[ruby-cvs:60190] normal:r53037 (trunk): hash.c (rb_num_hash_start): avoid pathological behavior

normal at ruby-lang.org normal at ruby-lang.org
Fri Dec 11 17:23:48 JST 2015


normal	2015-12-11 17:23:46 +0900 (Fri, 11 Dec 2015)

  New Revision: 53037

  http://svn.ruby-lang.org/cgi-bin/viewvc.cgi?view=revision&revision=53037

  Log:
    hash.c (rb_num_hash_start): avoid pathological behavior
    
    The OR-ing itself is bad for a hash function, and shifting 3 bits
    left was not enough to undo the damage done by shifting
    (RUBY_SPECIAL_SHIFT+3) bits right.  Experimentally, shifting 16-17
    bits seemed to work well in preparing the number for murmur hash.
    
    Add a few more benchmarks to based on bm_hash_shift to ensure
    we don't hurt performance too much with tweaks.
    
    I'm pretty confident about this change and commiting it now;
    especially since we're still using Murmur behind it (but perhaps
    we can update to a newer hash from Murmur...)
    
    [ruby-core:72028] [Feature #11405]
    
    target 0: a (ruby 2.3.0dev (2015-12-11 trunk 53027) [x86_64-linux]) at "/home/ew/rrrr/b/ruby"
    target 1: b (ruby 2.3.0dev (2015-12-11 master 53027) [x86_64-linux]) at "/home/ew/ruby/b/ruby"
    
    benchmark results:
    minimum results in each 5 measurements.
    Execution time (sec)
    name	a	b
    hash_aref_dsym	0.279	0.276
    hash_aref_dsym_long	4.951	4.936
    hash_aref_fix	0.281	0.283
    hash_aref_flo	0.060	0.060
    hash_aref_miss	0.409	0.410
    hash_aref_str	0.387	0.385
    hash_aref_sym	0.275	0.270
    hash_aref_sym_long	0.410	0.411
    hash_flatten	0.252	0.237
    hash_ident_flo	0.035	0.032
    hash_ident_num	0.254	0.251
    hash_ident_obj	0.252	0.256
    hash_ident_str	0.250	0.252
    hash_ident_sym	0.259	0.270
    hash_keys	0.267	0.267
    hash_shift	0.016	0.015
    hash_shift_u16	0.074	0.072
    hash_shift_u24	0.071	0.071
    hash_shift_u32	0.073	0.072
    hash_to_proc	0.008	0.008
    hash_values	0.263	0.264
    
    Speedup ratio: compare with the result of `a' (greater is better)
    name	b
    hash_aref_dsym	1.009
    hash_aref_dsym_long	1.003
    hash_aref_fix	0.993
    hash_aref_flo	1.001
    hash_aref_miss	0.996
    hash_aref_str	1.006
    hash_aref_sym	1.017
    hash_aref_sym_long	0.998
    hash_flatten	1.061
    hash_ident_flo	1.072
    hash_ident_num	1.012
    hash_ident_obj	0.987
    hash_ident_str	0.993
    hash_ident_sym	0.959
    hash_keys	0.997
    hash_shift	1.036
    hash_shift_u16	1.039
    hash_shift_u24	1.001
    hash_shift_u32	1.017
    hash_to_proc	1.001
    hash_values	0.995

  Added files:
    trunk/benchmark/bm_hash_shift_u16.rb
    trunk/benchmark/bm_hash_shift_u24.rb
    trunk/benchmark/bm_hash_shift_u32.rb
  Modified files:
    trunk/ChangeLog
    trunk/hash.c


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