ruby-core

* Author: eightbitraptor (Matthew Valentine-House)
* Status: Open
----------------------------------------
[[Github PR #10598]](https://github.com/ruby/ruby/pull/10598)

## Background

Ruby's GC running during Rails requests can have negative impacts on currently
running requests, causing applications to have high tail-latency.

A technique to mitigate this high tail-latency is Out-of-band GC (OOBGC). This
is basically where the application is run with GC disabled, and then GC is
explicitly started after each request, or when no requests are in progress.

This can reduce the tail latency, but also introduces problems of its own. Long
GC pauses after each request reduce throughput. This is more pronounced on
threading servers like Puma because all the threads have to finish processing
user requests and be "paused" before OOBGC can be triggered.

This throughput decrease happens for a couple of reasons:

1. There are few heuristics available for users to determine when GC should run,
this means that in OOBGC scenarios, it's possible that major GC's are being run
more than necessary. 2. The lack of any GC during a request means that lots of
garbage objects have been created and not cleaned up, so the process is using
more memory than it should - requiring major GC's run as part of OOBGC to do
more work and therefore take more time.

This ticket attempts to address these issues by:

1. Provide `GC.disable_major` and its antonym `GC.enable_major` to disable and
enable only major GC 2. Provide `GC.needs_major?` as a basic heuristic allowing
users to tell when Ruby should run a Major GC.

These ideas were originally proposed by @ko1 and @byroot in [this rails
issue](https://github.com/rails/rails/issues/50449)

Disabling GC major's would still allow minor GC's to run during the request,
avoiding the ballooning memory usage caused by not running GC at all, and
reducing the time that a major takes when we do run it, because the nursery
objects have been cleaned up during the request already so there is less work
for a major GC to do.

This can be used in combination with `GC.needs_major?` to selectively run an
OOBGC only when necessary

## Implementation

This PR adds 3 new methods to the `GC` module

- `GC.disable_major` This prevents major GC's from running automatically. It
does not restrict minors. When `objspace->rgengc.need_major_gc` is set and a
GC is run, instead of running a major, new heap pages will be allocated and a
minor run instead. `objspace->rgengc.need_major_gc` will remain set until a
major is manually run. If a major is not manually run then the process will
eventually run out of memory.

When major GC's are disabled, object promotion is disabled. That is, no
objects will increment their ages during a minor GC. This is to attempt to
minimise heap growth during the period between major GC's, by restricting the
number of old-gen objects that will remain unconsidered by the GC until the
next major.

When `GC.start` is run, then major GC's will be enabled, a GC triggered with
the options passed to `GC.start`, and then `disable_major` will be set to the
state it was in before `GC.start` was called.

- `GC.enable_major` This simply unsets the bit preventing major GC's. This will
revert the GC to normal generational behaviour. Everything behaves as default
again.

- `GC.needs_major?` This exposes the value of `objspace->rgengc.need_major_gc`
to the user level API. This is already exposed in
`GC.latest_gc_info[:need_major_by]` but I felt that a simpler interface would
make this easier to use and result in more readable code. eg.

```ruby out_of_band do GC.start if GC.needs_major? end ```

Because object aging is disabled when majors are disabled it is recommended to
use this in conjunction with `Process.warmup`, which will prepare the heap by
running a major GC, compacting the heap, and promoting every remaining object to
old-gen. This ensures that minor GC's are running over the smallets possible set
of young objects when `GC.disable_major` is true.

## Benchmarks

We ran some tests in production on Shopify's core monolith over a weekend and
found that:

**Mean time spent in GC, as well as p99.9 and p99.99 GC times are all
improved.**

**p99 GC time is slightly higher.**

We're running far fewer OOBGC major GC's now that we have `GC.needs_major?` than
we were before, and we believe that this is contributing to a slightly increased
number of minor GC's. raising the p99 slightly.

**App response times are all improved**

We see a 9% reduction in average and p99 response times when compared against
standard GC (4% p99.9 and p99.99).

This drops slightly to an 8% reduction in average and p99 response times when
compared against standard OOBGC (3.59 p99.9 and 4% p99.99)

--
https://bugs.ruby-lang.org/
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