Freeze string literals when not mutated.

[schneems]

I wrote a utility that helps find areas where you could optimize your program using a frozen string instead of a string literal, it's called let_it_go. After going through the output and adding .freeze I was able to eliminate the creation of 1,114 string objects on EVERY request to codetriage. How does this impact execution?

To look at memory:

require 'get_process_mem'

mem = GetProcessMem.new
GC.start
GC.disable
1_114.times { " " }
before = mem.mb

after = mem.mb
GC.enable
puts "Diff: #{after - before} mb"

Creating 1,114 string objects results in Diff: 0.03125 mb of RAM allocated on every request. Or 1mb every 32 requests.

To look at raw speed:

require 'benchmark/ips'

number_of_objects_reduced = 1_114

Benchmark.ips do |x|
  x.report("freeze")    { number_of_objects_reduced.times { " ".freeze } }
  x.report("no-freeze") { number_of_objects_reduced.times { " " } }
end

We get the results

Calculating -------------------------------------
              freeze     1.428k i/100ms
           no-freeze   609.000  i/100ms
-------------------------------------------------
              freeze     14.363k (± 8.5%) i/s -     71.400k
           no-freeze      6.084k (± 8.1%) i/s -     30.450k

Now we can do some maths:

ips = 6_226k # iterations / 1 second
call_time_before = 1.0 / ips # seconds per iteration 

ips = 15_254 # iterations / 1 second
call_time_after = 1.0 / ips # seconds per iteration 

diff = call_time_before - call_time_after

number_of_objects_reduced * diff * 100

# => 0.4530373333993266 miliseconds saved per request

So we're shaving off 1 second of execution time for every 220 requests.

Is this going to be an insane speed boost to any Rails app: nope. Should we merge it: yep.

p.s. If you know of a method call that doesn't modify a string input such as String#gsub please give me a pull request to the appropriate file, or open an issue in LetItGo so we can track and freeze more strings.

Keep those strings Frozen

[schneems]

Wanted to mention that I did the above math wrong. Since each "iteration" of the test created 1_114 objects. So the actual time saved would be what I reported divided by 1_114.

Also the Ram test I did, returns the same result even if you remove the string allocation lines. Benchmarking is hard.

Over a really really long time, it will have an impact, although a small one. Since the change is so straightforward, i'm comfortable with even modest gains here.