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@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.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55686 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

2 files changed, 8 insertions, 2 deletions

diff --git a/ChangeLog b/ChangeLog
index 8ddd4ad666..9e378c1088 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Fri Jul 15 08:25:15 2016  Jeremy Evans <code@jeremyevans.net>
+
+	* 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
+
 Thu Jul 14 17:26:00 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* Makefile.in (enc/unicode/name2ctype.h): remove stale recipe,
diff --git a/string.c b/string.c
index ff3792ffb2..4bc4ebfd99 100644
--- a/string.c
+++ b/string.c
@@ -1188,7 +1188,7 @@ str_new_empty(VALUE str)
     return v;
 }
 
-#define STR_BUF_MIN_SIZE 128
+#define STR_BUF_MIN_SIZE 127
 
 VALUE
 rb_str_buf_new(long capa)
@@ -2590,7 +2590,7 @@ str_buf_cat(VALUE str, const char *ptr, long len)
 		    capa = (total + 4095) / 4096 * 4096;
 		    break;
 		}
-		capa = 2 * capa;
+		capa = 2 * capa + 1;
 	    }
 	}
 	else {