[ruby-core:81460] [Ruby trunk Feature#13378] Eliminate 4 of 8 syscalls when requiring file by absolute path

Issue #13378 has been updated by burke (Burke Libbey).

File 0001-reduce-syscalls-on-require-v2.patch added

Thank you for the feedback! I've attached an updated patch to address the issues.

As for testing it, I haven't been able to think of a reasonable method to verify the behaviour without using dtrace/strace, since the only observable effect without system-level instrumentation should be a slight reduction in runtime. I *could* add a case to `DTrace::TestRequire`, but it feels wrong, since this file is concerned with testing the dtrace probes implemented by ruby, not testing ruby using built-in dtrace probes.

Suggestions?

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Feature #13378: Eliminate 4 of 8 syscalls when requiring file by absolute path
https://bugs.ruby-lang.org/issues/13378#change-65163

* Author: burke (Burke Libbey)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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Don't open file twice when specified by absolute path.

When invoking `require '/a.rb'` (i.e. via an absolute path), ruby generates this sequence of syscalls:

    open    /a.rb
    fstat64 /a.rb
    close   /a.rb
    open    /a.rb
    fstat64 /a.rb
    fstat64 /a.rb
    read    /a.rb
    close   /a.rb

It is apparent that the only inherently necessary members of this sequence are:

    open    /a.rb
    fstat64 /a.rb
    read    /a.rb
    close   /a.rb

(the fstat64 isn't *obviously* necessary, but it does serve a purpose and probably shouldn't be removed).

The first open/fstat64/close is used to check whether the file is loadable. This is important when scanning the `$LOAD_PATH`, since it is used to determine when a file has been found. However, when we've already unambiguously identified a file before invoking `require`, this serves no inherent purpose, since we can move whatever work is happening as a result of that `fstat64` into the second open/close sequence.

This change bypasses the first open/fstat64/close in the case of an absolute path to `require`. It also removes one of the doubled-up `fstat64` calls later in the sequence. As a result, the number of syscalls to require a file changes:

* From 8 to 4 when specified by absolute path;
* From 5+3n to 4+3n otherwise *(where n is the number of `$LOAD_PATH` items scanned)*.

In future work, it would be possible to re-use the file descriptor opened while searching the `$LOAD_PATH` without the close/open sequence, but this would cause some ugly layering issues.

---

*We intend to use this in conjunction with something like https://github.com/shopify/bootscale, which pre-resolves required features to absolute paths before calling `require`. This change reduces our total number of filesystem accesses by 13% during application boot.*

*Various notes and rationale at http://notes.burke.libbey.me/ruby-require-optimization*

---Files--------------------------------
0001-reduce-syscalls-on-require.patch (7.56 KB)
0001-reduce-syscalls-on-require-fixed.patch (6.94 KB)
0001-reduce-syscalls-on-require-v2.patch (6.44 KB)


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