I have been all over the internet with no one directly answering this question. So I have a struct in my main process and I need multiple threads to modify it. I know that I am going to need mutexs to protect the data, but is there any way to share pointers with out passing it as an arg in pthread_create. I understand that threads share the same memory address and there for if I allocate in one the other should have access to it. But with out passing a pointer how do the threads know that a certain pointer already exists.
The typical answer to your question is that rather than passing the pointer directly to the thread when you call pthread_create, you instead pass a pointer to some higher-level structure that serves as your communication with the thread.
For example, say you are writing a network server that receives requests from clients and then needs to process those requests. The threads that process requests from clients would typically be passed the address of some kind of waitable queue. When a thread receives a request from a client, it would put that request in the waitable queue. The threads that process client requests are already waiting for things to be put on that queue, and they get the information about the request they need to process from the queue.
I am learning about threads. And I need to understand how threads communicate between each other, so what does it mean when we say something like "let Thread A send a message to Thread B"?
I can think of the following:
Thread B is blocking on some sort of queue, and Thread A places a new
entry in this queue, which causes Thread B to unblock, and retrieve
this entry.
Thread B is blocking on an event (for example, in Windows API there
is the Event object), and Thread A signals this event which will
cause Thread B to wake up (or is this called notifying a thread and
not sending a message to it?)
The "threads" world is subject of many ambiguity due to different nomenclature coming from different environments, sometimes using same words to mean different things.
Your first assertion makes sense in very general terms: the "message" is what makes the thread to wake-up and get some "input".
Depending on the OS and its own API, your second assertion makes sense and is nothing more then a way to implement the first using the Win32 API.
Another possible interpretation can be that the thread is blocked on a message loop (see GetMessage) and the other one calls PostThreadMessage.
In a more general term, you can think of a "message" as an "event" that carries a "state" with it: an event simply happens (and that's all the information it gives). A message "happens", and has some parameter associated with it.
Link to example Windows code that uses two threads to copy a file, the original thread reads, a created thread writes. There's a custom messaging system that uses Windows mutexes and semaphores. Other than the overhead to create and delete the mutexes and semaphores, the actual functions are fairly small. I've worked on embedded multi-threaded devices, using a similar messaging interface scheme.
http://rcgldr.net/misc/mtcopy.zip
I am writing a small server that will receive data from multiple sources and process this data. The sources and data received is significant, but no more than epoll should be able to handle quite well. However, all received data must be parsed and run through a large number of tests which is time consuming and will block a single thread despite epoll multiplexing. Basically, the pattern should be something like follows: IO-loop receives data and bundles it into a job, sends to the first thread available in the pool, the bundle is processed by the job and the result is passed pack to the IO loop for writing to file.
I have decided to go for a single IO thread and N worker threads. The IO thread for accepting tcp connections and reading data is easy to implement using the example provided at:
http://linux.die.net/man/7/epoll
Thread are also usually easy enough to deal with, but I am struggling to combine the epoll IO loop with a threadpool in an elegant manner. I am unable to find any "best practice" for using epoll with a worker pool online either, but quite a few questions regarding the same topic.
I therefore have some question I hope someone can help me answering:
Could (and should) eventfd be used as a mechanism for 2-way synchronization between the IO thread and all the workers? For instance, is it a good idea for each worker thread to have its own epoll routine waiting on a shared eventfd (with a struct pointer, containing data/info about the job) i.e. using the eventfd as a job queue somehow? Also perhaps have another eventfd to pass results back into the IO thread from multiple worker threads?
After the IO thread is signaled about more data on a socket, should the actual recv take place on the IO thread, or should the worker recv the data on their own in order to not block the IO thread while parsing data frames etc.? In that case, how can I ensure safety, e.g. in case recv reads 1,5 frames of data in a worker thread and another worker thread receives the last 0,5 frame of data from the same connection?
If the worker thread pool is implemented through mutexes and such, will waiting for locks block the IO thread if N+1 threads are trying to use the same lock?
Are there any good practice patterns for how to build a worker thread pool around epoll with two way communication (i.e. both from IO to workers and back)?
EDIT: Can one possible solution be to update a ring buffer from the IO-loop, after update send the ring buffer index to the workers through a shared pipe for all workers (thus giving away control of that index to the first worker that reads the index off the pipe), let the worker own that index until end of processing and then send the index number back into the IO-thread through a pipe again, thus giving back control?
My application is Linux-only, so I can use Linux-only functionality in order to achieve this in the most elegant way possible. Cross platform support is not needed, but performance and thread safety is.
In my tests, one epoll instance per thread outperformed complicated threading models by far. If listener sockets are added to all epoll instances, the workers would simply accept(2) and the winner would be awarded the connection and process it for its lifetime.
Your workers could look something like this:
for (;;) {
nfds = epoll_wait(worker->efd, &evs, 1024, -1);
for (i = 0; i < nfds; i++)
((struct socket_context*)evs[i].data.ptr)->handler(
evs[i].data.ptr,
evs[i].events);
}
And every file descriptor added to an epoll instance could have a struct socket_context associated with it:
void listener_handler(struct socket_context* ctx, int ev)
{
struct socket_context* conn;
conn->fd = accept(ctx->fd, NULL, NULL);
conn->handler = conn_handler;
/* add to calling worker's epoll instance or implement some form
* of load balancing */
}
void conn_handler(struct socket_context* ctx, int ev)
{
/* read all available data and process. if incomplete, stash
* data in ctx and continue next time handler is called */
}
void dummy_handler(struct socket_context* ctx, int ev)
{
/* handle exit condition async by adding a pipe with its
* own handler */
}
I like this strategy because:
very simple design;
all threads are identical;
workers and connections are isolated--no stepping on each other's toes or calling read(2) in the wrong worker;
no locks are required (the kernel gets to worry about synchronization on accept(2));
somewhat naturally load balanced since no busy worker will actively contend on accept(2).
And some notes on epoll:
use edge-triggered mode, non-blocking sockets and always read until EAGAIN;
avoid dup(2) family of calls to spare yourself from some surprises (epoll registers file descriptors, but actually watches file descriptions);
you can epoll_ctl(2) other threads' epoll instances safely;
use a large struct epoll_event buffer for epoll_wait(2) to avoid starvation.
Some other notes:
use accept4(2) to save a system call;
use one thread per core (1 for each physical if CPU-bound, or 1 for each each logical if I/O-bound);
poll(2)/select(2) is likely faster if connection count is low.
I hope this helps.
When performing this model, because we only know the packet size once we have fully received the packet, unfortunately we cannot offload the receive itself to a worker thread. Instead the best we can still do is a thread to receive the data which will have to pass off pointers to fully received packets.
The data itself is probably best held in a circular buffer, however we will want a separate buffer for each input source (if we get a partial packet we can continue receiving from other sources without splitting up the data. The remaining question is how to inform the workers of when a new packet is ready, and to give them a pointer to the data in said packet. Because there is little data here, just some pointers the most elegant way of doing this would be with posix message queues. These provide the ability for multiple senders and multiple receivers to write and read messages, always ensuring every message is received and by precisely 1 thread.
You will want a struct resembling the one below for each data source, I shall go through the fields purposes now.
struct DataSource
{
int SourceFD;
char DataBuffer[MAX_PACKET_SIZE * (THREAD_COUNT + 1)];
char *LatestPacket;
char *CurrentLocation
int SizeLeft;
};
The SourceFD is obviously the file descriptor to the data stream in question, the DataBuffer is where Packets contents are held while being processed, it is a circular buffer. The LatestPacket pointer is used to temporarily hold a pointer to the most resent packet in case we receive a partial packet and move onto another source before passing the packet off. The CurrentLocation stores where the latest packet ends so that we know where to place the next one, or where to carry on in case of partial receive. The size left is the room left in the buffer, this will be used to tell if we can fit the packet or need to circle back around to the beginning.
The receiving function will thus effectively
Copy the contents of the packet into the buffer
Move CurrentLocation to point to the end of the packet
Update SizeLeft to account for the now decreased buffer
If we cannot fit the packet in the end of the buffer we cycle around
If there is no room there either we try again a bit later, going to another source meanwhile
If we had a partial receive store the LatestPacket pointer to point to the start of the packet and go to another stream until we get the rest
Send a message using a posix message queue to a worker thread so it can process the data, the message will contain a pointer to the DataSource structure so it can work on it, it also needs a pointer to the packet it is working on, and it's size, these can be calculated when we receive the packet
The worker thread will do its processing using the received pointers and then increase the SizeLeft so the receiver thread will know it can carry on filling the buffer. The atomic functions will be needed to work on the size value in the struct so we don't get race conditions with the size property (as it is possible it is written by a worker and the IO thread simultaneously, causing lost writes, see my comment below), they are listed here and are simple and extremely useful.
Now, I have given some general background but will address the points given specifically:
Using the EventFD as a synchronization mechanism is largely a bad idea, you will find yourself using a fair amount of unneeded CPU time and it is very hard to perform any synchronization. Particularly if you have multiple threads pick up the same file descriptor you could have major problems. This is in effect a nasty hack that will work sometimes but is no real substitute for proper synchronization.
It is also a bad idea to try and offload the receive as explained above, you can get around the issue with complex IPC but frankly it is unlikely receiving IO will take enough time to stall your application, your IO is also likely much slower than CPU so receiving with multiple threads will gain little. (this assumes you do not say, have several 10 gigabit network cards).
Using mutexes or locks is a silly idea here, it fits much better into lockless coding given the low amount of (simultaneously) shared data, you are really just handing off work and data. This will also boost performance of the receive thread and make your app far more scalable. Using the functions mentioned here http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Atomic-Builtins.html you can do this nice and easily. If you did do it this way, what you would need is a semaphore, this can be unlocked every time a packet is received and locked by each thread which starts a job to allow dynamically more threads in if more packets are ready, that would have far less overhead then a homebrew solution with mutexes.
There is not really much difference here to any thread pool, you spawn a lot of threads then have them all block in mq_receive on the data message queue to wait for messages. When they are done they send their result back to the main thread which adds the results message queue to its epoll list. It can then receive results this way, it is simple and very efficient for small data payloads like pointers. This will also use little CPU and not force the main thread to waste time managing workers.
Finally your edit is fairly sensible, except for the fact as I ave suggested, message queues are far better than pipes here as they very efficiently signal events , guarantee a full message read and provide automatic framing.
I hope this helps, however it is late so if I missed anything or you have questions feel free to comment for clarification or more explanation.
I post the same answer in other post: I want to wait on both a file descriptor and a mutex, what's the recommended way to do this?
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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.
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(), 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 wait 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 worker_thread){
sock, resp = worker->pop_result();
sock.send(resp);
}
if(fd is client_socket){
req = fd.read();
worker->add_task(fd, req);
}
}
}
Usage in worker thread:
fd, req = queue->pop_task();
resp = proc(req);
queue->add_result(fd, resp);
I wrote a simple program that implements master/worker scheme where the master is the main thread, and workers are created by it.
The main thread writes something to a shared buffer, and the worker threads read this shared buffer, writing and reading to shared buffer are organized by read/write lock.
Unfortunately, this scheme definitely leads to starvation of main thread, since a single write has to wait on several reads to complete. One possible solution is increasing the priority of the master thread, so if it wants to write something, it will get immediate access to the shared buffer.
According to a great post to a similar issue, I discovered that probably manipulating the priority of a thread under SCHED_OTHER policy is not allowed, what can be changed is the nice value only.
I wrote a procedure to give worker threads lower priority than master thread, but it seems not to work correctly.
void assignWorkerThreadPriority(pthread_t* worker)
{
struct sched_param* worker_sched_param = (struct sched_param*)malloc(sizeof(struct sched_param));
worker_sched_param->sched_priority =0; //any value other than 0 gives error?
int policy = SCHED_OTHER;
pthread_setschedparam(*worker, policy, worker_sched_param);
printf("Result of changing priority is: %d - %s\n", errno, strerror(errno));
}
I have a two-fold question:
How can I set the nice value of a worker threads to avoid main thread starvation.
If not possible, then how can I change the scheduling policy to a one that allows changing the priority.
Edit: I managed to run the program using other policies, such as SCHED_FIFO, all I had to do was running the program as a super user
You cannot avoid problems using a read/write lock when the read and write usage is so even. You need a different method. You need a lock-free message queue or independent work queues or one of many other techniques.
Here is another way to do the job, the way I would do it. The worker can take the buffer away and work on it rather than keeping it shared:
Write thread:
Create work item.
Lock the mutex or CriticalSection protecting the current queue and pointer to queue.
Add work item to queue.
Release the lock.
Optionally signal a condition variable or Event. Another option is for worker threads to check for work on a timer.
Worker thread:
Create a new queue.
Wait for a condition variable or event or other signal, or wait on a timer.
Lock the mutex or CriticalSection protecting the current queue and pointer to queue.
Set the current queue pointer to the new queue.
Release the lock.
Proceed to work on the now private queue.
Delete the queue when all work items complete.
Now write thread creates more work items. When all the worker threads have their own copies of a queue to work on it will be able to write many items in peace.
You can modify this. For example, a worker thread may lock the queue and move a limited number of work items off into its own internal queue instead of taking the whole thing.
There is a way to serialize the C write() so that I can write bytes on a socket, shared between k-threads, with no data-loss? I imagine that a solution to this problem includes user-space locking, and what about scalability? Thank you in advance.
I think the right answer depends on whether your threads need to synchronously wait for a response or not. If they just need to write some message to a socket and not wait for the peer to respond, I think the best answer is to have a single thread that is dedicated to writing messages from a queue that the other threads place messages on. That way, the worker threads can simply place their messages on the queue and get on with doing something else.
Of course, the queue has to be protected by a mutex but any one thread only has to hold the lock for as long as it is manipulating the queue (guaranteed to be quite a short time). The more obvious alternative of letting every thread write directly to the socket requires each thread to hold the lock for as long as it takes the write operation to complete. This will always be much longer than just adding an item to a queue since write is a system call and potentially, it could block for a long period.
Even if your threads need a response to their messages, it may still pay to do something similar. Your socket servicing thread becomes more complex because you'll have to do something like select() on the socket for reads and writes to stop it from blocking and you'll also need a way to match up messages to responses and a way to inform the threads when their responses have arrived.
Since POSIX does not seem to specify atomicity guarantees on send(2), you will likely have to use a mutex. Scalability of course goes down the drain with this sort of serialization.
One possible approach would be to use the locking mechanism. Every thread should wait for a lock before writing any thing on the socket and should release the lock, once it is done.
If all of your threads are sending exactly the same kind of messages, the receiver end would not have any problem in reading the data, but if different threads can send different kind of data with possible different info, you should have an unique message id associated with each kind of data and its better to send the thread id as well (although not necessary, but might help you in debugging small issues).
You can have a structure like:
typedef struct my_socket_data_st
{
int msg_id;
#ifdef __debug_build__
int thread_id;
#endif
size_t data_size_in_bytes;
.... Followed by your data ....
} my_socket_data_t
Scalability depends on a lot things including the hardware resources on which your application would be running. Since it is a network application, you will have to think about the network bandwidth as well. Although there is no (there are a few, but I think you can ignore them for now for your application) limitation from OS on sending/receiving data over a socket, but you will have to consider about making the send synchronous or asynchronous based on your requirement. Also since, you are taking a lock, you will have to think about lock congestion as well. If the lock is not available easily for other threads, that will degrade the performance by a huge factor.