Well my problem is the following. I have a piece of code that runs on several virtual machines, and each virtual machine has N interfaces(a thread per each). The problem itself is receiving a message on one interface and redirect it through another interface in the fastest possible manner.
What I'm doing is, when I receive a message on one interface(Unicast), calculate which interface I want to redirect it through, save all the information about the message(Datagram, and all the extra info I want) with a function I made. Then on the next iteration, the program checks if there are new messages to redirect and if it is the correct interface reading it. And so on... But this makes the program exchange information very slowly...
Is there any mechanism that can speed things up?
Somebody has already invented this particular wheel - it's called MPI
Take a look at either openMPI or MPICH
Why don't you use queuing? As the messages come in, put them on a queue and notify each processing module to pick them up from the queue.
For example:
MSG comes in
Module 1 puts it on queue
Module 2,3 get notified
Module 2 picks it up from queue and saved it in the database
In parallel, Module 3 picks it up from queue and processes it
The key is "in parallel". Since these modules are different threads, while Module 2 is saving to the db, Module 3 can massage your message.
You could use JMS or MQ or make your own queue.
It sounds like you're trying to do parallel computing across multiple "machines" (even if virtual). You may want to look at existing protocols, such as MPI - Message Passing Interface to handle this domain, as they have quite a few features that help in this type of scenario
Related
Looking for the best approach to sending the same message to multiple destinations using TCP/IP sockets. I'm working with an existing VS 2010 C++ application on Windows. Hoping to use a standard library/design pattern approach that has many of the complexities already worked out if possible.
Here's one approach I'm thinking about.. One main thread retrieves messages from a database and adds them to some sort of thread safe queue. The application also has one thread for each client socket connection to some destination server. Each one of these threads would read from the thread safe queue, and send the message over a tcp/ip socket.
There may be better/simpler/more robust approaches than this one though..
The issues I have to be concerned about mostly are latency. The destinations could be anywhere, and there may be significant latency between one socket connection and another.
The messages must go in an exact FIFO order to all the destinations.
Also one destination will be considered the primary destination.. all messages must get to this destination, no exceptions. For the other destinations, i.e. non-primary, the messages are just copies and it's not absolutely critical if the non-primary destinations do not receive a few messages. At any point, one of the non-primary destinations could become the primary destination. If one of the destinations falls too far behind, then that thread would need to catch up to the primary destination, but skipping some messages.
Looking for any suggestions. Preliminary research so far, my situation appears to be something akin to a single producer and multiple consumers pattern, or possibly master-worker pattern in Java.
I need to implement this in C++ on Windows, and the application must use tcp/ip sockets using an existing defined protocol.
Any help at all would be greatly appreciated.
You need exactly two threads, one that saturates the IO channel to the database and another that saturates the IO channel to the network leading to the 12 servers. Unless you have multiple network interfaces (which you should think about!) you don't send things faster by using multiple threads. Also, since you don't have multiple threads taking care of the network, you don't have to sync them.
What you definitely need to know about is select(). In the case of WinSock, also take a look at WSAEventSelect/WaitForMultipleObjects. Basically, you take a message from the queue and then send it to all clients when they're ready. select() tells you when one of a set of sockets is ready to accept data, so you don't waste time waiting or block trying to send data. What you need to come up with is a schema to reconnect after broken connections, when to drop messages to lagging clients etc. Also, in case the throughput to the different targets varies a lot, you need to think about handling multiple messages in parallel. If they are small (less than a network packet's payload) it makes sense combining them anyway to avoid overhead.
I hope this short overview helps getting you started, otherwise I can elaborate on the details.
I'm working on a little client that interfaces with a game server. The server sends messages to the connected client over HTTP. Its relatively easy to parse the text messages coming into the client and form responses to send back.
Now what I'm trying to figure out is how to break up the process. I want to have a thread receiving the messages, parsing them into some data object, and placing them into an "incoming" queue to be processed. Then another thread reads messages from this queue and processes them (the brains or AI of the client) and makes responses back to the server.
I want to have the thread that watches the incoming data to do process the text (break up the messages, pull the important data out, etc.) so the AI thread doesn't have that overhead. But the problem is that the server can send a couple hundred different types of messages to the client (what the client can see, other players, if you are firing etc). I want to package this data into a neat little structure so the AI can handle it quickly, and the AI can be rewritten easily.
But how do I write a function that can pull something off a queue and know what type of message it is (so I know what data is contained within the message)?
Example messages:
ALIVE (tells you if you are alive)
It has only one data object, the current game time
DAM (tells if you are damaged)
Has a whole bunch of data, who damaged you, how much, what gun it is, if you can see them, etc.
It is possible to make an object that can handle all of these different message types and be interpreted by a single function? Very few messages have common attributes, so I don't think inheriting or just making one really big message class would be very good...
I'm not looking for a full solution here, just point me in the right direction and hopefully I'll be able to learn a bit on the way :-)
Basically what you're asking about is called a protocol: how data is exchanged and interpreted. Traditionally you'd define your own (and odds are they'd tend to start out rather naive -- sending plain text data with newlines to indicate the end of a command, or something like that). After a while you begin to realize that more is needed (how do you handle binary data? how do you handle errors? etc, etc)
Fortunately there are libraries out there to make life easier for you. These days I tend to favor simple RPC-like libraries for most of my needs. Examples include protocol buffers (by Google), Apache Thrift (by Facebook) and Apache Avro.
I have two applications running on my machine. One is supposed to hand in the work and other is supposed to do the work. How can I make sure that the first application/process is in wait state. I can verify via the resources its consuming, but that does not guarantee so. What tools should I use?
Your 2 applications shoud communicate. There are a lot of ways to do that:
Send messages through sockets. This way the 2 processes can run on different machines if you use normal network sockets instead of local ones.
If you are using C you can use semaphores with semget/semop/semctl. There should be interfaces for that in other languages.
Named pipes block until there is both a read and a write operation in progress. You can use that for synchronisation.
Signals are also good for this. In C it is called sendmsg/recvmsg.
DBUS can also be used and has bindings for variuos languages.
Update: If you can't modify the processing application then it is harder. You have to rely on some signs that indicate the progress. (I am assuming you processing application reads a file, does some processing then writes the result to an output file.) Do you know the final size the result should be? If so you need to check the size repeatedly (or whenever it changes).
If you don't know the size but you know how the processing works you may be able to use that. For example the processing is done when the output file is closed. You can use strace to see all the system calls including the close. You can replace the close() function with the LD_PRELOAD environment variable (on windows you have to replace dlls). This way you can sort of modify the processing program without actually recompiling or even having access to its source.
you can use named pipes - the first app will read from it but it will be blank and hence it will keep waiting (blocked). The second app will write into it when it wants the first one to continue.
Nothing can guarantee that your application is in waiting state. You have to pass it some work and get back a response. It might be transactions or not - application can confirm that it got the message to process before it starts to process it or after it was processed (successfully or not). If it does not wait, passing a piece of work should fail. Whether when trying to write to a TCP/IP socket or other means, or if timeout occurs. This depends on implementation, what kind of transport you are using and other requirements.
There is actually a way of figuring out if the process (thread) is in blocking state and waiting for data on a socket (or other source), but that means that client should be on the same computer and have access privileges required to do that, but that makes no sense other than debugging, which you can do using any debugger anyway.
Overall, the idea of making sure that application is waiting for data before trying to pass it that data smells bad. Not to mention the racing condition - what if you checked and it was OK, and when you actually tried to send the data, you found out that application is not waiting at that time (even if that is microseconds).
I have a simple c++ application that generates reports on the back end of my web app (simple LAMP setup). The problem is the back end loads a data file that takes about 1.5GB in memory. This won't scale very well if multiple users are running it simultaneously, so my thought is to split into several programs :
Program A is the main executable that is always running on the server, and always has the data loaded, and can actually run reports.
Program B is spawned from php, and makes a simple request to program A to get the info it needs, and returns the data.
So my questions are these:
What is a good mechanism for B to ask A to do something?
How should it work when A has nothing to do? I don't really want to be polling for tasks or otherwise spinning my tires.
Use a named mutex/event, basically what this does is allows one thread (process A in your case) to sit there hanging out waiting. Then process B comes along, needing something done, and signals the mutex/event this wakes up process A, and you proceed.
If you are on Microsoft :
Mutex, Event
Ipc on linux works differently, but has the same capability:
Linux Stuff
Or alternatively, for the c++ portion you can use one of the boost IPC libraries, which are multi-platform. I'm not sure what PHP has available, but it will no doubt have something equivalent.
Use TCP sockets running on localhost.
Make the C++ application a daemon.
The PHP front-end creates a persistent connection to the daemon. pfsockopen
When a request is made, the PHP sends a request to the daemon which then processes and sends it all back. PHP Sockets C++ Sockets
EDIT
Added some links for reference. I might have some really bad C code that uses sockets of interprocess communication somewhere, but nothing handy.
IPC is easy on C++, just call the POSIX C API.
But what you're asking would be much better served by a queue manager. Make the background daemon wait for a message on the queue, and the frontend PHP just add there the specifications of the task it wants processed. Some queue managers allow the result of the task to be added to the same object, or you can define a new queue for the finish messages.
One of the best known high-performance queue manager is RabbitMQ. Another one very easy to use is MemcacheQ.
Or, you could just add a table to MySQL for tasks, the background process just queries periodically for unfinished ones. This works and can be very reliable (sometimes called Ghetto queues), but break down at high tasks/second.
I'm programming an online game for two reasons, one to familiarize myself with server/client requests in a realtime environment (as opposed to something like a typical web browser, which is not realtime) and to actually get my hands wet in that area, so I can proceed to actually properly design one.
Anywho, I'm doing this in C++, and I've been using winsock to handle my basic, basic network tests. I obviously want to use a framelimiter and have 3D going and all of that at some point, and my main issue is that when I do a send() or receive(), the program kindly idles there and waits for a response. That would lead to maybe 8 fps on even the best internet connection.
So the obvious solution to me is to take the networking code out of the main process and start it up in its own thread. Ideally, I would call a "send" in my main process which would pass the networking thread a pointer to the message, and then periodically (every frame) check to see if the networking thread had received the reply, or timed out, or what have you. In a perfect world, I would actually have 2 or more networking threads running simultaneously, so that I could say run a chat window and do a background download of a piece of armor and still allow the player to run around all at once.
The bulk of my problem is that this is a new thing to me. I understand the concept of threading, but I can see some serious issues, like what happens if two threads try to read/write the same memory address at the same time, etc. I know that there are already methods in place to handle this sort of thing, so I'm looking for suggestions on the best way to implement something like this. Basically, I need thread A to be able to start a process in thread B by sending a chunk of data, poll thread B's status, and then receive the reply, also as a chunk of data., ideally without any major crashing going on. ^_^ I'll worry about what that data actually contains and how to handle dropped packets, etc later, I just need to get that happening first.
Thanks for any help/advice.
PS: Just thought about this, may make the question simpler. Is there a way to use the windows event handling system to my advantage? Like, would it be possible to have thread A initialize data somewhere, then trigger an event in thread B to have it pick up the data, and vice versa for thread B to tell thread A it was done? That would probably solve a lot of my problems, since I don't really need both threads to be able to work on the data at the same time, more of a baton pass really. I just don't know if this is possible between two different threads. (I know one thread can create its own messages for the event handler.)
The easiest thing
for you to do, would be to simply invoke the windows API QueueUserWorkItem. All you have to specify is the function that the thread will execute and the input passed to it. A thread pool will be automatically created for you and the jobs executed in it. New threads will be created as and when is required.
http://msdn.microsoft.com/en-us/library/ms684957(VS.85).aspx
More Control
You could have a more detailed control using another set of API's which can again manage the thread pool for you -
http://msdn.microsoft.com/en-us/library/ms686980(VS.85).aspx
Do it yourself
If you want to control all aspects of your thread creation and the pool management you would have to create the threads yourself, decide how they should end , how many to create etc (beginthreadex is the api you should be using to create threads. If you use MFC you should use AfxBeginThread function).
Send jobs to worker threads - Io completion Ports
In this case, you would also have to worry about how to communicate your jobs - i would recommend IoCOmpletionPorts to do that. It is the most scalable notification mechanism that i currently know of made for this purpose. It has the additional advantage that it is implemented in the kernel so you avoid all kinds of dead loack sitautions you would encounter if you decide to handroll something yourself.
This article will show you how with code samples -
http://blogs.msdn.com/larryosterman/archive/2004/03/29/101329.aspx
Communicate Back - Windows Messages
You could use windows messages to communicate the status back to your parent thread since it is doing the message wait anyway. use the PostMessage function to do this. (and check for errors)
ps : You could also allocate the data that needs to be sent out on a dedicated pointer and then the worker thread could take care of deleting it after sending it out. That way you avoid the return pointer traffic too.
BlodBath's suggestion of non-blocking sockets is potentially the right approach.
If you're trying to avoid using a multithreaded approach, then you could investigate the use of setting up overlapped I/O on your sockets. They will not block when you do a transmit or receive, but have the added bonus of giving you the option of waiting for multiple events within your single event loop. When your transmit has finished, you will receive an event. (see this for some details)
This is not incompatible with a multithreaded approach, so there's the option of changing your mind later. ;-)
On the design of your multithreaded app. the best thing to do is to work out all of the external activities that you want to be alerted to. For example, so far in your question you've listed network transmits, network receives, and user activity.
Depending on the number of concurrent connections you're going to be dealing with you'll probably find it conceptually simpler to have a thread per socket (assuming small numbers of sockets), where each thread is responsible for all of the processing for that socket.
Then you can implement some form of messaging system between your threads as RC suggested.
Arrange your system so that when a message is sent to a particular thread and event is also sent. Your threads can then be sent to sleep waiting for one of those events. (as well as any other stimulus - like socket events, user events etc.)
You're quite right that you need to be careful of situations where more than one thread is trying to access the same piece of memory. Mutexes and semaphores are the things to use there.
Also be aware of the limitations that your gui has when it comes to multithreading.
Some discussion on the subject can be found in this question.
But the abbreviated version is that most (and Windows is one of these) GUIs don't allow multiple threads to perform GUI operations simultaneously. To get around this problem you can make use of the message pump in your application, by sending custom messages to your gui thread to get it to perform gui operations.
I suggest looking into non-blocking sockets for the quick fix. Using non-blocking sockets send() and recv() do not block, and using the select() function you can get any waiting data every frame.
See it as a producer-consumer problem: when receiving, your network communication thread is the producer whereas the UI thread is the consumer. When sending, it's just the opposite. Implement a simple buffer class which gives you methods like push and pop (pop should be blocking for the network thread and non-blocking for the UI thread).
Rather than using the Windows event system, I would prefer something that is more portable, for example Boost condition variables.
I don't code games, but I've used a system similar to what pukku suggested. It lends nicely to doing things like having the buffer prioritize your messages to be processed if you have such a need.
I think of them as mailboxes per thread. You want to send a packet? Have the ProcessThread create a "thread message" with the payload to go on the wire and "send" it to the NetworkThread (i.e. push it on the NetworkThread's queue/mailbox and signal the condition variable of the NetworkThread so he'll wake up and pull it off). When the NetworkThread receives the response, package it up in a thread message and send it back to the ProcessThread in the same manner. Difference is the ProcessThread won't be blocked on a condition variable, just polling on mailbox.empty( ) when you want to check for the response.
You may want to push and pop directly, but a more convenient way for larger projects is to implement a toThreadName, fromThreadName scheme in a ThreadMsg base class, and a Post Office that threads register their Mailbox with. The PostOffice then has a send(ThreadMsg*); function that gets/pushes the messages to the appropriate Mailbox based on the to and from. Mailbox (the buffer/queue class) contains the ThreadMsg* = receiveMessage(), basically popping it off the underlying queue.
Depending on your needs, you could have ThreadMsg contain a virtual function process(..) that could be overridden accordingly in derived classes, or just have an ordinary ThreadMessage class with a to, from members and a getPayload( ) function to get back the raw data and deal with it directly in the ProcessThread.
Hope this helps.
Some topics you might be interested in:
mutex: A mutex allows you to lock access to specific resources for one thread only
semaphore: A way to determine how many users a certain resource still has (=how many threads are accessing it) and a way for threads to access a resource. A mutex is a special case of a semaphore.
critical section: a mutex-protected piece of code (street with only one lane) that can only be travelled by one thread at a time.
message queue: a way of distributing messages in a centralized queue
inter-process communication (IPC) - a way of threads and processes to communicate with each other through named pipes, shared memory and many other ways (it's more of a concept than a special technique)
All topics in bold print can be easily looked up on a search engine.