netdev - Re: [RFC] [PATCH] net: Add support for ndo_select_queue() functions to cache the queue mapping

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Message-Id: <1264703676.2783.44.camel@achroite.uk.solarflarecom.com>
Date:	Thu, 28 Jan 2010 18:34:34 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	netdev@...r.kernel.org, linux-net-drivers@...arflare.com
Subject: Re: [RFC] [PATCH] net: Add support for ndo_select_queue()
	functions to cache the queue mapping

On Thu, 2010-01-28 at 10:00 -0800, Stephen Hemminger wrote:
> On Thu, 28 Jan 2010 17:38:37 +0000
> Ben Hutchings <bhutchings@...arflare.com> wrote:
> 
> > We think it's worth matching up RX and TX queue selection for a socket
> > when the queues share an interrupt.  Currently the default TX queue
> > selection is unlikely to match RX queue selection.  TX queue selectionc
> > can be overridden by the driver to match RX queue selection, but at the
> > expense of caching.  We found that without caching the cost of
> > recalculating the the hash in software for each packet outweighed the
> > benefit of good queue selection.
> 
> Will this work with RPS and device driver hashing?

If the device only has a single TX queue this can't possibly help it.

In the case where a device has multiple RX and TX queues, I think it is
preferable that contention for TX queues is likely to be among cores
that are close together, not spread over the whole system.  I assume
that RPS will normally be configured to spread traffic from each
hardware RX queue to a group of cores that are close together (same NUMA
mode, maybe sharing some caches).  In that case it would also be
desirable to limit contention for a TX queue to within these same groups
of cores, so TX queue selection ought to match hardware RX queue
selection.  So I don't think this should conflict with RPS.

> I am thinking of the problem where the hardware hash function is
> Toeplitz (per NDIS spec), and the software hash function is Jhash
> (what kernel uses because it is cheaper).
> 
> Therefore the transmit hash and receiver hash will be different.

The intent here is to make it worthwhile to calculate that expensive
hash on the TX side.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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