netdev - Re: [PATCH net-next v2 2/2] net: ovs: use CRC32 accelerated flow hash if available

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Message-ID: <CAEP_g=8R5T71tXOwW-s15jNsD5k2Pv6vD0JZEfEoAARC9BRZ_g@mail.gmail.com>
Date:	Fri, 13 Dec 2013 13:12:12 -0800
From:	Jesse Gross <jesse@...ira.com>
To:	Francesco Fusco <ffusco@...hat.com>
Cc:	netdev <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 v2 2/2] net: ovs: use CRC32 accelerated flow hash
 if available

On Fri, Dec 13, 2013 at 1:55 AM, Francesco Fusco <ffusco@...hat.com> wrote:
> On 12/12/2013 09:20 PM, Jesse Gross wrote:
>>
>> Out of curiosity, did you try using crc32q? OVS data structures are
>> already aligned to 8 bytes. It would also be interesting to know if a
>> parallelized implementation is worthwhile, although my guess is that
>> the OVS flow key is not quite long enough.
>
>
> We did not try with crc32q yet because we had the very same concerns.
> In the general case, when the length of the key is not a multiple of 8
> bytes, we will have to use the crc32q to process multiple 64 bit numbers,
> then process the the rest with a crc32l. My guess is that the
> performance benefit won't be that high.

On 64-bit platforms, OVS already enforces alignment and multiples of 8
bytes for flow keys. If we introduced a hash primitive that does 8
bytes at a time instead of 4, there should be nothing extra to process
(although I don't know how big the upside would be).
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