Bug #15625

Module#name performance has exponential-time worst case by aliased constants

Added by nelhage (Nelson Elhage) over 1 year ago. Updated about 1 year ago.

Target version:
ruby -v:
ruby 2.6.1p33 (2019-01-30 revision 66950) [x86_64-darwin18]


It's well-known (c.f #11119) that Module#name has poor performance on anonymous classes, due to searching the entire constant namespace of the VM.

However, we recently discovered that it's even worse than that. In the presence of aliased constants (e.g. module M; end; Alias = M), the find_class_path method actually searches every path to a given module (e.g. it will visit that module under both the M and Alias names). This can lead to exponential-time behavior in cases of repeated aliases. See the attached test case, where performance gets twice as slow with every additional layer of aliasing:

On my laptop:

       user     system      total        real
before  2.183899   0.012151   2.196050 (  2.355187)
       user     system      total        real
a19  5.614596   0.024394   5.638990 (  5.702878)
       user     system      total        real
a10  9.204399   0.052817   9.257216 (  9.391010)
       user     system      total        real
a11 16.184035   0.060854  16.244889 ( 16.561101)

Our house style makes extensive use of aliases as an import-like mechanism to provide short names within a given scope. We discovered this bug while exploring an actual performance problem -- this is not just a theoretical report.


classname.rb (854 Bytes) classname.rb nelhage (Nelson Elhage), 02/27/2019 05:35 PM
bug-15625.patch (1.38 KB) bug-15625.patch nobu (Nobuyoshi Nakada), 03/04/2019 11:52 AM

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