From: Bob Hutchison <hutch@...>
Date: 2006-09-05T21:45:30+09:00
Subject: Re: The real difference between Mutex and Sync


On Sep 5, 2006, at 4:25 AM, khaines@enigo.com wrote:

> On Tue, 5 Sep 2006, Kent Sibilev wrote:
>
>> This patch will make Mutex a bit slower, but much better in terms of
>> garbage collection:
>>
>> Index: lib/thread.rb
>> ===================================================================
>> RCS file: /src/ruby/lib/thread.rb,v
>> retrieving revision 1.16.2.2
>> diff -r1.16.2.2 thread.rb
>> 99c99
>> <       @waiting.push Thread.current
>> ---
>>>       @waiting.unshift Thread.current
>> 115c115
>> <       t = @waiting.shift
>> ---
>>>       t = @waiting.pop
>
> While I am not benchmarking with performance in mind, that change  
> doesn't seem to have any significant effect on overall speed, which  
> doesn't surprise me.  Speed-wise, it is just flipping the location  
> of the expensive array operation from the unlock to the lock.
>
> And yes, that simple change does seem to make a significant  
> difference, because pop will realloc the array.


I've attached a trivial script that demonstrates on OS X and likely  
linux (unlikely windows because of the ps stuff) the problem. To see  
the bug, run it like:

	ruby blowup2.rb

A *simple* fix is to just remove the reference to whatever is pointed  
to by the first element. To see this work, run the script as:

	ruby blowup2.rb fix

This trick will probably lead to a quicker fix (and if you are daring  
you might actually patch the C implementation of shift to do it for  
you). Might want to hear what Matz has to say about that.

Cheers,
Bob

----
Bob Hutchison                  -- blogs at <http://www.recursive.ca/ 
hutch/>
Recursive Design Inc.          -- <http://www.recursive.ca/>
Raconteur                      -- <http://www.raconteur.info/>
xampl for Ruby                 -- <http://rubyforge.org/projects/xampl/>



$VERBOSE = nil
STDOUT.sync = true
trap('INT'){ exit }

fixit = 0 < ARGV.size

m = 1000000
n = 1000

all_arrays = []
first = true

n.times do |i|
   if 0 < m then
     a = ["x" * m ]
     all_arrays << a
     a[0] = nil if fixit and 0 < a.size
     a.shift
   else
     a = []
     all_arrays << a
   end

   if 0 == (all_arrays.size % 10) then
     GC.start
     stdout = `ps v -p #{ Process.pid }`
     stdout = stdout.split(%r/\n/)
     if first then
       printf("\n         %s\n", stdout.first)
       first = false
     end
     printf("%6d:: %s\n", all_arrays.size, stdout.last)
   end
end