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Make hash collisions in the registry much less likely #657

Merged
merged 3 commits into from Oct 15, 2019
Merged

Make hash collisions in the registry much less likely #657

merged 3 commits into from Oct 15, 2019

Commits on Oct 14, 2019

  1. Expose bug #633 

    Signed-off-by: beorn7 <beorn@grafana.com>
    @beorn7
    beorn7 committed Oct 14, 2019
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  2. Minimal “fix” for hash collisions 

    This makes the collisions a bit less likely by XOR'ing descIDs rather
than adding them up for the collectorID.

Signed-off-by: beorn7 <beorn@grafana.com>
    @beorn7
    beorn7 committed Oct 14, 2019
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  3. Move registry hashing to xxhash 

    This is a much stronger hash function than fnv64a and comparably fast
(with super-fast assembly implementation for amd64).

Performance is not critical here anyway.

The old fnv64a is kept for vectors, where collision detection is in
place and the weakness of the hashing doesn't matter that much. I
implemented a vector version with xxhash and found that xxhash is
slower in all cases except very very high cardinality (where it is
only slightly faster). Also, ``xxhash.New`` comes with an allocation
of 80 bytes. Thus, to keep vectors alloc-free, we needed to add a
`sync.Pool`, which would have an additional performance overhead.

Signed-off-by: beorn7 <beorn@grafana.com>
    @beorn7
    beorn7 committed Oct 14, 2019
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