From: "H.Yamamoto" <ocean@...2.ccsnet.ne.jp>
Date: 2006-02-27T13:55:51+09:00
Subject: Re: Memory Question

Hello.

Ruby's GC is conservative, so there is no guarantee object is freed even if
object is not reachable from GC's root.

But anyway, probably I found the problem on ELTS_SHARED.

/////////////////////////////////

def pause
  GC.start
  $stdout.puts "measure memory and hit any key..."
  $stdout.flush
  $stdin.getc
end

pause

a = Array.new(10000){ "." * 1000 } # huge memory

pause

a.map!{|s| s[-100..-1]} # memory stays large

pause

a.map!{|s| s[-3..-1]} # reduces memory

pause


/////////////////////////////////

This is because rb_str_substr (string.c) 's

    else if (len > sizeof(struct RString)/2 &&
	beg + len == RSTRING(str)->len && !FL_TEST(str, STR_ASSOC)) {
	str2 = rb_str_new3(rb_str_new4(str));
	RSTRING(str2)->ptr += RSTRING(str2)->len - len;
	RSTRING(str2)->len = len;
    }

is executed at

  a.map!{|s| s[-100..-1]} # memory stays large

rb_str_new3 generates ELS_SHARED RString which holds original RString.

When original string becomes unreachable, it should be garbage collected.
But ELTS_SHARED substring references it (RString#aux->shared), so not collected
until substring itself becomes unreachable.

I haven't confirmed this is really cause of your problem, but there is possibility
this hidden huge string eats memory. (maybe same thing happens on Array)