[ruby-core:33809] [Ruby 1.9-Feature#4184] String that has the same object_id in an each occurrence in a code

Issue #4184 has been updated by Kurt  Stephens.



The cost of GC increases with the number of allocated and referenced objects.  The copy-on-write internal String buffers reduces the needless copying of the String buffers, if they are likely to be dup'ed and not mutated, but does not improve collection times.

  FOO = 'foobar'.freeze
  def foo
    FOO.sub('bar', 'baz')
  end

performs much better than:

  def foo
    'foobar'.sub('bar', 'baz')
  end

because FOO.object_id always == FOO.object_id, where as 'foobar'.object_id != 'foobar'.object_id.  'foobar' immediately becomes unreachable after String#sub; it's allocation is pointless.  Every lexical String "constant" allocates a new object.

The same is true for ARRAY = [ :foo, :bar ].freeze .vs. inline [ :foo, :bar ].  

I've been able to get 2-3% improvements in Rails apps by simply rewriting some 'constant's and inline Arrays as CONSTANTs.

I have patches to MRI that use cached, immutable Strings for the internal #to_s messages on immutable objects; e.g. changing Symbol#to_s, Float#to_s, Bignum#to_s, Rational#to_s, etc. to return the same frozen String instance.  I measured 1-6% performance improvement in the standard MRI tests.

The cost of stop-the-world, mark/sweep GC is not in the allocation, it's in collection.  Allocating fewer objects improves both phases.

A generic, thread-safe, "memoize expression" lexical syntax would very useful.  Maybe something like %m('foo') or %m([ :foo, :bar ]) and %M('foo') for the %m('foo'.freeze) variant.



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http://redmine.ruby-lang.org/issues/show/4184

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