All the non-GC objects (i.e. immediates) have addresses such that
`obj % RUBY_IMMEDIATE_MASK != 0` (except for `Qfalse`, which is 0). We
can define `OBJ_ID_INCREMENT` as `RUBY_IMMEDIATE_MASK + 1` which should
guarantee that GC objects never have conflicting object IDs with
immediates.
This feature provides a new method `GC.config` that configures internal
GC configuration variables provided by an individual GC implementation.
Implemented in this PR is the option `full_mark`: a boolean value that
will determine whether the Ruby GC is allowed to run a major collection
while the process is running.
It has the following semantics
This feature configures Ruby's GC to only run minor GC's. It's designed
to give users relying on Out of Band GC complete control over when a
major GC is run. Configuring `full_mark: false` does two main things:
* Never runs a Major GC. When the heap runs out of space during a minor
and when a major would traditionally be run, instead we allocate more
heap pages, and mark objspace as needing a major GC.
* Don't increment object ages. We don't promote objects during GC, this
will cause every object to be scanned on every minor. This is an
intentional trade-off between minor GC's doing more work every time,
and potentially promoting objects that will then never be GC'd.
The intention behind not aging objects is that users of this feature
should use a preforking web server, or some other method of pre-warming
the oldgen (like Nakayoshi fork)before disabling Majors. That way most
objects that are going to be old will have already been promoted.
This will interleave major and minor GC collections in exactly the same
what that the Ruby GC runs in versions previously to this. This is the
default behaviour.
* This new method has the following extra semantics:
- `GC.config` with no arguments returns a hash of the keys of the
currently configured GC
- `GC.config` with a key pair (eg. `GC.config(full_mark: true)` sets
the matching config key to the corresponding value and returns the
entire known config hash, including the new values. If the key does
not exist, `nil` is returned
* When a minor GC is run, Ruby sets an internal status flag to determine
whether the next GC will be a major or a minor. When `full_mark:
false` this flag is ignored and every GC will be a minor.
This status flag can be accessed at
`GC.latest_gc_info(:needs_major_by)`. Any value other than `nil` means
that the next collection would have been a major.
Thus it's possible to use this feature to check at a predetermined
time, whether a major GC is necessary and run one if it is. eg. After
a request has finished processing.
```ruby
if GC.latest_gc_info(:needs_major_by)
GC.start(full_mark: true)
end
```
[Feature #20443]
This feature is no longer possible under current design; now that our GC
is pluggable, we cannot assume what was achieved by this compiler flag
is always possble by the dynamically-loaded GC implementation.
