netdev - Re: TCP Delayed ACK in FIN/ACK

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Date:	Sat, 4 Aug 2012 21:20:45 +0200
From:	Sławek Janecki <janecki@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	richard -rw- weinberger <richard.weinberger@...il.com>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: TCP Delayed ACK in FIN/ACK

I have no such info in dmsg.
I think I will file a issue to Node.js Dev Team.
Thanks for reply.

On Sat, Aug 4, 2012 at 6:34 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> 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@...il.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.
>
>
>



-- 

pozdrawiam
Sławomir Janecki
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