rails/rails — pull request #17093 by phiggins (2014-09-29 03:27:34+00:00)
Title: Reduce allocations when running AR callbacks.

Inspired by @tenderlove's work in c363fff29f060e6a2effe1e4bb2c4dd4cd805d6e, this reduces the number of strings allocated when running callbacks for ActiveRecord instances. I measured that using this script:

```
require 'objspace'
require 'active_record'
require 'allocation_tracer'

ActiveRecord::Base.establish_connection adapter: "sqlite3",
database: ":memory:"

ActiveRecord::Base.connection.instance_eval do
create_table(:articles) { |t| t.string :name }
end

class Article < ActiveRecord::Base; end
a = Article.create name: "foo"
a = Article.find a.id

N = 10
result = ObjectSpace::AllocationTracer.trace do
N.times { Article.find a.id }
end

result.sort.each do |k,v|
p k => v
end
puts "total: #{result.values.map(&:first).inject(:+)}"
```

When I run this against master and this branch I get this output:

```
pete@balloon:~/projects/rails/activerecord$ git checkout master
M Gemfile
Switched to branch 'master'
pete@balloon:~/projects/rails/activerecord$ bundle exec ruby benchmark_allocation_with_callback_send.rb > allocations_before
pete@balloon:~/projects/rails/activerecord$ git checkout remove-dynamic-send-on-built-in-callbacks
M Gemfile
Switched to branch 'remove-dynamic-send-on-built-in-callbacks'
pete@balloon:~/projects/rails/activerecord$ bundle exec ruby benchmark_allocation_with_callback_send.rb > allocations_after
pete@balloon:~/projects/rails/activerecord$ diff allocations_before allocations_after
39d38
<
{["/home/pete/projects/rails/activesupport/lib/active_support/callbacks.rb",
81]=>[40, 0, 0, 0, 0, 0]}
42c41
< total: 630
---
> total: 590

```

In addition to this, there are two micro-optimizations present:

* Using `block.call if block` vs `yield if block_given?` when the block was being captured already.

```
pete@balloon:~/projects$ cat benchmark_block_call_vs_yield.rb
require 'benchmark/ips'

def block_capture_with_yield &block
yield if block_given?
end

def block_capture_with_call &block
block.call if block
end

def no_block_capture
yield if block_given?
end

Benchmark.ips do |b|
b.report("block_capture_with_yield") { block_capture_with_yield }
b.report("block_capture_with_call") { block_capture_with_call }
b.report("no_block_capture") { no_block_capture }
end
pete@balloon:~/projects$ ruby benchmark_block_call_vs_yield.rb
Calculating -------------------------------------
block_capture_with_yield
124979 i/100ms
block_capture_with_call
138340 i/100ms
no_block_capture 136827 i/100ms
-------------------------------------------------
block_capture_with_yield
5703108.9 (±2.4%) i/s - 28495212 in 4.999368s
block_capture_with_call
6840730.5 (±3.6%) i/s - 34169980 in 5.002649s
no_block_capture 5821141.4 (±2.8%) i/s - 29144151 in 5.010580s
```

* Defining and calling methods instead of using send.

```
pete@balloon:~/projects$ cat benchmark_method_call_vs_send.rb
require 'benchmark/ips'

class Foo
def tacos
nil
end
end

my_foo = Foo.new

Benchmark.ips do |b|
b.report('send') { my_foo.send('tacos') }
b.report('call') { my_foo.tacos }
end
pete@balloon:~/projects$ ruby benchmark_method_call_vs_send.rb
Calculating -------------------------------------
send 97736 i/100ms
call 151142 i/100ms
-------------------------------------------------
send 2683730.3 (±2.8%) i/s - 13487568 in 5.029763s
call 8005963.9 (±2.7%) i/s - 40052630 in 5.006604s
```

The result of this is making typical ActiveRecord operations slightly faster:

https://gist.github.com/phiggins/e46e51dcc7edb45b5f98