[#100309] How to use backport custom field — Jun Aruga <jaruga@...>
Please allow my ignorance.
9 messages
2020/10/06
[#100310] Re: How to use backport custom field
— "NARUSE, Yui" <naruse@...>
2020/10/06
IkJhY2twb3J0IGN1c3RvbSBmaWVsZCIgaXMgb25seSBhdmFpbGFibGUgZm9yIHRpY2tldHMgd2hv
[#100311] Re: How to use backport custom field
— Jun Aruga <jaruga@...>
2020/10/06
On Tue, Oct 6, 2020 at 4:44 PM NARUSE, Yui <[email protected]> wrote:
[#100314] Re: How to use backport custom field
— "NARUSE, Yui" <naruse@...>
2020/10/06
VGhhbmsgeW91IGZvciBjb25maXJtYXRpb24uCkkgY2hlY2tlZCBhZ2FpbiBhbmQgdG8gZWRpdCBi
[#100322] Re: How to use backport custom field
— Jun Aruga <jaruga@...>
2020/10/07
On Tue, Oct 6, 2020 at 7:25 PM NARUSE, Yui <[email protected]> wrote:
[#100326] Re: How to use backport custom field
— "NARUSE, Yui" <naruse@...>
2020/10/07
SSBhZGRlZCB5b3UgdG8gIlJlcG9ydGVyIiByb2xlIGluIHRoZSBwcm9qZWN0CgoyMDIw5bm0MTDm
[#100327] Re: How to use backport custom field
— Jun Aruga <jaruga@...>
2020/10/07
On Wed, Oct 7, 2020 at 1:42 PM NARUSE, Yui <[email protected]> wrote:
[ruby-core:100449] [Ruby master Feature#17176] GC.auto_compact / GC.auto_compact=(flag)
From:
tenderlove@...
Date:
2020-10-20 19:39:33 UTC
List:
ruby-core #100449
Issue #17176 has been updated by tenderlovemaking (Aaron Patterson).
I made another pull request that sets the default to "ON":
https://github.com/ruby/ruby/pull/3316
It looks like there is one optional ruby spec test that is failing, but all of the other tests pass. Since the test suites pass with automatic compaction enabled, it makes me pretty confident about the implementation (though I'm sure there are bugs somewhere).
----------------------------------------
Feature #17176: GC.auto_compact / GC.auto_compact=(flag)
https://bugs.ruby-lang.org/issues/17176#change-88068
* 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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