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

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Date:	Tue, 15 May 2007 13:52:34 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	David Miller <davem@...emloft.net>
Cc:	ak@...e.de, krkumar2@...ibm.com, netdev@...r.kernel.org
Subject: Re: [RFC] New driver API to speed up small packets xmits

 > > I'll have to think about implementing that for IPoIB.  One issue I see
 > > is if I have, say, 4 free entries in my send queue and skb_gso_segment()
 > > gives me back 5 packets to send.  It's not clear I can recover at that
 > > point -- I guess I have to check against gso_segs in the xmit routine
 > > before actually doing the segmentation.
 > 
 > I'd suggest adding a fudge factor to your free TX space, which
 > is advisable anyways so that when TX is woken up, more of
 > the transfer from queue to device can happen in a batch-like
 > fashion.

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?

 - R.
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