From: zdennis <zdennis@...>
Date: 2006-02-28T05:37:16+09:00
Subject: Re: Memory Question

H. Yamamoto,

I posted my post and your reply to ruby-core. Hopefully this can get clarified or as you suspect, 
the problem you potentially found can be fixed, Thank you for your reply,

Zach

H.Yamamoto wrote:
> 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)
> 
>