From: ko1@... Date: 2019-09-20T07:45:58+00:00 Subject: [ruby-core:95000] [Ruby master Bug#16121] Stop making a redundant hash copy in Hash#dup Issue #16121 has been updated by ko1 (Koichi Sasada). Thank you. Completely fine. Do you want to merge via github PR or attached patch by my commit? ---------------------------------------- Bug #16121: Stop making a redundant hash copy in Hash#dup https://bugs.ruby-lang.org/issues/16121#change-81628 * Author: dylants (Dylan Thacker-Smith) * Status: Open * Priority: Normal * Assignee: ko1 (Koichi Sasada) * Target version: * ruby -v: ruby 2.7.0dev (2019-08-23T16:41:09Z master b38ab0a3a9) [x86_64-darwin18] * Backport: 2.5: UNKNOWN, 2.6: UNKNOWN ---------------------------------------- ## Problem I noticed while profiling object allocations that Hash#dup was allocating 2 objects instead of only 1 as expected. I looked for alternatives for comparison and found that `Hash[hash]` created a copy with only a single object allocation and seemed to be more than twice as fast. Reading the source code revealed the difference was that Hash#dup creates a copy of the Hash, then rehashes the copy. However, rehashing is done by making a copy of the hash, so the first copy before rehashing was unnecessary. ## Solution I changed the code to just use rehashing to make the copy of the hash to improve performance while also preserving the existing behaviour. ## Benchmark ```ruby require 'benchmark' N = 100000 def report(x, name) x.report(name) do N.times do yield end end end hashes = { small_hash: { a: 1 }, larger_hash: 20.times.map { |i| [('a'.ord + i).chr.to_sym, i] }.to_h } Benchmark.bmbm do |x| hashes.each do |name, hash| report(x, "#{name}.dup") do hash.dup end end end ``` results on master ``` user system total real small_hash.dup 0.401350 0.001638 0.402988 ( 0.404608) larger_hash.dup 7.218548 0.433616 7.652164 ( 7.695990) ``` results with the attached patch ``` user system total real small_hash.dup 0.336733 0.002425 0.339158 ( 0.341760) larger_hash.dup 6.617343 0.398407 7.015750 ( 7.070282) ``` ---Files-------------------------------- 0001-Remove-redundant-Check_Type-after-to_hash.diff.txt (624 Bytes) 0002-Fix-freeing-and-clearing-destination-hash-in-Hash.diff.txt (1.57 KB) 0003-Remove-dead-code-paths-in-rb_hash_initialize_copy.diff.txt (1.12 KB) 0004-Stop-making-a-redundant-hash-copy-in-Hash-dup.diff.txt (1.35 KB) -- https://bugs.ruby-lang.org/ Unsubscribe: