Re: [RFC] tcp: use order-3 pages in tcp_sendmsg()

[Eric Dumazet <eric.dumazet@gmail.com>]

On Thu, 2012-09-20 at 13:06 -0700, Rick Jones wrote:

> 
> Yes, I was being too fast and loose with my wording, paying more 
> attention to the netperf tests than the rest of it.  While loopback may 
> be lossless, TCP retransmissions over loopback shouldn't be all *that* 
> surprising.

Sending perfect packets (large packets) should trigger no retransmits.

In your tests, you send one-byte packets, so obviously the receiver will
drop some of them, because sk_rcvbuf limit (or the backlog limit) is hit
very fast.

(This should be less frequent with TCP coalescing that was recently
introduced : We are able to coalesce about 1600 'one-byte packets' into
a single one.)

netperf -t TCP_STREAM over loopback should not drop packets or
retransmit them.

# netstat -s|grep TCPRcvCoalesce
    TCPRcvCoalesce: 0

# netperf -t TCP_RR -- -b 1024 -D -S 16K -o
local_transport_retrans,remote_transport_retrans MIGRATED TCP
REQUEST/RESPONSE TEST from 0.0.0.0 () port 0 AF_INET to localhost ()
port 0 AF_INET : nodelay : first burst 1024
Local Transport Retransmissions,Remote Transport Retransmissions
0,0

# netstat -s|grep TCPRcvCoalesce
    TCPRcvCoalesce: 2072191