On Sep 5, 2007, at 8:50 PM, Cd Cd wrote:

> Shouldn't the output be all done at once since the code is running in
> parallel?

what would that look like exactly though?  ;-)

seriously there are more than one reason that cannot happen.

1) the console being printed to can only allow one thing to write to  
it at a time.  generally speaking the console is line buffered up to  
some limit so, as long as a program writes a chunk of chars less that  
that limit the lines will not appear to intermixed.  things get more  
complicated when programs are not writing to the console, but to file  
instead.

2) threads are never run in parallel from the perspective of the  
computer: the cpu can only run one program at once.  it simply does  
very intelligent switching very quickly to make you think it's doing  
more than one thing ;-).  with ruby's threads, which are known as  
'green' threads, ruby itself does this switching for you.  with  
'native' threads the switching is done by the operating system.  in  
either case the concept of 'parallel' is really from the perspective  
of the programmer.  this isn't strictly true as sometimes a program  
might be able to write to disk or the network but not need the cpu -  
in that case it might do two things at once.  the central concept is  
basically that a thread is a programming abstraction for  
*programmers* to imagine themselves getting more done.  sometimes  
it's a useful one.

now, in a multi-cpu machine this gets even murkier - threads (native  
ones not green ones) are actually very close to a whole process and  
the operating system may in fact allow to bits of your program to run  
on two cpus.  in the end you must assume that only one piece of a  
program is using a given piece of computer hardware at once, since we  
simply cannot defy physics,  but there are various approaches and  
abstractions that help us let the computer help us, like threads, by  
structuring our program such that the operating system *might* be  
able to run two bit s of our code on two bits of hardware at once.

3) threads are evil and best understood via meditation - not actual  
thinking.

kind regards.

a @ http://drawohara.com/
--
we can deny everything, except that we have the possibility of being  
better. simply reflect on that.
h.h. the 14th dalai lama