Feature #15997
Updated by ioquatix (Samuel Williams) over 5 years ago
https://github.com/ruby/ruby/pull/2224
This PR improves the performance of fiber allocation and reuse by implementing a better stack cache. As per @ko1's request, we also increased fiber stack size to be the same as thread stack size.
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.