Re: speed difference between using hard-linked and modular drives?

Andrew Morton
2001-11-09T07:00:39Z

Andi Kleen wrote:
>
> On Thu, Nov 08, 2001 at 10:04:44PM -0800, David S. Miller wrote:
> > From: Andi Kleen <ak@suse.de>
> > Date: Fri, 9 Nov 2001 06:45:40 +0100
> >
> > Sounds like you need a better hash function instead.
> >
> > Andi, please think about the problem before jumping to conclusions.
> > N_PAGES / N_CHAINS > 1 in his situation. A better hash function
> > cannot help.
>
> I'm assuming that walking on average 5-10 pages on a lookup is not too big a
> deal, especially when you use prefetch for the list walk. It is a tradeoff
> between a big hash table thrashing your cache and a smaller hash table that
> can be cached but has on average >1 entries/buckets. At some point the the
> smaller hash table wins, assuming the hash function is evenly distributed.
>
> It would only get bad if the average chain length would become much bigger.
>
> Before jumping to real conclusions it would be interesting to gather
> some statistics on Anton's machine, but I suspect he just has an very
> unevenly populated table.

I played with that earlier in the year. Shrinking the hash table
by a factor of eight made no measurable difference to anything on
a Pentium II. The hash distribution was all over the place though.
Lots of buckets with 1-2 pages, lots with 12-13.

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