netdev - Re: Where exactly will arch_fast

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Date:	Mon, 08 Dec 2014 17:32:09 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	George Spelvin <linux@...izon.com>
Cc:	davem@...emloft.net, dborkman@...hat.com,
	herbert@...dor.apana.org.au, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, tgraf@...g.ch, tytso@....edu
Subject: Re: Where exactly will arch_fast_hash be used



On Mon, Dec 8, 2014, at 17:19, George Spelvin wrote:
> >>> In case of openvswitch it shows a performance improvment. The seed
> >>> parameter could be used as an initial biasing of the crc32 function, but
> >>> in case of openvswitch it is only set to 0.
>  
> >> NACK. [...]
> 
> > Sorry for being unclear, I understood that and didn't bother patching
> > that '0' with a random seed exactly because of this.
> 
> And I'm sorry for delivering a long lecture on a subject you already
> understood perfectly well.

I learned something, so your time wasn't completely wasted. ;)

> I'd just been thinking about it because of Herbert's comments, so it was
> conveniently at hand. :-)
> 
> Out of curiousity, what *were* you referring to when you talked
> about biasing the crc32 function?  "Biasing" is a good term becuase
> it just applies an offset, but what do you gain from doing that?

Actually, I don't know why the seed parameter was added. I just wanted
to mention that there is a way to bias the crc32 function which fits
into the style of the other hashing functions, like jhash with its
initval parameter.

I just kept it around during the rewrite.

The only use case I can imagine would be if one would like to calculate
a crc32c over a non-contiguous array, thus feeding the result of one crc
operation into the next one.

> There are nifty things one can do with the CRC32 instruction, however.
> A lot of ciphers these days use an ARX (add, rotate, XOR) kernel.
> A crc32 instruction, although linear, does some very powerful rotate &
> xor operations, and could replace the XOR and rotate.

Yes, I have seen it being used in cityhash.

There is also a proposal by Intel, but the hash seems too weak, too:
http://www.intel.com/content/dam/www/public/us/en/documents/white-papers/hash-method-performance-paper.pdf

Bye,
Hannes
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