netdev - Re: [PATCH 3/3] tcp: Slightly optimize tcp

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Date:	Thu, 10 Dec 2009 15:59:45 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
Cc:	David Miller <davem@...emloft.net>, Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH 3/3] tcp: Slightly optimize tcp_sendmsg

Hi Ilpo,

"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi> wrote on 12/10/2009 01:47:30
PM:

> Subject: Re: [PATCH 3/3] tcp: Slightly optimize tcp_sendmsg>
>
> > Slightly optimize tcp_sendmsg since NETIF_F_SG is used many
> > times iteratively in the loop. The only other modification is
> > to change:
> >          } else if (i == MAX_SKB_FRAGS ||
> >                (!i &&
> >                !(sk->sk_route_caps & NETIF_F_SG))) {
> >    to:
> >          } else if (i == MAX_SKB_FRAGS || !sg) {
>
> I can see that you make this change from the patch itself but could you
> give a justification on why dropping the !i is possible? ...I couldn't
> see what would allow that.

>From what *I understood*, this code (other than the MAX_SKB_FRAGS case)
executes only due to the else part of "if (skb_tailroom(skb) > 0) {" -
there was no space in the skb to put the data inline. Hence SG is false
is a sufficient condition and there is no way to add a fragment (i == 0
since skb_fill_page_desc cannot be called when !SG).

thanks,

- KK

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