in the filesystem tree will be arriving.

I'm going to isolate the code that searches for stuff in the filesystem tree in one or more threads. I would like to take requests that result in a need to search the tree and put them in a thread-safe queue of things to be done by the searcher threads. The results will be put into a queue of completed searches.

I would like to be able to service all the non-search requests quickly while the searches are going on. I would like to be able to act on the search results in a timely fashion.

Servicing the incoming requests would generally imply some kind of event-driven architecture that uses epoll. The queue of disk-search requests and the return queue of results would imply a thread-safe queue that uses mutexes or semaphores to implement the thread safety.

The standard way to wait on an empty queue is to use a condition variable. But that won't work if I need to service other requests while I'm waiting. Either I end up polling the results queue all the time (and delaying the results by half the poll interval, on average), blocking and not servicing requests.

Answer 1

Whenever one uses an event driven architecture, one is required to have a single mechanism to report event completion. On Linux, if one is using files, one is required to use something from the select or poll family meaning that one is stuck with using a pipe to initiate all none file related events.

Edit: Linux has eventfd and timerfd. These can be added to your epoll list and used to break out of the epoll_wait when either triggered from another thread or on a timer event respectively.

There is another option and that is signals. One can use fcntl modify the file descriptor such that a signal is emitted when the file descriptor becomes active. The signal handler may then push a file-ready message onto any type of queue of your choosing. This may be a simple semaphore or mutex/condvar driven queue. Since one is now no longer using select/poll, one no longer needs to use a pipe to queue none file based messages.

Health warning: I have not tried this and although I cannot see why it will not work, I don't really know the performance implications of the signal approach.

Edit: Manipulating a mutex in a signal handler is probably a very bad idea.

Answer 2

I've solved this exact problem using what you mention, pipe() and libevent (which wraps epoll). The worker thread writes a byte to its pipe FD when its output queue goes from empty to non-empty. That wakes up the main IO thread, which can then grab the worker thread's output. This works great is actually very simple to code.

Answer 3

You have the Linux tag so I am going to throw this out: POSIX Message Queues do all this, which should fulfill your "built-in" request if not your less desired cross-platform wish.

The thread-safe synchronization is built-in. You can have your worker threads block on read of the queue. Alternatively MQs can use mq_notify() to spawn a new thread (or signal an existing one) when there is a new item put in the queue. And since it looks like you are going to be using select(), MQ's identifier (mqd_t) can be used as a file descriptor with select.

Answer 4

Duck's and twk's are actually better answers than doron's (the one selected by the OP), in my opinion. doron suggests writing to a message queue from within the context of a signal handler, and states that the message queue can be "any type of queue." I would strongly caution you against this since many C library/system calls cannot safely be called from within a signal handler (see async-signal-safe).

In particuliar, if you choose a queue protected by a mutex, you should not access it from a signal handler. Consider this scenario: your consumer thread locks the queue to read it. Immediately after, the kernel delivers the signal to notify you that a file descriptor now has data on it. You signal handler runs in the consumer thread, necessarily), and tries to put something on your queue. To do this, it first has to take the lock. But it already holds the lock, so you are now deadlocked.

select/poll is, in my experience, the only viable solution to an event-driven program in UNIX/Linux. I wish there were a better way inside a mutlithreaded program, but you need some mechanism to "wake up" your consumer thread. I have yet to find a method that does not involve a system call (since the consumer thread is on a waitqueue inside the kernel during any blocking call such as select).

EDIT: I forgot to mention one Linux-specific way to handle signals when using select/poll: signalfd(2). You get a file descriptor you can select/poll on, and you handling code runs normally instead of in a signal handler's context.

Answer 5

It seems nobody has mentioned this option yet:

Don't run select/poll/etc. in your "main thread". Start a dedicated secondary thread which does the I/O and pushes notifications into your thread-safe queue (the same queue which your other threads use to communicate with the main thread) when I/O operations complete.

Then your main thread just needs to wait on the notification queue.

Answer 6

This is a very common seen problem, especially when you are developing network server-side program. Most Linux server-side program's main look will loop like this:

epoll_add(serv_sock);
while(1){
    ret = epoll_wait();
    foreach(ret as fd){
        req = fd.read();
        resp = proc(req);
        fd.send(resp);
    }
}

It is single threaded(the main thread), epoll based server framework. The problem is, it is single threaded, not multi-threaded. It requires that proc() should never blocks or runs for a significant time(say 10 ms for common cases).

If proc() will ever runs for a long time, WE NEED MULTI THREADS, and executes proc() in a separated thread(the worker thread).

We can submit task to the worker thread without blocking the main thread, using a mutex based message queue, it is fast enough.

epoll_add(serv_sock);
while(1){
    ret = epoll_wait();
    foreach(ret as fd){
        req = fd.read();
        queue.add_job(req); // fast, non blockable
    }
}

Then we need a way to obtain the task result from a worker thread. How? If we just check the message queue directly, before or after epoll_wait().

epoll_add(serv_sock);
while(1){
    ret = epoll_wait(); // may blocks for 10ms
    resp = queue.check_result(); // fast, non blockable
    foreach(ret as fd){
        req = fd.read();
        queue.add_job(req); // fast, non blockable
    }
}

However, the checking action will execute after epoll_wait() to end, and epoll_wait() usually blocks for 10 micro seconds(common cases) if all file descriptors it waits are not active.

For a server, 10 ms is quite a long time! Can we signal epoll_wait() to end immediately when task result is generated?

Yes! I will describe how it is done in one of my open source project:

Create a pipe for all worker threads, and epoll waits on that pipe as well. Once a task result is generated, the worker thread writes one byte into the pipe, then epoll_wait() will end in nearly the same time! - Linux pipe has 5 us to 20 us latency.

---

In my project SSDB(a Redis protocol compatible in-disk NoSQL database), I create a SelectableQueue for passing messages between the main thread and worker threads. Just like its name, SelectableQueue has an file descriptor, which can be wait by epoll.

SelectableQueue: https://github.com/ideawu/ssdb/blob/master/src/util/thread.h#L94

Usage in main thread:

epoll_add(serv_sock);
epoll_add(queue->fd());
while(1){
    ret = epoll_wait();
    foreach(ret as fd){
        if(fd is queue){
            sock, resp = queue->pop_result();
            sock.send(resp);
        }
        if(fd is client_socket){
            req = fd.read();
            queue->add_task(fd, req);
        }
    }
}

Usage in worker thread:

fd, req = queue->pop_task();
resp = proc(req);
queue->add_result(fd, resp);

Answer 7

C++11 has std::mutex and std::condition_variable. The two can be used to have one thread signal another when a certain condition is met. It sounds to me like you will need to build your solution out of these primitives. If you environment does not yet support these C++11 library features, you can find very similar ones at boost. Sorry, can't say much about python.

Answer 8

One way to accomplish what you're looking to do is by implementing the Observer Pattern

You would register your main thread as an observer with all your spawned threads, and have them notify it when they were done doing what they were supposed to (or updating during their run with the info you need).

Basically, you want to change your approach to an event-driven model.