netdev - Re: [RFC] New driver API to speed up small packets xmits

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Message-ID: <1179266728.25622.46.camel@dell>
Date:	Tue, 15 May 2007 15:05:28 -0700
From:	"Michael Chan" <mchan@...adcom.com>
To:	"Roland Dreier" <rdreier@...co.com>
cc:	"David Miller" <davem@...emloft.net>, ak@...e.de,
	krkumar2@...ibm.com, "netdev" <netdev@...r.kernel.org>
Subject: Re: [RFC] New driver API to speed up small packets xmits

On Tue, 2007-05-15 at 14:08 -0700, Roland Dreier wrote:
>  > > Well, IPoIB doesn't do netif_wake_queue() until half the device's TX
>  > > queue is free, so we should get batching.  However, I'm not sure that
>  > > I can count on a fudge factor ensuring that there's enough space to
>  > > handle everything skb_gso_segment() gives me -- is there any reliable
>  > > way to get an upper bound on how many segments a given gso skb will
>  > > use when it's segmented?
>  > 
>  > Take a look at tg3.c.  I use (gso_segs * 3) as the upper bound.
> 
> Thanks for the pointer... I noticed that code, but could you tell me
> where the "* 3" comes from?
> 
For each gso_seg, there will be a header and the payload may span 2
pages for 1500-byte packets.  We always assume 1500-byte packets because
the buggy chips do not support jumbo frames.

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