Re: TCP Delayed ACK in FIN/ACK

On Sat, 2012-08-04 at 16:51 +0200, richard -rw- weinberger wrote:

On Sat, Aug 4, 2012 at 4:45 PM, Sławek Janecki [off-list ref] wrote:

quoted

I have a node.js client (10.177.62.7) requesting some data from http
rest service from server (10.177.0.1).
Client is simply using nodejs http.request() method (agent=false).
Client is on Ubuntu 11.10 box.
Why client sends FIN ACK after 475ms? Why so slow? He should send FIN
ACK immediately.
I have many situations like this. About 1% of whole traffic is request
with delayed FIN ACK.
Cpu idle on the client is about 99%, so nothing is draining CPU.
How to debug this? What could it be? Is there any sysctl option I need to tune?
I think this behaviour is the Delayed ACK feature of RFC1122 TCP stack.

Link to tcpdump picture (done on a client machine) :
http://i48.tinypic.com/35cpogx.png

Can you tell why kernel delayed that FIN/ACK.
In tcpflow data there is exacly one ACK per packet comming from server.
Why kernel delayed client FIN/ACK.
It could avoid sending ACK every 'data' packet.
But it choose to delay FIN/ACK?
Is this possible? Is this a bug?

I've also posted question on stackexchange:
http://stackoverflow.com/questions/11711218/client-sends-delayed-fin-ack-500ms-to-server

Please help.

CC'ing netdev.

I see nothing wrong in this tcpdump. You should strace the application
instead.

FIN/ACK is sent when client closes its socket (or calls shutdown()), and
not in reply of FIN sent by the server.

Kernel has no additional delay. I suspect your client is slow processing
the server answer, then close() its socket _after_ data processing.

Its possible tcp_send_fin() has to loop while allocating one skb under
very high memory pressure, and it seems we have no counters for this
case. But if it _was_ ever happening, you would have lot of messages in
kernel log (dmesg) about alloc_skb_clone() failures.