Bug #20462: Native threads are no longer reused

• Status: Open
• Assignee: -
• Target version: -
• Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN

[tenderlovemaking (Aaron Patterson)]

Ruby used to reuse native threads in order to amortize the cost of making a pthread.

For example this program:

ntids = 1000.times.map {
  Thread.new {
    Thread.current.native_thread_id
  }.value
}

p ntids.uniq.length

With Ruby 3.2.0, this would return 1. With Ruby 3.3.x, it returns 1000. It means we cannot amortize the cost of a pthread for short lived threads.

I was able to bisect this to commit be1bbd5b7d40ad863ab35097765d3754726bbd54. But the change is big so I don't know how to fix it.

[jhawthorn (John Hawthorn)]

In a benchmark of thread creation we see that 3.3 is slower than 3.2

jhawthorn@zergling:~   [ruby 3.2.2]
$ time ruby --disable-gems -e '100_000.times { Thread.new{}.join }'
ruby --disable-gems -e '100_000.times { Thread.new{}.join }'  0.73s user 1.16s system 107% cpu 1.751 total

jhawthorn@zergling:~   [ruby 3.3.1]
$ time ruby --disable-gems -e '100_000.times { Thread.new{}.join }'
ruby --disable-gems -e '100_000.times { Thread.new{}.join }'  0.55s user 1.67s system 70% cpu 3.126 total