Re: hashed waitqueues

Daniel Phillips
2002-01-05T00:34:45Z

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