netdev - Re: Socket buffer sizes with autotuning

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Message-ID: <d1c2719f0805071154t20004e9drecd3d9b7b9b3470c@mail.gmail.com>
Date:	Wed, 7 May 2008 11:54:01 -0700
From:	"Jerry Chu" <hkchu@...gle.com>
To:	"David Miller" <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: Socket buffer sizes with autotuning

On Tue, May 6, 2008 at 9:28 PM, David Miller <davem@...emloft.net> wrote:
> From: "Jerry Chu" <hkchu@...gle.com>
>
> Date: Tue, 6 May 2008 20:57:46 -0700
>
>
> > It may be getting a copy (e.g., when GSO is on?) hence losing all
>  > its connection to the original tp and any chance to have the pkt
>  > properly accounted for as host_infligh by TCP. The skb may also be
>  > cloned more than once (e.g., due to tcpdump)...
>
>  It only gets a copy if the SKB is already cloned and that clone is
>  alive somewhere (f.e. stuck in the device, a rare occurance by
>  the time we retransmit).

This is one of many things about skb that I still don't completely understand.
Why in tcp_transmit_skb() we'll have to pskb_copy() if skb_cloned()?
Can't we clone a skb mulitple times? Is it due to some special optimization
from skb->fclone stuff... that imposes this restriction?

>
>  It is a clone %99.999999 of the time.

When I turned on GSO but not TSO, I believe it's a copy %99.999999 of
the time. (I must confess I don't understand GSO code yet. I temporarily
ran out of stream after studying bunch of other code.)

Jerry

>
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