I had something like
while(j<nOSlaves)
{
//Iterate through all the slaves.
for(int i=1;i<nOSlaves && j<nOSlaves;i++)
{
//Create a taskMessage which contains length and distance.
MPI_Status st;
MPI_Recv(&buffer, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, &st);
if (buffer > 0)
{ //Handle the message.... }
}
}
The problem now is that I have to wait everyone until the message arrives, I wanted it faster and tried it async.
MPI_Irecv(&buffer, 1, MPI_INT, i, 0, MPI_COMM_WORLD, &rq);
int flag = 0;
MPI_Test(&rq, &flag, &st);
//If the asynchronous message has been received advance, else try again later.
if (flag)
{ //Handle the message.... }
But after each iteration of the for I will lose the request.
Is there a way to iterate throught all the "slaves" and look if some already answered?
With that error you have some block. Your sends and receives are not happening at the same time.
So MPI_Irecv is the non blocking receive function while MPI_Recv is a blocking function. Since you do not include your sending functions it is difficult to tell what is causing the blocking here (but given that error message this is likely the case). I suggest looking at Hristo Ilieve's (he seems to be active here) tutorials. One of the hardest things in MPI is the blocking. A way you could ensure that everything is caught up is to make use of MPI_Barrier though I mostly use that for debugging. If you are concerned with speed then passing a single integer is not a great idea, unless you are using it for indexing. You can also use MPI_Scatterv if you want to have uneven chunks sent out. If you are sending the same section of buffer, which it looks like you are, you might try MPI_Bcast.
I find that it helps to write down some parts of the code to make sure that they are not blocking each other.
Related
I am currently working on a project written in C++ involving UDP real time connection. I receive UDP packets from a control computer containing commands to start/stop an infinite while loop that reads data from an IMU and sends that data to the control computer.
My problem is the following: First I implemented an exit condition from the loop using recvfrom() and read(), but the control computer sends a UDP packet every second, which was delaying the whole loop and made sending the data in the desired time interval of 5ms impossible.
I tried to fix this problem by usingfcntl(fd, F_SETFL, O_NONBLOCK);and using only read(), which actually works fine, but I am unsure whether this is a wise idea or not, since I am not checking for errors anymore. Is there any elegant way how to solve this problem? I thought about using Pthreads or something like that, however I have never worked with threads or parallel programming so I would have to spend some time learning that.
I appreciate any advice on that problem you could give me.
Here is a code example:
//include
...
int main() {
RNet cmd; //RNet: struct that contains all the information of the UDP header and the command
RNet* pCmd = &cmd;
ssize_t b;
int fd2;
struct sockaddr_in snd; // sender is control computer
socklen_t length;
// further declaration of variables, connecting to socket, etc...
...
fcntl(fd2, F_SETFL, O_NONBLOCK);
while (1)
{
// read messages from control computer
if ((b = read(fd2, pCmd, 19)) > 0) {
memcpy(&cmd, pCmd, b);
}
// transmission
while (cmd.CLout.MotionCommand == 1) // MotionCommand: 1 - send messages; 0 - do nothing
{
if(time_elapsed >= 5) // elapsed time in ms
{
// update sensor values
...
//sendto ()
...
// update control time, timestamp, etc.
...
}
if (recvfrom(fd2, pCmd, (int)sizeof(pCmd), 0, (struct sockaddr*) &snd, &length) < 0) {
perror("error receiving data");
return 0;
}
// checking Control Model Command
if ((b = read(fd2, pCmd, 19)) > 0) {
memcpy(&cmd, pCmd, b);
}
}
}
}
I really like the "blocking calls on multiple threads" design. It enables you to have distinct independent tasks, and you don't have to worry about how each task can disturb another. It can have some drawbacks but it is usually a good fit for many needs.
To do that, just use pthread_create to create a new thread for each task (you may keep the main thread for one task). In your case, you should have a thread to receive commands, and another one to send your data. You also need for the receiving thread to notify the sending thread of the commands. To do that, you can use some synchronization tool, like a mutex.
Overall, you should have your receiving thread blocking on recvfrom, and the sending thread waiting for a signal from the mutex (wait for the mutex to be freed, technically). When the receiving thread receive a start command, it signals the mutex and go back to recvfrom (optionally you can set a variable to provide more information to the other thread).
As a comment, remember that UDP are 1-to-many, thus your code here will react to any packet sent to you (even from some random or malicious host). You may want to filter with the remote sockaddr after recvfrom, or use connect + recv. It depends on what you want.
Hoping someone can help me out. I'm wanting to implement some sort of timeout if my socket is not able to receive data in a certain amount of time... I've looked up ways online but the examples I've looked at doesn't have their recv() in a while loop like mine, they typically just receive the whole buffer that is waiting. Maybe mine just isn't very efficient and someone could point me in a better direction in receiving all the data.
The string that is to be received is not a fixed length which is why I receive 1 at a time because I don't know how big the string might be. As you can see my recv() will receive data until it finds the End of text character (). The examples with select() I found would use the select before calling receive, but should I be doing that for each go around of my while loop? or maybe call select() before I even enter the while loop?
Anyways, any help is appreciated.
string recv_data(int socket){
bool endfound = false;
char temp[1];
string recvstring ;
while(endfound == false)//receives 1 character at a time until ETX(\x03)character is found
{
if(recv(socket,temp,sizeof(temp),0)<0)
{
perror("error in recv data");
}
if(memchr(temp,'\x03',1) != NULL)
{
endfound = true;
}
recvstring += temp;
temp[0] = 0;
}
return formatting(recvstring);
}
You must put your socket into non-blocking mode, and use poll().
poll() technically works on regular blocking sockets too; however it's been my experience that there are various subtle differences in semantics and race conditions, when using poll() with blocking sockets, and for best portability I always used non-blocking mode sockets, together with poll(), and careful inspection of the returning value from recv() or read().
Some Google food for you: fcntl(), F_SETFL, O_NONBLOCK.
I read that it should be safe from different threads concurrently, but my program has some weird behaviour and I don't know what's wrong.
I have concurrent threads communicating with a client socket
one doing send to a socket
one doing select and then recv from the same socket
As I'm still sending, the client has already received the data and closed the socket.
At the same time, I'm doing a select and recv on that socket, which returns 0 (since it is closed) so I close this socket. However, the send has not returned yet...and since I call close on this socket the send call fails with EBADF.
I know the client has received the data correctly since I output it after I close the socket and it is right. However, on my end, my send call is still returning an error (EBADF), so I want to fix it so it doesn't fail.
This doesn't always happen. It happens maybe 40% of the time. I don't use sleep anywhere. Am I supposed to have pauses between sends or recvs or anything?
Here's some code:
Sending:
while(true)
{
// keep sending until send returns 0
n = send(_sfd, bytesPtr, sentSize, 0);
if (n == 0)
{
break;
}
else if(n<0)
{
cerr << "ERROR: send returned an error "<<errno<< endl; // this case is triggered
return n;
}
sentSize -= n;
bytesPtr += n;
}
Receiving:
while(true)
{
memset(bufferPointer,0,sizeLeft);
n = recv(_sfd,bufferPointer,sizeLeft, 0);
if (debug) cerr << "Receiving..."<<sizeLeft<<endl;
if(n == 0)
{
cerr << "Connection closed"<<endl; // this case is triggered
return n;
}
else if (n < 0)
{
cerr << "ERROR reading from socket"<<endl;
return n;
}
bufferPointer += n;
sizeLeft -= n;
if(sizeLeft <= 0) break;
}
On the client, I use the same receive code, then I call close() on the socket.
Then on my side, I get 0 from the receive call and also call close() on the socket
Then my send fails. It still hasn't finished?! But my client already got the data!
I must admit I'm surprised you see this problem as often as you do, but it's always a possibility when you're dealing with threads. When you call send() you'll end up going into the kernel to append the data to the socket buffer in there, and it's therefore quite likely that there'll be a context switch, maybe to another process in the system. Meanwhile the kernel has probably buffered and transmitted the packet quite quickly. I'm guessing you're testing on a local network, so the other end receives the data and closes the connection and sends the appropriate FIN back to your end very quickly. This could all happen while the sending machine is still running other threads or processes because the latency on a local ethernet network is so low.
Now the FIN arrives - your receive thread hasn't done a lot lately since it's been waiting for input. Many scheduling systems will therefore raise its priority quite a bit and there's a good chance it'll be run next (you don't specify which OS you're using but this is likely to happen on at least Linux, for example). This thread closes the socket due to its zero read. At some point shortly after this the sending thread will be re-awoken, but presumably the kernel notices that the socket is closed before it returns from the blocked send() and returns EBADF.
Now this is just speculation as to the exact cause - among other things it heavily depends on your platform. But you can see how this could happen.
The easiest solution is probably to use poll() in the sending thread as well, but wait for the socket to become write-ready instead of read-ready. Obviously you also need to wait until there's any buffered data to send - how you do that depends on which thread buffers the data. The poll() call will let you detect when the connection has been closed by flagging it with POLLHUP, which you can detect before you try your send().
As a general rule you shouldn't close a socket until you're certain that the send buffer has been fully flushed - you can only be sure of this once the send() call has returned and indicates that all the remaining data has gone out. I've handled this in the past by checking the send buffer when I get a zero read and if it's not empty I set a "closing" flag. In your case the sending thread would then use this as a hint to do the close once everything is flushed. This matters because if the remote end does a half-close with shutdown() then you'll get a zero read even if it might still be reading. You might not care about half closes, however, in which case your strategy above is OK.
Finally, I personally would avoid the hassle of sending and receiving threads and just have a single thread which does both - that's more or less the point of select() and poll(), to allow a single thread of execution to deal with one or more filehandles without worrying about performing an operation which blocks and starves the other connections.
Found the problem. It's with my loop. Notice that it's an infinite loop. When I don't have anymore left to send, my sentSize is 0, but I'll still loop to try to send more. At this time, the other thread has already closed this thread and so my send call for 0 bytes returns with an error.
I fixed it by changing the loop to stop looping when sentSize is 0 and it fixed the problem!
I have a program that maintains a list of "streaming" sockets. These sockets are configured to be non-blocking sockets.
Currently, I have used a list to store these streaming sockets. I have some data that I need to send to all these streaming sockets hence I used the iterator to loop through this list of streaming sockets and calling the send_TCP_NB function below:
The issue is that my own program buffer that stores the data before sending to this send_TCP_NB function slowly decreases in free size indicating that the send is slower than the rate at which data is put into the program buffer. The rate at which the program buffer is about 1000 data per second. Each data is quite small, about 100 bytes.
Hence, i am not sure if my send_TCP_NB function is working efficiently or correct?
int send_TCP_NB(int cs, char data[], int data_length) {
bool sent = false;
FD_ZERO(&write_flags); // initialize the writer socket set
FD_SET(cs, &write_flags); // set the write notification for the socket based on the current state of the buffer
int status;
int err;
struct timeval waitd; // set the time limit for waiting
waitd.tv_sec = 0;
waitd.tv_usec = 1000;
err = select(cs+1, NULL, &write_flags, NULL, &waitd);
if(err==0)
{
// time limit expired
printf("Time limit expired!\n");
return 0; // send failed
}
else
{
while(!sent)
{
if(FD_ISSET(cs, &write_flags))
{
FD_CLR(cs, &write_flags);
status = send(cs, data, data_length, 0);
sent = true;
}
}
int nError = WSAGetLastError();
if(nError != WSAEWOULDBLOCK && nError != 0)
{
printf("Error sending non blocking data\n");
return 0;
}
else
{
if(nError == WSAEWOULDBLOCK)
{
printf("%d\n", nError);
}
return 1;
}
}
}
One thing that would help is if you thought out exactly what this function is supposed to do. What it actually does is probably not what you wanted, and has some bad features.
The major features of what it does that I've noticed are:
Modify some global state
Wait (up to 1 millisecond) for the write buffer to have some empty space
Abort if the buffer is still full
Send 1 or more bytes on the socket (ignoring how much was sent)
If there was an error (including the send decided it would have blocked despite the earlier check), obtain its value. Otherwise, obtain a random error value
Possibly print something to screen, depending on the value obtained
Return 0 or 1, depending on the error value.
Comments on these points:
Why is write_flags global?
Did you really intend to block in this function?
This is probably fine
Surely you care how much of the data was sent?
I do not see anything in the documentation that suggests that this will be zero if send succeeds
If you cleared up what the actual intent of this function was, it would probably be much easier to ensure that this function actually fulfills that intent.
That said
I have some data that I need to send to all these streaming sockets
What precisely is your need?
If your need is that the data must be sent before proceeding, then using a non-blocking write is inappropriate*, since you're going to have to wait until you can write the data anyways.
If your need is that the data must be sent sometime in the future, then your solution is missing a very critical piece: you need to create a buffer for each socket which holds the data that needs to be sent, and then you periodically need to invoke a function that checks the sockets to try writing whatever it can. If you spawn a new thread for this latter purpose, this is the sort of thing select is very useful for, since you can make that new thread block until it is able to write something. However, if you don't spawn a new thread and just periodically invoke a function from the main thread to check, then you don't need to bother. (just write what you can to everything, even if it's zero bytes)
*: At least, it is a very premature optimization. There are some edge cases where you could get slightly more performance by using the non-blocking writes intelligently, but if you don't understand what those edge cases are and how the non-blocking writes would help, then guessing at it is unlikely to get good results.
EDIT: as another answer implied, this is something the operating system is good at anyways. Rather than try to write your own code to manage this, if you find your socket buffers filling up, then make the system buffers larger. And if they're still filling up, you should really give serious thought to the idea that your program needs to block anyways, so that it stops sending data faster than the other end can handle it. i.e. just use ordinary blocking sends for all of your data.
Some general advice:
Keep in mind you are multiplying data. So if you get 1 MB/s in, you output N MB/s with N clients. Are you sure your network card can take it ? It gets worse with smaller packets, you get more general overhead. You may want to consider broadcasting.
You are using non blocking sockets, but you block while they are not free. If you want to be non blocking, better discard the packet immediately if the socket is not ready.
What would be better is to "select" more than one socket at once. Do everything that you are doing but for all the sockets that are available. You'll write to each "ready" socket, then repeat again while there are sockets that are not ready. This way, you'll proceed with the sockets that are available first, and then with some chance, the busy sockets will become themselves available.
the while (!sent) loop is useless and probably buggy. Since you are checking only one socket FD_ISSET will always be true. It is wrong to check again FD_ISSET after a FD_CLR
Keep in mind that your OS has some internal buffers for the sockets and that there are way to extend them (not easy on Linux, though, to get large values you need to do some config as root).
There are some socket libraries that will probably work better than what you can implement in a reasonable time (boost::asio and zmq for the ones I know).
If you need to implement it yourself, (i.e. because for instance zmq has its own packet format), consider using a threadpool library.
EDIT:
Sleeping 1 millisecond is probably a bad idea. Your thread will probably get descheduled and it will take much more than that before you get some CPU time again.
This is just a horrible way to do things. The select serves no purpose but to waste time. If the send is non-blocking, it can mangle data on a partial send. If it's blocking, you still waste arbitrarily much time waiting for one receiver.
You need to pick a sensible I/O strategy. Here is one: Set all sockets non-blocking. When you need to send data to a socket, just call write. If all the data writes, lovely. If not, save the portion of data that wasn't sent for later and add the socket to your write set. When you have nothing else to do, call select. If you get a hit on any socket in your write set, write as many bytes as you can from what you saved. If you write all of them, remove that socket from the write set.
(If you need to write to a data that's already in your write set, just add the data to the saved data to be sent. You may need to close the connection if too much data gets buffered.)
A better idea might be to use a library that already does all these things. Boost::asio is a good one.
You are calling select() before calling send(). Do it the other way around. Call select() only if send() reports WSAEWOULDBLOCK, eg:
int send_TCP_NB(int cs, char data[], int data_length)
{
int status;
int err;
struct timeval waitd;
char *data_ptr = data;
while (data_length > 0)
{
status = send(cs, data_ptr, data_length, 0);
if (status > 0)
{
data_ptr += status;
data_length -= status;
continue;
}
err = WSAGetLastError();
if (err != WSAEWOULDBLOCK)
{
printf("Error sending non blocking data\n");
return 0; // send failed
}
FD_ZERO(&write_flags);
FD_SET(cs, &write_flags); // set the write notification for the socket based on the current state of the buffer
waitd.tv_sec = 0;
waitd.tv_usec = 1000;
status = select(cs+1, NULL, &write_flags, NULL, &waitd);
if (status > 0)
continue;
if (status == 0)
printf("Time limit expired!\n");
else
printf("Error waiting for time limit!\n");
return 0; // send failed
}
return 1;
}
I am learning how to network program using c/c++ and I have created a server(TCP) that is suppose to respond in specific ways to messages from a client in order to do this I created a class that the server class passes the message to and returns a string to report back to the client.
Here is my problem sometimes it reports the correct string back other times if just repeats what I sent to the message handler. Which no where in the code do I have it return what was passed in. So I am wondering am I handling getting the message correctly?
Secondly, I am unsure of how to keep a connection open in a while loop to continually pass messages back and forth. You can see how I did it in the code below but I am pretty sure this is incorrect, any help on this would be great. Thanks!
if (!fork())
{ // this is the child process
close(sockfd); // child doesn't need the listener
while ((numbytes = recv(new_fd, buf, MAXDATASIZE-1, 0)) > 0)
{
//numbytes = recv(new_fd, buf, MAXDATASIZE-1, 0);
buf[numbytes-1] = '\0';
const char* temp = ash.handleMessage(buf).c_str();
int size_of_temp = ash.handleMessage(buf).length();
send(new_fd, temp, size_of_temp, 0);
//send(new_fd, temp, size_of_temp+1, 0);
}
}//end if
Please excuse my ghetto code
Handles the message
Class Method handler uses
If your learning about sockets you should also know that you can't assume that what you send() as a "complete message", will be delivered as a complete message.
If you send() some big data from your client you might need to use multiple recv() on the server (or vice versa) to read it all. This is a big difference of how files usually work...
If you'r designing your own protocol you can opt to also send the messages length, like [LEN][message]. An easy example would be if the strings you send are limited to 256 bytes you can start with send()ing a short representing the strings length,
Or easier, decide that you use line-feeds (newline - \n) to terminate messages. The the protocol would look like
"msg1\nmsg2\n"
then you would have to recv(), and append the data, until you get a newline. This is all I can muster right now, there are a lot of great examples on the internet, but I would recommend getting the source of some "real" program and look at how it handles its network.
You are calling handleMessage twice. You didn't post the code, but it looks like you're returning a string. It might be better to do:
string temp = ash.handleMessage(buf);
int size_of_temp = temp.length();
This would avoid repeating any action that takes place in handleMessage.