netdev - Re: [PATCH net-next] net: ovs: use CRC32 accelerated flow hash if available

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Message-ID: <CAEP_g=9m9iSM8eKrTCfZpomRGKJA7+cLG8KQOzpuVGCDAr7gsg@mail.gmail.com>
Date:	Tue, 10 Dec 2013 13:40:39 -0800
From:	Jesse Gross <jesse@...ira.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	David Miller <davem@...emloft.net>,
	Francesco Fusco <ffusco@...hat.com>,
	Linux Netdev List <netdev@...r.kernel.org>,
	"dev@...nvswitch.org" <dev@...nvswitch.org>,
	Daniel Borkmann <dborkman@...hat.com>,
	Thomas Graf <tgraf@...hat.com>
Subject: Re: [PATCH net-next] net: ovs: use CRC32 accelerated flow hash if available

On Tue, Dec 10, 2013 at 1:27 PM, Tom Herbert <therbert@...gle.com> wrote:
> On Tue, Dec 10, 2013 at 11:36 AM, David Miller <davem@...emloft.net> wrote:
>> From: Jesse Gross <jesse@...ira.com>
>> Date: Tue, 10 Dec 2013 11:28:08 -0800
>>
>>> I think this is definitely a good optimization to do given that so
>>> much of the work that OVS does is hashing. However, isn't there a
>>> library where there would be a more appropriate place to put this?
>>
>> I also honestly don't see why we want to special case OVS at all
>> here.  This faster hashing would be useful for socket demux and
>> other locations in the kernel.
>>
>
> Also, we already do a lot of work to compute flow hashes for packets
> in both TX and RX.  Can these be leveraged? For instance, if OVS is
> computing a 5-tuple hash on an RX packet it's probably redundant to
> skb->rxhash.

That's true if we care about exactly the 5-tuple. However, the set of
fields that are included in the OVS flow lookup is dynamically
generated down to a bit mask so I don't know if it's worth special
casing. (In fact, I don't think that current OVS userspace will ever
actually emit exactly a 5-tuple.)
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