[ruby-core:47588] [ruby-trunk - Feature #905] Add String.new(fixnum) to preallocate large buffer

Issue #905 has been updated by shyouhei (Shyouhei Urabe).


Just a technical comment, not for the feature itself:

headius (Charles Nutter) wrote:
>      to by ptr, realloc() creates a new allocation, copies as much of the old data

This "copy" is done by mremap(2) system call, which just reassembles OS's process-private virtual memory map to move a region of memory to another, in O(1).  That is what mame said in "This feature is meaningless with MRI, at least, on Linux."
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Feature #905: Add String.new(fixnum) to preallocate large buffer
https://bugs.ruby-lang.org/issues/905#change-29511

Author: headius (Charles Nutter)
Status: Feedback
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: 
Target version: 2.0.0


=begin
 Because Strings are used in ruby as arbitrary byte buffers, and because the cost of growing a String increases as it gets larger (especially when it starts small), String.new should support a form that takes a fixnum and ensures the backing store will have at least that much room. This is analogous to Array.new(fixnum) which does the same thing.
 
 The simple implementation of this would just add a Fixnum check to the String.new method, and the result would be an empty string with that size buffer. This would allow heavy string-appending algorithms and libraries (like ERb) to avoid doing so many memory copies while they run.
=end



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