[ruby-core:68990] [Ruby trunk - Feature #11098] [Open] Thread-level allocation counting

Issue #11098 has been reported by Jason Clark.

----------------------------------------
Feature #11098: Thread-level allocation counting
https://bugs.ruby-lang.org/issues/11098

* Author: Jason Clark
* Status: Open
* Priority: Normal
* Assignee: 
----------------------------------------
This patch introduces a thread-local allocation count. Today you can get a
global allocation count from `GC.stat`, but in multi-threaded contexts that
can give a muddied picture of the allocation behavior of a particular piece of
code.

Usage looks like this:

```
[2] pry(main)> Thread.new do
[2] pry(main)*   1000.times do
[2] pry(main)*     Object.new
[2] pry(main)*   end
[2] pry(main)*   puts Thread.current.allocated_objects
[2] pry(main)* end
1000
```

This would be of great interest to folks profiling Ruby code in cases where we
can't turn on more detailed object tracing tools. We currently use GC activity
as a proxy for object allocations, but this would let us be way more precise.

Obviously performance is a big concern. Looking at GET_THREAD, this doesn't
appear to have any clearly large overhead. To check this out, I ran the
following benchmark:

```
require 'benchmark/ips'

Benchmark.ips do |benchmark|
  benchmark.report "Object.new" do
    Object.new
  end

  benchmark.report "Object.new" do
    Object.new
  end

  benchmark.report "Object.new" do
    Object.new
  end
end
```

Results from a few run-throughs locally:

Commit 9955bb0 on trunk:

```
Calculating -------------------------------------
          Object.new   105.244k i/100ms
          Object.new   105.814k i/100ms
          Object.new   106.579k i/100ms
-------------------------------------------------
          Object.new      4.886M (賊 4.5%) i/s -     24.417M
          Object.new      4.900M (賊 1.9%) i/s -     24.549M
          Object.new      4.835M (賊 7.4%) i/s -     23.980M
```

With this patch:

```
Calculating -------------------------------------
          Object.new   114.248k i/100ms
          Object.new   114.508k i/100ms
          Object.new   114.472k i/100ms
-------------------------------------------------
          Object.new      4.776M (賊 5.1%) i/s -     23.878M
          Object.new      4.767M (賊 5.2%) i/s -     23.818M
          Object.new      4.818M (賊 1.5%) i/s -     24.154M
```

I don't have a good sense of whether this is an acceptable level of change or
not, but I figured without writing the code to test there was no way
to know. What do you think?

---Files--------------------------------
thread-local.patch (2.04 KB)


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