netdev - Re: [PATCH net-next-2.6] net: pskb_expand

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Message-ID: <20100912204552.GA2585@del.dom.local>
Date:	Sun, 12 Sep 2010 22:45:52 +0200
From:	Jarek Poplawski <jarkao2@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	eric.dumazet@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next-2.6] net: pskb_expand_head() optimization

On Sun, Sep 12, 2010 at 08:58:33AM -0700, David Miller wrote:
> From: Jarek Poplawski <jarkao2@...il.com>
> Date: Sun, 12 Sep 2010 12:45:34 +0200
> 
> > Then a few more questions:
> > 1) if doubly linked lists really require such pskb_copying, isn't it
> >    all too costly?
> 
> In the common case the data reference will be one, so we will not
> copy.

Even if so, one such a case on the fast path should hit performance,
so it would need special reviewing.

> 
> > 2) why skb_clone isn't enough instead of pskb_copy?
> 
> Can't share the metadata.

I'd really like to understand why the change in handling next/prev
should affect more than skb pointers wrt. current sharing.

> 
> > 3) since skb_clone has some cost too, why e.g. saving only the pointer
> >    to the tail of the list in skb_shared_info isn't enough?
> 
> Then we won't get the rest of the advantages of using list_head such
> as prefetching during traversals, automatic debugging facilities, et al.

Right, we need to sum pros and cons. So, what's the pros? ;-)

Jarek P.
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