Use mmap in ReadAll if possible

[tgulacsi]

PR Details

Use mmap instead of io.ReadAll, if possible

Description

Introduce readAll function which uses mmap if possible, io.ReadAll otherwise.

Related Issue

#2121

Motivation and Context

Memory inefficiency.

How Has This Been Tested

go test

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[shcabin]

Have you compared it with io.Copy? It seems that io.Copy is more compatible and not fallback to io.ReadAll().

In my work environment there is only Fd() uintptr （not found Fd() int), and I am not sure why, so I am unable to conduct effective testing mmap.

[tgulacsi]

My bad, you're right, Fd() returns uintptr.

This mmap is not for speed, it's for decreasing RSS by using the kernel-supplied probably-in-filecache []byte of the file.