linux-kernel - Re: [patch 2/7] lib/hashmod: Add modulo based hash mechanism

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Date:	Mon, 2 May 2016 09:11:34 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	George Spelvin <linux@...izon.com>
cc:	eric.dumazet@...il.com, linux-kernel@...r.kernel.org,
	riel@...hat.com, torvalds@...ux-foundation.org
Subject: Re: [patch 2/7] lib/hashmod: Add modulo based hash mechanism

On Sun, 1 May 2016, George Spelvin wrote:
> But I noticed a much greater difference.
> 
>  Wang      * PHI    % 4093   Wang      * PHI    % 4093
>  13599486  3494816  5238442  13584166  3460266  5239463
>  13589552  3466764  5237327  13582381  3422304  5276253
>  13569409  3407317  5236239  13566738  3393215  5267909
>  13575257  3413736  5237708  13596428  3379811  5280118
>  13583607  3540416  5325609  13650964  3380301  5265210
> 
> At 3.7 GHz, that's 
> 
> * PHI:     1059 M ops/second
> * Modulo:   706 M ops/second
> * Wang:     271 M ops/second
> 
> Of course, that's a tight loop hashing; I presume your test case
> has more overhead.

Indeed.
 
> Anyway, my recommendation (I'll write the patches if you like) is:
> 
> * Modify the multiplicative constants to be
>   #define COLDEN_RATIO_32 0x61C88647
>   #define COLDEN_RATIO_64 0x61C8864680B583EB

Works for me. I ran them through my test case and they behaved reasonably
well.
 
> * Change the prototype of hash_64 to return u32.

Makes sense.
 
> * Create a separate 32-bit implementation of hash_64() for the
>   BITS_PER_LONG < 64 case.  This should not be Wang's or anything
>   similar because 64-bit shifts are slow and awkward on 32-bit
>   machines.
>   One option is something like __jhash_final(), but I think
>   it will suffice to do:
> 
>   static __always_inline u32 hash_64(u64 val, unsigned int bits)
>   {
> 	u32 hash = (u32)(val >> 32) * GOLDEN_RATIO_32 + (u32)val;
> 	hash *= GOLDEN_RATIO_32;
>         return hash >> (32 - bits);
>   }

Works. That's more or less the same overhead as the modulo one, which behaved
well on 32bit.
 
Thanks,

	tglx