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Date:	Wed, 12 Mar 2008 15:10:52 +0000
From:	Ben Hutchings <>
To:	Tom Herbert <>
Cc:	"Brandeburg, Jesse" <>,
	Max Krasnyanskiy <>,
	David Miller <>,,
	Steven Rostedt <>,
	Ingo Molnar <>
Subject: Re: RFC [PATCH net-2.6 1/6] net: Scheduling softirqs between CPUSs

Tom Herbert wrote:
> >  Recent versions of Windows that are aware of RSS will instruct network
> >  drivers to update their RSS indirection tables so that received
> >  packets are handled on the same CPU as the thread likely to use the
> >  received data.  This requires that the network stack knows the hash
> >  function, so Microsoft specified a "Toeplitz hash" for IPv4 and IPv6.
> >  You can expect that most hardware RSS implementations will use this
> >  function and not a simple XOR.
> >
> Thanks.  I am planning to do the Toeplitz hash in the softRSS.  Also,
> I was planning to add support for devices that provide the Toeplitz
> hash but don't do full RSS.
> One nice feature about Microsoft RSS seems to be the ability for the
> stack to dynamically re-balance networking load of a device using
> an indirection table that is set up in the device.  I was wondering if
> this is supported in Linux stack or if anyone is working on that?

It's not supported, but it's on David Miller's to-do list[1].

By the way, Microsoft suddenly decided that RSS is too problematic to
enable by default[2] because software and hardware can disagree about
hash values in the presence of some local NAT implementations.
Hopefully we can avoid that particular pitfall in Linux.



Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
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