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From: Daniel Phillips <phillips@bonn-fries.net>
To: William Lee Irwin III <wli@holomorphy.com>, linux-kernel@vger.kernel.org
Subject: Re: hashed waitqueues
Date: Sat, 5 Jan 2002 01:37:40 +0100
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On January 4, 2002 06:40 pm, William Lee Irwin III wrote:

> +       unsigned long hash = (unsigned long)page;

Just to be anal here, 'long' doesn't add anything useful to this 
declaration.  In fact, u32 is what you want since you've chosen your 
multiplier with a 32 bit register in mind - on 64bit arch I think you'll get 
distinctly non-random results as it stands.

> +	hash *= GOLDEN_RATIO_PRIME;
> +	hash >>= BITS_PER_LONG - zone->wait_table_bits;
> +	hash &= zone->wait_table_size - 1;

Nice hash!  For arches with expensive multiplies you might want to look 
for a near-golden ratio multiplier that has a simple contruction in terms of 
1's & 0's so it can be computed with 2 or 3 shift-adds, dumping the 
efficiency problem on the compiler.  You don't have to restrict your search 
to the neighbourhood of 32 bits, you can go a few bits less than that and 
substract from a smaller value than BITS_PER_LONG (actually, you should just 
write '32' there, since that's what you really have in mind).

--
Daniel


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