Messages in this thread | | | Subject | Re: TCP Delayed ACK in FIN/ACK | From | Eric Dumazet <> | Date | Sat, 04 Aug 2012 18:34:43 +0200 |
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On Sat, 2012-08-04 at 16:51 +0200, richard -rw- weinberger wrote: > On Sat, Aug 4, 2012 at 4:45 PM, Sławek Janecki <janecki@gmail.com> wrote: > > 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.
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