Messages in this thread | | | Subject | Re: [PATCH 3/3] x86, extable: Handle early exceptions | From | "H. Peter Anvin" <> | Date | Thu, 19 Apr 2012 11:55:00 -0700 |
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Either way I suggest picking up David's presorting patchset since it is already done and use its infrastructure for any further improvements.
As far as a linear probe you get an average of n lookups with a packing density of 1-1/n so you are right; a linear probe with a density of say 1/2 is probably best.
Linus Torvalds <torvalds@linux-foundation.org> wrote:
>On Thu, Apr 19, 2012 at 10:59 AM, H. Peter Anvin <hpa@zytor.com> wrote: >> >> I would argue that the O(1) hash makes things simpler as there is no >> need to deal with collisions at all. > >Most of the O(1) hashes I have seen more than made up for the trivial >complexity of a few linear lookups by making the hash function way >more complicated. > >A linear probe with a step of one really is pretty simple. Sure, you >might want to make the initial hash "good enough" to not often hit the >probing code, but doing a few linear probes is cheap. > >In contrast, the perfect linear hashes do crazy things like having >table lookups *JUST TO COMPUTE THE HASH*. > >Which is f*cking stupid, really. They'll miss in the cache just at >hash compute time, never mind at hash lookup. The table-driven >versions look beautiful in microbenchmarks that have the tables in the >L1 cache, but for something like the exception handling, I can >guarantee that *nothing* is in L1, and probably not even L2. > >So what you want is: > - no table lookups for hashing > - simple code (ie a normal "a multiply and a shift/mask or two") to >keep the I$ footprint down too > - you *will* take a cache miss on the actual hash table lookup, that >cannot be avoided, but linear probing at least hopefully keeps it to >that single cache miss even if you have to do a probe or two. > >Remember: this is very much a "cold-cache behavior matters" case. We >would never ever call this in a loop, at most we have loads that get a >fair amount of exceptions (but will go through the exception code, so >the L1 is probably blown even then). > > Linus
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