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

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Date:	Tue, 15 May 2007 14:32:07 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	xma@...ibm.com
Cc:	rdreier@...co.com, ak@...e.de, krkumar2@...ibm.com,
	netdev@...r.kernel.org, netdev-owner@...r.kernel.org
Subject: Re: [RFC] New driver API to speed up small packets xmits

From: Shirley Ma <xma@...ibm.com>
Date: Tue, 15 May 2007 14:22:57 -0700

>       I just wonder without TSO support in HW, how much benefit we
> can get by pushing GSO from interface layer to device layer besides
> we can do multiple packets in IPoIB.

I bet the gain is non-trivial.

I'd say about half of the gain from TSO comes from only calling down
into the driver from TCP one time as opposed to N times.  That's
the majority of the "CPU work" involved in TCP sending.

The rest of the gain comes from only transmitting the packet headers
once rather than N times, which conserves I/O bus bandwidth.

GSO will not help the case of lots of UDP applications sending
small packets, or something like that.  An efficient qdisc-->driver
transfer during netif_wake_queue() could help solve some of that,
as is being discussed here.
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