netdev - Re: RFC [PATCH net-2.6 1/6] net: Scheduling softirqs between CPUSs

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Message-ID: <20080312151051.GI26963@solarflare.com>
Date:	Wed, 12 Mar 2008 15:10:52 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	"Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
	Max Krasnyanskiy <maxk@...lcomm.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>
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.

1. http://vger.kernel.org/~davem/net_todo.html
2. http://support.microsoft.com/kb/948496

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