Feature #12025
Updated by naruse (Yui NARUSE) about 8 years ago
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.