UDP Multiple sockets Receive data and process efficiently - C & Linux

Question

I have to receive data from 15 different clients each of them sending on 5 different ports. totally 15 *5 sockets.

for each client port no is defined and fixed. example client 1 ,ports 3001 to 3005. client 2 ,ports 3051 to 3055 etc. They have one thing in common say first port (3001 , 3051) is used to send commands. other ports send some data.

After receiving the data i have to check for checksum. keep track of recvd packets, Re request the packet if lost and also have to write to files on hard disk.

Restriction I cannot change the above design and i cannot change from UDP to TCP. The two methods i'm aware of after reading are

• asynchronous multiplexing using select().
• Thread per socket.

I tried the first one and i'm stuck at the point when i get the data. I'm able to receive data. I have some processing to do so i want to start a thread for each socket (or) for sockets to handle (say all first ports, all second, etc ..i.e.3001,3051 etc) But here if client sends any data then FD_ISSET becomes true , so if i start a thread ,then it becomes thread for every message. Question: How to add thread code here, Say if i include pthread_create inside if(FD_ISSET .. ) then for every message that i receive i create a thread. But i wanted a thread per socket.

  while(1)
   {
      int nready=0;
      read_set = active_set;

      if((nready = select(fdmax+1,&read_set,NULL,NULL,NULL)) == -1)
      {
        printf("Select Errpr\n");
        perror("select");
        exit(EXIT_FAILURE);
      }
      printf("number of ready desc=%d\n",nready);

      for(index=1;index <= 15*5;index++)
      {
         if(FD_ISSET(sock_fd[index],&read_fd_set))
         {              
           rc = recvfrom(sock_fd[index],clientmsgInfo,MSG_SIZE,0,
                    (struct sockaddr *)&client_sockaddr_in,
                      &sockaddr_in_length);
           if(rc < 0)
               printf("socket %d down\n",sock_fd[index]);

           printf("Recieved packet from %s: %d\nData: %s\n\n", inet_ntoa(client_sockaddr_in.sin_addr), ntohs(client_sockaddr_in.sin_port), recv_client_message);                                      
                }
         } //for
     } //while

Answer 1

create the threads at the startup of program and divide them to handle data, commmands e.t.c.

how?

1. lets say you created 2 threads, one for data and another for the commands.
2. make them sleep in the thread handler or let them wait on a lock that the main thread
   acquired, seems to be that mainthread got two locks one for each of them.
3. when any client data or command that got into the recvfrom at mainthread, depending on the 
   type of the buffer(data, commands), copy the buffer into the shared data by mainthread and 
   other threads and unlock the mutex. 
4. at threads lock the mutex so that mainthread wont' corrupt the data and once processing is 
   done at the threads unlock and sleep.

The better one would be to have a queue, that fills up by main thread and can be accessed element wise by the other threads.

Answer 2

I assume that each client context is independent of the others, ie. one client socket group can be managed on its own, and the data pulled from the sockets can be processed alone.

You express two possibilities of handling the problem:

1. Asynchronous multiplexing: in this setting, the sockets are all managed by one single thread. This threads selects which socket must be read next, and pulls data out of it

2. Thread per socket: in this scenario, you have as many threads as there are sockets, or more probably group of sockets, ie. clients - this the interpretation I will build from.

In both cases, threads must keep ownership of their respective resources, meaning sockets. If you start moving sockets around between threads, you will make things more difficult that it needs to be.

Outside the work that needs to be done, you will need to handle thread management:

• How do threads get started?
• How and when are they stopped?
• What are the error handling policies?

Your question doesn't cover these issues, but they might play a significant role in your final design.

Scenario (2) seems simpler: you have one main "template" (I use the word in a general meaning here) for handling a group of sockets using select on them, and in the same thread receive and process the data. It's quite straightforward to implement, with a struct to contain the context specific data (socket ports, pointer to function for packet processing), and a single function looping on select and process, plus perhaps some other checks for errors and thread life management.

Scenario (1) requires a different setup: one I/O thread reads all the packets and pass them on to specialized worker threads to do the processing. If processing error occurs, worker threads will have to generate the adhoc packet to be sent to the client, and pass it to the I/O thread for sending. You will need packet queues both ways to allow communication between I/O and workers, and have the I/O thread check the worker queues somehow for resend requests. So this solution is a bit more expensive in terms of developement, but reduce the I/O contention to one single point. It's also more flexible, in case some processing must be done against data coming from several clients, or if you want to chain up processing somehow. For instance, you could have instead one thread per client socket, and then one other thread per client group of socket further down the work pipeline, with each step of the pipeline interconnected by packet queue.

A blend of both solution can of course be implemented, with one IO thread per client, and pipelined worker threads.

The advantage of both outlined solutions is the fixed number of threads: no need to spawn and destroy threads on demand (although you could design a thread pool to handle that as well).

For a solution involving moving sockets between threads, the questions are:

• When should these resources be passed on? What happens after a worker thread has read a packet? Should it return the socket to the IO thread, or risk a blocking read on the socket for the next packet? If it does a select to poll the socket for more packets, we fall in scenario (2), where each client will has its own I/O thread when there is network trafic from all of them, in which case what is the gain of the initial I/O thread doing the select?

• If it passes the socket back, should the IO thread wait for all workers to give back their socket before initiating another select? If it waits, it takes the risk of making unserved client wait for packets already in the network buffers, inducing processing lag. If it does not wait, and return to select to avoid lag on unserved sockets, then the served ones will have to wait for the next wake up to see their sockets back in the select pool.

As you can see, the problem is difficult to handle. That's the reason why I recommend exclusive sockets ownership by threads as described in scenarii (1) and (2).

Answer 3

Your solution requires a fixed, relatively small, number of connections.

Create a help procedure that creates thread procedures that listen to each of the five ports and block on the recvfrom(), process the data, and block again. You can then call the helper 15 times to create the threads.

This avoids all polling, and allows Linux to schedule each thread when the I/O completes. No CPU used while waiting, and this can scale to somewhat larger solutions.

If you need to scale massively, why not use a single set of ports, and get the partner address from the client_sockaddr_in structure. If the processing takes a material amount of time, you could extend it by keeping a pool of threads available and assign a new one each time a message is received and continue processing the message thereafter, and adding the thread back to the pool after the response is sent.