[ruby-core:93818] [Ruby master Feature#15997] Improve performance of fiber creation by using pool allocation strategy.

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


There is some kind of performance regression in 2.6.3 -> 2.7.0-master.

So, I'm trying with 2.7.0-preview1 to see if it's better or worse.

```
Server Software:        
Server Hostname:        localhost
Server Port:            9294

Document Path:          /small
Document Length:        1200 bytes

Concurrency Level:      256
Time taken for tests:   17.464 seconds
Complete requests:      100000
Failed requests:        0
Total transferred:      126000000 bytes
HTML transferred:       120000000 bytes
Requests per second:    5726.11 [#/sec] (mean)
Time per request:       44.708 [ms] (mean)
Time per request:       0.175 [ms] (mean, across all concurrent requests)
Transfer rate:          7045.80 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:        0   20 137.8      1    1029
Processing:     4   24   7.8     21     428
Waiting:        0   10   8.5      9     420
Total:          4   44 138.5     23    1452

Percentage of the requests served within a certain time (ms)
  50%     23
  66%     24
  75%     28
  80%     30
  90%     34
  95%     36
  98%     45
  99%   1032
 100%   1452 (longest request)
```

2.7.0-preview1 is much worse, relatively speaking.

----------------------------------------
Feature #15997: Improve performance of fiber creation by using pool allocation strategy.
https://bugs.ruby-lang.org/issues/15997#change-79685

* 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.



---Files--------------------------------
Screen Shot 2019-07-16 at 8.30.59 PM.png (138 KB)


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