select vs poll Post: 302115861

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Special Forums IP Networking select vs poll Post 302115861 by Perderabo on Saturday 28th of April 2007 07:25:47 PM

04-28-2007

Administrator Emeritus

You sure don't mention many details. Not even which OS you use. "Connections"... so you are awaiting connections from a listen(2)? How many fd's are involved? Lots of connections to lots of fd's is a different problem than lots of connections to a few fd's.

For lots of fd's, select/poll bogs down because massive structures to describe the fd's must be copied between user/kernel space. This is one of several problems addressed be the epoll facility which I have only seen in Linux. (But note that fd_set can be large with modern implementions. People need to stop assuming it is an int!)

Lots of connections to a few fd's can be addressed by not allowing select to get involved with every connection... instead drain the listen queue on the first select. This paper argues that when select(2) returns, accept(2) should be called in loop until it gets EWOULDBLOCK. The authors describe a considerable performance boost. You might want to try this... I thought it sounded very interesting.

Or are you reading and writing on established connections? Assuming your OS has a decent thread implementation, I would try one thread per socket in each direction that data flows.

You ask about asyncronous I/O. Traditionally this is a problem if more than one fd is involved since there is only one SIGPOLL signal to deliver to a process. Sure, you can get the signal and then do poll(2)/select(2) but this is only useful for very sparse data arriving on multiple fd's. And now with threads, perhaps it is not useful at all. Posix defines some new async i/o stuff, but your OS may not have and I have never used the new stuff.

Perderabo

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listen

listen(3XNET)					   X/Open Networking Services Library Functions 				     listen(3XNET)

NAME

       listen - listen for socket connections and limit the queue of incoming connections

SYNOPSIS

       cc [ flag ... ] file ... -lxnet [ library ... ]
       #include <sys/socket.h>

       int listen(int socket, int backlog);

DESCRIPTION

       The  listen() function marks a connection-mode socket, specified by the socket argument, as accepting connections, and limits the number of
       outstanding connections in the socket's listen queue to the value specified by the backlog argument.

       If listen() is called with a backlog argument value that is less than 0, the function sets the length of the socket's listen queue to 0.

       The implementation may include incomplete connections in the queue subject to the queue limit. The implementation  may  also  increase  the
       specified queue limit internally if it includes such incomplete connections in the queue subject to this limit.

       Implementations	may  limit the length of the socket's listen queue.  If backlog exceeds the implementation-dependent maximum queue length,
       the length of the socket's listen queue will be set to the maximum supported value.

       The socket in use may require the process to have appropriate privileges to use the listen() function.

RETURN VALUES

       Upon successful completions, listen() returns 0.  Otherwise, -1 is returned and errno is set to indicate the error.

ERRORS

       The listen() function will fail if:

       EBADF	       The socket argument is not a valid file descriptor.

       EDESTADDRREQ    The socket is not bound to a local address, and the protocol does not support listening on an unbound socket.

       EINVAL	       The socket is already connected.

       ENOTSOCK        The socket argument does not refer to a socket.

       EOPNOTSUPP      The socket protocol does not support listen().

       The listen() function may fail if:

       EACCES	  The calling process does not have the appropriate privileges.

       EINVAL	  The socket has been shut down.

       ENOBUFS	  Insufficient resources are available in the system to complete the call.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Standard			   |
       +-----------------------------+-----------------------------+
       |MT-Level		     |MT-Safe			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       accept(3XNET), connect(3XNET), socket(3XNET), attributes(5), standards(5)

SunOS 5.11							    10 Jun 2002 						     listen(3XNET)