netdev - Re: [net-next RFC PATCH 0/5] Series short description

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Date:	Wed, 7 Dec 2011 17:02:04 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Jason Wang <jasowang@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>
CC:	<krkumar2@...ibm.com>, <kvm@...r.kernel.org>, <mst@...hat.com>,
	<netdev@...r.kernel.org>,
	<virtualization@...ts.linux-foundation.org>,
	<levinsasha928@...il.com>
Subject: Re: [net-next RFC PATCH 0/5] Series short description

On Wed, 2011-12-07 at 19:31 +0800, Jason Wang wrote:
> On 12/07/2011 03:30 PM, Rusty Russell wrote:
> > On Mon, 05 Dec 2011 16:58:37 +0800, Jason Wang<jasowang@...hat.com>  wrote:
> >> multiple queue virtio-net: flow steering through host/guest cooperation
> >>
> >> Hello all:
> >>
> >> This is a rough series adds the guest/host cooperation of flow
> >> steering support based on Krish Kumar's multiple queue virtio-net
> >> driver patch 3/3 (http://lwn.net/Articles/467283/).
> > Is there a real (physical) device which does this kind of thing?  How do
> > they do it?  Can we copy them?
> >
> > Cheers,
> > Rusty.
> As far as I see, ixgbe and sfc have similar but much more sophisticated 
> mechanism.
> 
> The idea was originally suggested by Ben and it was just borrowed form 
> those real physical nic cards who can dispatch packets based on their 
> hash. All of theses cards can filter the flow based on the hash of 
> L2/L3/L4 header and the stack would tell the card which queue should 
> this flow goes.

Solarflare controllers (sfc driver) have 8192 perfect filters for
TCP/IPv4 and UDP/IPv4 which can be used for flow steering.  (The filters
are organised as a hash table, but matched based on 5-tuples.)  I
implemented the 'accelerated RFS' interface in this driver.

I believe the Intel 82599 controllers (ixgbe driver) have both
hash-based and perfect filter modes and the driver can be configured to
use one or the other.  The driver has its own independent mechanism for
steering RX and TX flows which predates RFS; I don't know whether it
uses hash-based or perfect filters.

Most multi-queue controllers could support a kind of hash-based
filtering for TCP/IP by adjusting the RSS indirection table.  However,
this table is usually quite small (64-256 entries).  This means that
hash collisions will be quite common and this can result in reordering.
The same applies to the small table Jason has proposed for virtio-net.

> So in host, a simple hash to queue table were introduced in tap/macvtap 
> and in guest, the guest driver would tell the desired queue of a flow 
> through changing this table.

I don't think accelerated RFS can work well without the use of perfect
filtering or hash-based filtering with a very low rate of collisions.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
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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