Messages in this thread |  | | Date | Thu, 08 Nov 2001 22:54:30 -0800 | From | Andrew Morton <> | Subject | Re: speed difference between using hard-linked and modular drives? |
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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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