[ruby-cvs:62850] normal:r55686 (trunk): string.c: reduce malloc overhead for default buffer size

normal at ruby-lang.org normal at ruby-lang.org
Fri Jul 15 08:30:29 JST 2016

normal	2016-07-15 08:30:29 +0900 (Fri, 15 Jul 2016)

  New Revision: 55686


    string.c: reduce malloc overhead for default buffer size
    * string.c (STR_BUF_MIN_SIZE): reduce from 128 to 127
      [ruby-core:76371] [Feature #12025]
    * string.c (rb_str_buf_new): adjust for above reduction
    From Jeremy Evans <code at jeremyevans.net>:
    This changes the minimum buffer size for string buffers from 128 to
    127.  The underlying C buffer is always 1 more than the ruby buffer,
    so this changes the actual amount of memory used for the minimum
    string buffer from 129 to 128.  This makes it much easier on the
    malloc implementation, as evidenced by the following code (note that
    time -l is used here, but Linux systems may need time -v).
    $ cat bench_mem.rb
    i = ARGV.first.to_i
    Array.new(1000000){" " * i}
    $ /usr/bin/time -l ruby bench_mem.rb 128
            3.10 real         2.19 user         0.46 sys
        289080  maximum resident set size
         72673  minor page faults
            13  block output operations
            29  voluntary context switches
    $ /usr/bin/time -l ruby bench_mem.rb 127
            2.64 real         2.09 user         0.27 sys
        162720  maximum resident set size
         40966  minor page faults
             2  block output operations
             4  voluntary context switches
    To try to ensure a power-of-2 growth, when a ruby string capacity
    needs to be increased, after doubling the capacity, add one.  This
    ensures the ruby capacity will be odd, which means actual amount
    of memory used will be even, which is probably better than the
    current case of the ruby capacity being even and the actual amount
    of memory used being odd.
    A very similar patch was proposed 4 years ago in feature #5875. It
    ended up being rejected, because no performance increase was shown.
    One reason for that is that ruby does not use STR_BUF_MIN_SIZE
    unless rb_str_buf_new is called, and that previously did not have
    a ruby API, only a C API, so unless you were using a C extension
    that called it, there would be no performance increase.
    With the recently proposed feature #12024, String.buffer is added,
    which is a ruby API for creating string buffers.  Using
    String.buffer(100) wastes much less memory with this patch, as the
    malloc implementation can more easily deal with the power-of-2
    sized memory usage.  As measured above, memory usage is 44% less,
    and performance is 17% better.

  Modified files:

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