[ruby-core:93807] [Ruby master Feature#15997] Improve performance of fiber creation by using pool allocation strategy.

Issue #15997 has been updated by ioquatix (Samuel Williams).


On Darwin, comparing Fiber-pool with master:

```
> make benchmark COMPARE_RUBY="../../ruby/build/ruby --disable-gems" ITEM=vm2_fiber RUBY_SHARED_FIBER_POOL_FREE_STACKS=0
Calculating -------------------------------------
                           master  fiber-pool 
  vm2_fiber_allocate      99.329k    124.488k i/s -    100.000k times in 1.006759s 0.803293s
     vm2_fiber_count       3.621k     82.447k i/s -    100.000k times in 27.620062s 1.212895s
     vm2_fiber_reuse       55.039     615.402 i/s -     200.000 times in 3.633812s 0.324991s
    vm2_fiber_switch       8.803M      8.591M i/s -     20.000M times in 2.272063s 2.328041s

Comparison:
               vm2_fiber_allocate
          built-ruby:    124487.6 i/s 
        compare-ruby:     99328.6 i/s - 1.25x  slower

                  vm2_fiber_count
          built-ruby:     82447.4 i/s 
        compare-ruby:      3620.6 i/s - 22.77x  slower

                  vm2_fiber_reuse
          built-ruby:       615.4 i/s 
        compare-ruby:        55.0 i/s - 11.18x  slower

                 vm2_fiber_switch
        compare-ruby:   8802572.8 i/s 
          built-ruby:   8590914.0 i/s - 1.02x  slower

> make benchmark COMPARE_RUBY="../../ruby/build/ruby --disable-gems" ITEM=vm2_fiber RUBY_SHARED_FIBER_POOL_FREE_STACKS=1

Calculating -------------------------------------
                           master  fiber-pool 
  vm2_fiber_allocate      96.834k    121.823k i/s -    100.000k times in 1.032698s 0.820865s
     vm2_fiber_count       3.027k     80.419k i/s -    100.000k times in 33.035732s 1.243489s
     vm2_fiber_reuse       56.275     449.230 i/s -     200.000 times in 3.553979s 0.445206s
    vm2_fiber_switch       8.640M      8.255M i/s -     20.000M times in 2.314890s 2.422917s

Comparison:
               vm2_fiber_allocate
          built-ruby:    121822.7 i/s 
        compare-ruby:     96833.7 i/s - 1.26x  slower

                  vm2_fiber_count
          built-ruby:     80418.9 i/s 
        compare-ruby:      3027.0 i/s - 26.57x  slower

                  vm2_fiber_reuse
          built-ruby:       449.2 i/s 
        compare-ruby:        56.3 i/s - 7.98x  slower

                 vm2_fiber_switch
        compare-ruby:   8639719.4 i/s 
          built-ruby:   8254513.1 i/s - 1.05x  slower
```

----------------------------------------
Feature #15997: Improve performance of fiber creation by using pool allocation strategy.
https://bugs.ruby-lang.org/issues/15997#change-79676

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* Target version: 
----------------------------------------
https://github.com/ruby/ruby/pull/2224

This PR improves the performance of fiber allocation and reuse by implementing a better stack cache.

The fiber pool manages a singly linked list of fiber pool allocations. The fiber pool allocation contains 1 or more stack (typically more, e.g. 512). It uses N^2 allocation strategy, starting at 8 initial stacks, next is 8, 16, 32, etc.

```
//
// base = +-------------------------------+-----------------------+  +
//        |VM Stack       |VM Stack       |                       |  |
//        |               |               |                       |  |
//        |               |               |                       |  |
//        +-------------------------------+                       |  |
//        |Machine Stack  |Machine Stack  |                       |  |
//        |               |               |                       |  |
//        |               |               |                       |  |
//        |               |               | .  .  .  .            |  |  size
//        |               |               |                       |  |
//        |               |               |                       |  |
//        |               |               |                       |  |
//        |               |               |                       |  |
//        |               |               |                       |  |
//        +-------------------------------+                       |  |
//        |Guard Page     |Guard Page     |                       |  |
//        +-------------------------------+-----------------------+  v
//
//        +------------------------------------------------------->
//
//                                  count
//
```

The performance improvement depends on usage:

```
Calculating -------------------------------------
                     compare-ruby  built-ruby 
  vm2_fiber_allocate     132.900k    180.852k i/s -    100.000k times in 0.752447s 0.552939s
     vm2_fiber_count       5.317k    110.724k i/s -    100.000k times in 18.806479s 0.903145s
     vm2_fiber_reuse      160.128     347.663 i/s -     200.000 times in 1.249003s 0.575269s
    vm2_fiber_switch      13.429M     13.490M i/s -     20.000M times in 1.489303s 1.482549s

Comparison:
               vm2_fiber_allocate
          built-ruby:    180851.6 i/s 
        compare-ruby:    132899.7 i/s - 1.36x  slower

                  vm2_fiber_count
          built-ruby:    110724.3 i/s 
        compare-ruby:      5317.3 i/s - 20.82x  slower

                  vm2_fiber_reuse
          built-ruby:       347.7 i/s 
        compare-ruby:       160.1 i/s - 2.17x  slower

                 vm2_fiber_switch
          built-ruby:  13490282.4 i/s 
        compare-ruby:  13429100.0 i/s - 1.00x  slower
```

This test is run on Linux server with 64GB memory and 4-core Xeon (Intel Xeon CPU E3-1240 v6 @ 3.70GHz). "compare-ruby" is `master`, and "built-ruby" is `master+fiber-pool`.

Additionally, we conservatively use `madvise(free)` to avoid swap space usage for unused fiber stacks. However, if you remove this requirement, we can get 6x - 10x performance improvement in `vm2_fiber_reuse` benchmark. There are some options to deal with this (e.g. moving it to `GC.compact`) but as this is still a net win, I'd like to merge this PR as is.





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