[ruby-core:70156] [Ruby trunk - Bug #11396] Bad performance in ruby >= 2.2 for Hash with many symbol keys

Issue #11396 has been updated by David Unric.


I can report patch is functional and seems to be working also on current ruby_2_2 branch [ruby 2.2.3p147 (2015-07-04 revision 51143)] .

What about pushing commit also to this branch ? More testing needed ?
The speed-up is drastic, tempting to have it in next 2.2 release yet before 2.3 comes out.


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Bug #11396: Bad performance in ruby >= 2.2 for Hash with many symbol keys
https://bugs.ruby-lang.org/issues/11396#change-53583

* Author: Bruno Escherl
* Status: Closed
* Priority: Normal
* Assignee: 
* ruby -v: 
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: REQUIRED
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This started out as an issue on stackoverflow, where I found strange performance anomalies when comparing Set.include? and Array.include? in different ruby versions: http://stackoverflow.com/questions/31631284/performance-anomaly-in-ruby-set-include-with-symbols-2-2-2-vs-2-1-6

In the end it came down to problems with lookup of Hash keys. While for smaller Hashes the performance issues went away using ruby_2_2 branch, they staid for bigger Hashes. I'll attach a benchmark script (hash_bench_3.rb) I used that creates a Hash with 200000 keys and does a lookup of 10000 of them. 

Here my results:

ruby 2.1.6p336 (2015-04-13 revision 50298) [x86_64-darwin14.0]

              string    142.818  (賊 2.8%) i/s -    714.000 
              symbol    505.831  (賊 3.0%) i/s -      2.550k

ruby 2.2.2p95 (2015-04-13 revision 50295) [x86_64-darwin14]

              string    143.404  (賊 3.5%) i/s -    728.000 
              symbol     76.945  (賊 6.5%) i/s -    385.000 

ruby 2.2.3p147 (2015-07-04 revision 51143) [x86_64-darwin14] self-compiled

              string    138.349  (賊 2.2%) i/s -    702.000 
              symbol     77.495  (賊 3.9%) i/s -    392.000 

As you can see 2.2 is much slower than 2.1.6 for symbol keys. I was recommended to disable Garbage Collection for Symbols for testing and did so on the ruby_2_2 branch

ruby 2.2.3p147 (2015-07-04 revision 51143) [x86_64-darwin14] self-compiled, USE_SYMBOL_GC=0

              string    145.179  (賊 3.4%) i/s -    728.000 
              symbol    602.008  (賊 7.6%) i/s -      3.050k

I would have expected that symbol GC may have some performance impact, but this looks like it is too big. I can't say exactly at which point Garbage Collection really hurts, but the bigger the Hash and the bigger the number of include? calls, the slower it gets.

---Files--------------------------------
hash_bench_3.rb (605 Bytes)


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