On Nov 7, 2012, at 10:26 AM, abhinav narain <abhinavnarain10@xxxxxxxxx> wrote:
> I wanted to know why is MSG_PEEK used in the recv() call in mmap code and not recvfrom() with MSG_TRUNC flag.
> The reason i am asking is .. because I see my code takes a lot of CPU which is due to more looping, I suppose.
> The flag description for MSG_PEEK shows it doesn't discard the bytes even after reading from the queue.
> Can someone please explain.
> I would like to use recvfrom with MSG_TRUNC .. is that fine ?
The recv() is *not* reading a packet, it's reading an error code. There shouldn't even *be* any skbuffs to read from the socket in the mmapped code path - they should be in the memory-mapped buffer. That's why it's doing a recv() with MSG_PEEK.
The comment "A recv() will give us the actual error code." perhaps doesn't indicate that clearly enough, but that's what it's doing. A poll() that includes the descriptor for the socket should set the POLLERR flag if there's an error condition on the descriptor, such as a "this network interface has gone down" indication. You have to do a recv() from the socket to get the error code and clear the error indication so that a subsequent poll() that includes that descriptor won't set POLLERR for it.
If that code is being invoked a significant number of times, it means you have a problem - you're getting a lot of errors, not packets.
