ARC: Pre-convert zfs_arc_min_prefetch_ms

[amotin]

There is no need to do MSEC_TO_TICK() for each evicted ARC header. We can do it when tunables are set, since we already have separate internal variables for those.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[robn]

Yeah, bit easier to read. Nice.

FWIW, I went and read the Linux code to see if the __msecs_to_jiffies() we use to implement MSEC_TO_TICK() does anything other than simple math (I was wondering about frequency scaling or something that might change the output over time). Doesn't appear to be anything interesting though!