Re: How many copies to get from NIC RX to user read()?

[niv@us.ibm.com]

> I could've sworn I heard the stack was single-copy 
> on both the TX and RX sides. But, it doesn't look to 
> me like it is. Rather, it looks like there is one copy 
> in tcp_rcv_estabilshed() (via tcp_copy_to_iovec()), and a
> second copy in tcp_recvmsg() (which is called when the 
> user calls read()). Both of these copies are, I believe, 
> done by skb_copy_datagram_iovec().

tcp_recvmsg() only does the copy from the receive_queue
or the backlog queue. tcp_rcv_established() does the copy
directly into the iovec or queues it onto the receive_queue 
or backlog queue for tcp_recvmsg() to complete the work. So 
there arent two copies of the same data happening, just a 
question of one or the other function doing the work depending 
on whether there is currently a process doing a read or not..

hth,

thanks,
Nivedita