[ruby-core:101702] [Ruby master Bug#17373] Ruby 3.0 is slower at Discourse bench than Ruby 2.7

Issue #17373 has been updated by ioquatix (Samuel Williams).


In my early benchmarks:

## ruby 2.7.1p83 (2020-03-31 revision a0c7c23c9c) [x86_64-darwin19]

```
Running 2s test @ http://127.0.0.1:9294/
  8 threads and 8 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   228.46us   86.09us   3.82ms   83.40%
    Req/Sec     4.32k     1.07k    5.80k    50.60%
  72205 requests in 2.10s, 3.44MB read
Requests/sec:  34380.26
Transfer/sec:      1.64MB
```

## ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-darwin20]

```
Running 2s test @ http://127.0.0.1:9294/
  8 threads and 8 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   252.91us  107.48us   1.15ms   69.00%
    Req/Sec     3.90k     1.46k    5.87k    39.88%
  65106 requests in 2.10s, 3.10MB read
Requests/sec:  31001.91
Transfer/sec:      1.48MB
```

----------------------------------------
Bug #17373: Ruby 3.0 is slower at Discourse bench than Ruby 2.7
https://bugs.ruby-lang.org/issues/17373#change-89528

* Author: sam.saffron (Sam Saffron)
* Status: Open
* Priority: Normal
* ruby -v: 3.0
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
We have a continuous effort to keep https://rubybench.org/ up to date. Unfortunately we have lapsed a bit on updates so I decided to run Discourse bench by hand.

On my local system I am noticing the timing are much slower on Discourse bench than they were on Ruby 2.7: 

```
98c9120cc00ba691b4abcc13a49a30fa54638535

---
categories:
  50: 34
  75: 35
  90: 36
  99: 40
home:
  50: 47
  75: 50
  90: 56
  99: 92
topic:
  50: 53
  75: 55
  90: 58
  99: 92
categories_admin:
  50: 33
  75: 34
  90: 38
  99: 68
home_admin:
  50: 45
  75: 47
  90: 51
  99: 95
topic_admin:
  50: 51
  75: 52
  90: 54
  99: 59
timings:
  load_rails: 1449
ruby-version: 3.0.0-p-1
rss_kb: 293808
pss_kb: 284244

2.7.2


---
categories:
  50: 29
  75: 30
  90: 33
  99: 43
home:
  50: 42
  75: 43
  90: 47
  99: 73
topic:
  50: 47
  75: 47
  90: 52
  99: 54
categories_admin:
  50: 30
  75: 31
  90: 36
  99: 56
home_admin:
  50: 41
  75: 43
  90: 48
  99: 71
topic_admin:
  50: 47
  75: 48
  90: 52
  99: 55
timings:
  load_rails: 1339
ruby-version: 2.7.2-p137
rss_kb: 296836
pss_kb: 287310


2.6.6


---
categories:
  50: 30
  75: 30
  90: 31
  99: 51
home:
  50: 40
  75: 42
  90: 60
  99: 79
topic:
  50: 47
  75: 48
  90: 53
  99: 71
categories_admin:
  50: 30
  75: 31
  90: 35
  99: 50
home_admin:
  50: 41
  75: 43
  90: 62
  99: 100
topic_admin:
  50: 48
  75: 49
  90: 65
  99: 93
timings:
  load_rails: 1464
ruby-version: 2.6.6-p146
rss_kb: 332268
pss_kb: 322865
```

Concretely this means a typical "homepage" view is taking 47 milliseconds now, when in the past it was taking 42 or even 40 ms.

There may be some issues with the bench, you can see it here: 

https://github.com/discourse/discourse/tree/ruby-3

The script to run the bench is `ruby script/bench.rb`

I re-tested against a cleanly built Ruby 2.7 direct from the Git branch to ensure compilation was not at fault.


This is pretty concerning, Discourse bench usually tracks Rails performance pretty accurately. 

---Files--------------------------------
aaron — perf _home_aaron_git_railsbench — ssh whiteclaw.local — 175×54 2020-12-07 12-41-12.png (842 KB)
aaron — perf _home_aaron_git_railsbench — ssh whiteclaw.local — 175×54 2020-12-07 12-41-58.png (874 KB)
aaron — perf _home_aaron_git_railsbench — ssh whiteclaw.local — 299×65 2020-12-07 13-19-20.png (142 KB)


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