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    SubjectRe: 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 <> wrote:
    > > I have a node.js client ( requesting some data from http
    > > rest service from server (
    > > 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) :
    > >
    > >
    > > 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:
    > >
    > >
    > > Please help.
    > CC'ing netdev.

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

    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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