1. 10 Jul, 2017 1 commit
    • Sergey Senozhatsky's avatar
      lib/bsearch.c: micro-optimize pivot position calculation · 166a0f78
      Sergey Senozhatsky authored
      There is a slightly faster way (in terms of the number of instructions
      being used) to calculate the position of a middle element, preserving
      integer overflow safeness.
      
      ./scripts/bloat-o-meter lib/bsearch.o.old lib/bsearch.o.new
      add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-24 (-24)
      function                                     old     new   delta
      bsearch                                      122      98     -24
      
      TEST
      
      INT array of size 100001, elements [0..100000]. gcc 7.1, Os, x86_64.
      
      a) bsearch() of existing key "100001 - 2":
      
      BASE
      ====
      
      $ perf stat ./a.out
      
       Performance counter stats for './a.out':
      
              619.445196      task-clock:u (msec)       #    0.999 CPUs utilized
                       0      context-switches:u        #    0.000 K/sec
                       0      cpu-migrations:u          #    0.000 K/sec
                     133      page-faults:u             #    0.215 K/sec
           1,949,517,279      cycles:u                  #    3.147 GHz                      (83.06%)
             181,017,938      stalled-cycles-frontend:u #    9.29% frontend cycles idle     (83.05%)
              82,959,265      stalled-cycles-backend:u  #    4.26% backend cycles idle      (67.02%)
           4,355,706,383      instructions:u            #    2.23  insn per cycle
                                                        #    0.04  stalled cycles per insn  (83.54%)
           1,051,539,242      branches:u                # 1697.550 M/sec                    (83.54%)
              15,263,381      branch-misses:u           #    1.45% of all branches          (83.43%)
      
             0.620082548 seconds time elapsed
      
      PATCHED
      =======
      
      $ perf stat ./a.out
      
       Performance counter stats for './a.out':
      
              475.097316      task-clock:u (msec)       #    0.999 CPUs utilized
                       0      context-switches:u        #    0.000 K/sec
                       0      cpu-migrations:u          #    0.000 K/sec
                     135      page-faults:u             #    0.284 K/sec
           1,487,467,717      cycles:u                  #    3.131 GHz                      (82.95%)
             186,537,162      stalled-cycles-frontend:u #   12.54% frontend cycles idle     (82.93%)
              28,797,869      stalled-cycles-backend:u  #    1.94% backend cycles idle      (67.10%)
           3,807,564,203      instructions:u            #    2.56  insn per cycle
                                                        #    0.05  stalled cycles per insn  (83.57%)
           1,049,344,291      branches:u                # 2208.693 M/sec                    (83.60%)
                   5,485      branch-misses:u           #    0.00% of all branches          (83.58%)
      
             0.475760235 seconds time elapsed
      
      b) bsearch() of un-existing key "100001 + 2":
      
      BASE
      ====
      
      $ perf stat ./a.out
      
       Performance counter stats for './a.out':
      
              499.244480      task-clock:u (msec)       #    0.999 CPUs utilized
                       0      context-switches:u        #    0.000 K/sec
                       0      cpu-migrations:u          #    0.000 K/sec
                     132      page-faults:u             #    0.264 K/sec
           1,571,194,855      cycles:u                  #    3.147 GHz                      (83.18%)
              13,450,980      stalled-cycles-frontend:u #    0.86% frontend cycles idle     (83.18%)
              21,256,072      stalled-cycles-backend:u  #    1.35% backend cycles idle      (66.78%)
           4,171,197,909      instructions:u            #    2.65  insn per cycle
                                                        #    0.01  stalled cycles per insn  (83.68%)
           1,009,175,281      branches:u                # 2021.405 M/sec                    (83.79%)
                   3,122      branch-misses:u           #    0.00% of all branches          (83.37%)
      
             0.499871144 seconds time elapsed
      
      PATCHED
      =======
      
      $ perf stat ./a.out
      
       Performance counter stats for './a.out':
      
              399.023499      task-clock:u (msec)       #    0.998 CPUs utilized
                       0      context-switches:u        #    0.000 K/sec
                       0      cpu-migrations:u          #    0.000 K/sec
                     134      page-faults:u             #    0.336 K/sec
           1,245,793,991      cycles:u                  #    3.122 GHz                      (83.39%)
              11,529,273      stalled-cycles-frontend:u #    0.93% frontend cycles idle     (83.46%)
              12,116,311      stalled-cycles-backend:u  #    0.97% backend cycles idle      (66.92%)
           3,679,710,005      instructions:u            #    2.95  insn per cycle
                                                        #    0.00  stalled cycles per insn  (83.47%)
           1,009,792,625      branches:u                # 2530.660 M/sec                    (83.46%)
                   2,590      branch-misses:u           #    0.00% of all branches          (83.12%)
      
             0.399733539 seconds time elapsed
      
      Link: http://lkml.kernel.org/r/20170607150457.5905-1-sergey.senozhatsky@gmail.comSigned-off-by: 's avatarSergey Senozhatsky <sergey.senozhatsky@gmail.com>
      Cc: Peter Zijlstra <peterz@infradead.org>
      Signed-off-by: 's avatarAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: 's avatarLinus Torvalds <torvalds@linux-foundation.org>
      166a0f78
  2. 07 Mar, 2012 1 commit
  3. 19 May, 2011 1 commit