Issue #17176 has been updated by tenderlovemaking (Aaron Patterson).


ko1 (Koichi Sasada) wrote in #note-4:
> BTW as I mentioned in slack, GC.compact may have an issue (can access to T_MOVED objects from Ruby level) so please fix it before merge this large commit.

I think it's an issue with a "use-after-free" bug.  I believe it was fixed here: https://github.com/ruby/ruby/pull/3571

I was able to get the crash locally, but after applying ^^^ the crash goes away.

> Another idea is GC.enable(compact: true/false).

I like this better than `GC.auto_compact = true/false` because we can add more options.  I'll change to use that.

> Also do you have any performance results, for example memory consumption or GC time and so on?

I did a survey using RDoc.  The summary: Minor GC time is the same, major GC time is much slower, but I still think we should add this as an experimental feature.

Here are more details of the tests I did:

To benchmark compaction, I enabled `GC::Profiler` and then generated RDoc for Ruby.

I created a file `x.rb` that looks like this:

```ruby
BEGIN {
  $start = Process.clock_gettime(Process::CLOCK_MONOTONIC)
  # Uncomment for automatic compaction
  #GC.enable_autocompact
  GC::Profiler.enable
}
END {
  profile = GC::Profiler.raw_data.dup
  require "csv"
  CSV($stderr) do |csv|
    syms = %i[HEAP_USE_SIZE HEAP_TOTAL_SIZE HEAP_TOTAL_OBJECTS MOVED_OBJECTS]
    csv << (["major_by", "GC_TIME"] + syms.map(&:to_s))
    profile.each do |record|
      csv << ([record[:GC_FLAGS][:major_by], record[:GC_TIME] * 100] + syms.map { |s| record[s] })
    end
  end
  $stderr.puts
  $stderr.puts
  stats = %i[ heap_allocated_pages heap_eden_pages read_barrier_faults minor_gc_count major_gc_count ]
  stats.each { |s|
    $stderr.puts "#{s}: #{GC.stat(s)}"
  }
  elapsed = Process.clock_gettime(Process::CLOCK_MONOTONIC) - $start
  $stderr.puts "Elapsed: #{elapsed}"
}
```

Then I generated RDoc like this:

```
rm -rf .ext/rdoc
./ruby --disable-gems -I. -r x "./libexec/rdoc" --root "." --encoding=UTF-8 --all --ri --op ".ext/rdoc" --page-dir "./doc" --no-force-update  "." 2>logs/no_autocompact_$i.log
```

I did this 100 times with automatic compaction enabled, and with automatic compaction disabled.

This patch only adds automatic compaction to major GCs, so I don't expect minor GC times to change.
Here is a plot of the minor GC time:

![Minor GC Time](https://user-images.githubusercontent.com/3124/94854245-775c6180-03e1-11eb-8dcc-9c56988678c7.png)

X axis is the GC invoke count, Y axis is time in seconds.  As expected, there is really no change.

Here is a plot of the major GC time:

![Major GC Time](https://user-images.githubusercontent.com/3124/94854341-9e1a9800-03e1-11eb-9666-796ab8e0708c.png)

X axis is the GC invoke count, Y axis is time in seconds.  Adding compaction makes the major invocations significantly slower.  The algorithm for doing a major GC doesn't change, so the difference in these lines is completely due to the time it takes to compact the heap.

If we divide heap size by time to get "Objects Per Second", then we can get an idea of GC throughput:

![Major GC Throughput](https://user-images.githubusercontent.com/3124/94855783-e20e9c80-03e3-11eb-9d2e-cd79b452b044.png)

It's pretty clear that the throughput is lower with compaction enabled.

One interesting thing I've found.  If we plot "heap total objects" and compare it to GC time, it makes a graph like this:

![Major GC vs Compact Speed](https://user-images.githubusercontent.com/3124/94860534-0fab1400-03eb-11eb-8b46-e29cb1cef9f9.png)

Whenever the heap size remains stable, GC compaction gets significantly faster.  In other words, when the blue line stays level, the red line goes down.

My hypothesis is that this is due to the way the GC adds new pages when the heap expands.  When the heap expands, the GC adds pages on the left side of the heap.  So the top of "heap_eden" always points at the newest page.  The compaction algorithm packs to the left, so as we add new pages, more objects move (even though they didn't need to).

Anyway, I think that introducing this feature has value because it will help people test with more aggressive object movement.  I would like auto compaction to be enabled by default some day, but I think for now we can add this as an experimental feature.

Once the speed is acceptable and it seems like bugs are worked out, then we can enable it by default.


----------------------------------------
Feature #17176: GC.enable_autocompact / GC.disable_autocompact
https://bugs.ruby-lang.org/issues/17176#change-87849

* Author: tenderlovemaking (Aaron Patterson)
* Status: Open
* Priority: Normal
----------------------------------------
Hi,

I'd like to make compaction automatic eventually.  As a first step, I would like to introduce two functions:

* GC.enable_autocompact
* GC.disable_autocompact

One function enables auto compaction, the other one disables it.  Automatic compaction is *disabled* by default.  When it is enabled it will happen only on every major GC.

I've made a pull request here: https://github.com/ruby/ruby/pull/3547

This patch makes _object movement_ happen at the same time as page sweep.  When one page finishes sweeping, that page is filled.

## Sweep + Move Phase

During sweep, we keep a pointer to the current sweeping page.  This pointer is kept in [`heap->sweeping_page`](https://github.com/ruby/ruby/blob/8a4d8fa0ea463d44486bf2447ea9830593768fd7/gc.c#L4817).  At the beginning of sweep, this is the *first* element of the heap's linked list.

At the same time, the compaction process points at the *last* page in the heap, and that is stored in `heap->compact_cursor` [here](https://github.com/ruby/ruby/blob/8a4d8fa0ea463d44486bf2447ea9830593768fd7/gc.c#L5023).

Incremental sweeping sweeps one page at a time in the [`gc_page_sweep` function](https://github.com/ruby/ruby/blob/8a4d8fa0ea463d44486bf2447ea9830593768fd7/gc.c#L4624).  At the end of that function, we call [`gc_fill_swept_page`](https://github.com/ruby/ruby/blob/8a4d8fa0ea463d44486bf2447ea9830593768fd7/gc.c#L4738-L4742).  `gc_fill_swept_page` fills the page that was just swept and moves the movement cursor towards the sweeping cursor.

When the sweeping cursor and the movement cursor meet, sweeping is paused, and references are updated.  This can happen in 2 ways, the sweeping cursor "runs in to the moving cursor" which is [here](https://github.com/ruby/ruby/blob/8a4d8fa0ea463d44486bf2447ea9830593768fd7/gc.c#L4634-L4644).  Or the moving cursor runs in to the sweep cursor which happens [here](https://github.com/ruby/ruby/blob/8a4d8fa0ea463d44486bf2447ea9830593768fd7/gc.c#L4425-L4430).

Either way, the sweep step is paused and references are updated.

## Reference Updating

Reference updating hasn't changed, but since reference updating happens before the GC finishes a cycle, it must take in to account garbage objects [here](https://github.com/ruby/ruby/blob/8a4d8fa0ea463d44486bf2447ea9830593768fd7/gc.c#L8971-L8977).

## Read Barrier

During the sweep phase, some objects may touch other objects.  For example, `T_CLASS` [must remove itself from a parent class](https://github.com/ruby/ruby/blob/8a4d8fa0ea463d44486bf2447ea9830593768fd7/gc.c#L2769-L2770).

```ruby
class A; end
class B < A; end

const_set(:B, nil)
```

When `B` is freed, it must remove itself from `A`'s subclasses.  But what if `A` moved?  To fix this, I've introduced a read barrier.  The read barrier protects `heap_page_body` using `mprotect`.  If something tries to read from the page, an exception will occur and we can move all objects back to the page (invalidate the movement).

The lock function is [here](https://github.com/ruby/ruby/blob/8a4d8fa0ea463d44486bf2447ea9830593768fd7/gc.c#L4321-L4335).
The unlock function is [here](https://github.com/ruby/ruby/blob/8a4d8fa0ea463d44486bf2447ea9830593768fd7/gc.c#L4337-L4351).

It uses `sigaction` to catch the exception [here](https://github.com/ruby/ruby/blob/8a4d8fa0ea463d44486bf2447ea9830593768fd7/gc.c#L4514-L4530).

## Cross Platform

`mprotect` and `sigaction` are not cross platform, they doesn't work on Windows. On Windows the read barrier uses exception handlers that are built in to Windows.  I implemented them [here](https://github.com/ruby/ruby/blob/8a4d8fa0ea463d44486bf2447ea9830593768fd7/gc.c#L4471-L4503).

The read barrier seems to work on all platforms we're testing.

## Statistics

`GC.stat(:compact_count)` contains the number of times compaction has happened, so we can write things like this:

```ruby
GC.enable_autocompact

cc = GC.stat(:compact_count)
list = []
loop do
  500.times { list << Object.new }
  break if cc < GC.stat(:compact_count)
end

p GC.stat(:compact_count)
```

We can check when the read barrier is triggered with `GC.stat(:read_barrier_faults)`

I've also added `GC.latest_compact_info` so you can see what types of objects moved and how many.  For example:

```
[aaron@tc-lan-adapter ~/g/ruby (autocompact)]$ cat test.rb
list = []
500.times {
  list << Object.new
  Object.new
  Object.new
}

GC.enable_autocompact
count = GC.stat :compact_count
loop do
  list << Object.new
  break if GC.stat(:compact_count) > count
end

p GC.latest_compact_info
[aaron@tc-lan-adapter ~/g/ruby (autocompact)]$ make runruby
./miniruby -I./lib -I. -I.ext/common  ./tool/runruby.rb --extout=.ext  -- --disable-gems ./test.rb
{:considered=>{:T_OBJECT=>408}, :moved=>{:T_OBJECT=>408}}
[aaron@tc-lan-adapter ~/g/ruby (autocompact)]$
```

## Recap

New methods:

* GC.enable_autocompact
* GC.disable_autocompact
* GC.last_compact_info

New statistics in `GC.stat`:

* GC.stat(:read_barrier_faults)

Diff is here: https://github.com/ruby/ruby/pull/3547



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