Re: [PATCH 3/3] x86, extable: Handle early exceptions

H. Peter Anvin
2012-04-19T18:55:26Z

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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