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dmu_objset: replace dnode_hash impl with cityhash4 #16483

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dmu_objset: replace dnode_hash impl with cityhash4 #16483

wants to merge 3 commits into from

Commits on Sep 19, 2024

  1. dmu_objset: replace dnode_hash impl with cityhash4 

    As mentioned in PR openzfs#16131, replacing CRC-based hash with cityhash4
could slightly improve the performance by eliminating memory access.
Replacing algorightm is safe since the hash result is not persisted.

See: openzfs#16131

Signed-off-by: Shengqi Chen <[email protected]>
    @Harry-Chen
    Harry-Chen committed Sep 19, 2024
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  2. zcommon: add specialized versions of cityhash4 

    Specializing cityhash4 on 32-bit architectures can reduce the size
of stack frames as well as instruction count. This is a tiny but
useful optimization, since some callers invoke it frequently.

When specializing into 1/2/3/4-arg versions, the stack usage
(in bytes) on some 32-bit arches are listed as follows:

- x86: 32, 32, 32, 40
- arm-v7a: 20, 20, 28, 36
- riscv: 0, 0, 0, 16
- power: 16, 16, 16, 32
- mipsel: 8, 8, 8, 24

And each actual argument (even if passing 0) contributes evenly
to the number of multiplication instructions generated:

- x86: 9, 12, 15 ,18
- arm-v7a: 6, 8, 10, 12
- riscv / power: 12, 18, 20, 24
- mipsel: 9, 12, 15, 19

On 64-bit architectures, the tendencies are similar. But both stack
sizes and instruction counts are significantly smaller thus negligible.

See more discussion at openzfs#16483.

Acked-by: Alexander Motin <[email protected]>
Signed-off-by: Shengqi Chen <[email protected]>
    @Harry-Chen
    Harry-Chen committed Sep 19, 2024
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  3. cityhash: replace invocations with specialized versions when possible 

    So that we can get actual benefit from last commit.

See more discussion at openzfs#16483.

Acked-by: Alexander Motin <[email protected]>
Signed-off-by: Shengqi Chen <[email protected]>
    @Harry-Chen
    Harry-Chen committed Sep 19, 2024
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