netdev - Re: seeing strange values for tcp sk_rmem

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Message-ID: <4B155252.1040604@nortel.com>
Date:	Tue, 01 Dec 2009 11:28:50 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Eric Dumazet <dada1@...mosbay.com>
CC:	netdev@...r.kernel.org, Linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: seeing strange values for tcp sk_rmem_alloc

On 12/01/2009 10:58 AM, Eric Dumazet wrote:

> Me wondering why you think sk_rmem_alloc is about TX side.
> Its used in RX path. rmem means ReadMemory.

Yep, I realize this.

> You can send 1 Gbytes of data, and sk_rmem_alloc doesnt change, if your
> TCP stream is unidirectionnal.
> 
> sk_rmem_alloc grows when skb are queued into receive queue
> sk_rmem_alloc shrinks when application reads this receive queue.

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.

Chris
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