linux-kernel - Re: seeing strange values for tcp sk_rmem

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Message-ID: <4B1557CC.4060503@gmail.com>
Date:	Tue, 01 Dec 2009 18:52:12 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Chris Friesen <cfriesen@...tel.com>
CC:	netdev@...r.kernel.org, Linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: seeing strange values for tcp sk_rmem_alloc

Chris Friesen a écrit :

> I realize this.  I sent the data from a socket to itself.  It could just
> as easily be done with two tcp sockets.  The important thing is that I
> control both the tx and rx sides, so I know how much data should be
> present in the rx queue at any point in time.
> 
> The part that surprised me was that I could send multiple chunks of data
> without sk_rmem_alloc changing on the socket to which the data was being
> sent.  Then it would jump up by a large amount (up to 20K) all at once.
> 
> I'm starting to suspect that the discrepency might have something to do
> with the skb_copy_datagram_iovec() call in tcp_data_queue(), and how
> skb_set_owner_r() is only called if "eaten" is <= 0.  This could be
> totally off-base though.
> 

If you dont read() your socket, then skb_copy_datagram_iovec() is not called

But be careful of sender tcp stack : It might be delayed a bit,
because it waits for receiver to open its window (slow start)

You probably need something like

while (1) {
	send(fd1, buffer, 2Kbytes);
	sleep(2); // let tcp stack flush its write buffers
	display_sk_rmem_alloc(fd2);
}

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