From: Matt Armstrong <matt@...>
Date: 2002-04-21T04:08:05+09:00
Subject: Re: Threads creating threads creating threads...

Tobias Peters <tpeters@uni-oldenburg.de> writes:

> My previous posting was not clear enough, I'll try to enhance it.
>
> I include a ruby program in this message that should behave like
> this:
>
> 1) The main thread creates child thread with No n=1 
> 2) Child thread n creates child thread n+1
> 3) Child thread n exits and some other thread calls join upon it
> 4) Goto 2) with n=n+1
>
> So this is an endless loop, the example program should run forever,
> and threads are created and destroyed as it runs. The memory usage
> of this program should be constant, because at any given time, there
> are at most 3 threads living simultaneously.

Ideally, your program would run forever.

But when a new thread is created, the stack of the current thread is
included in the new thread.  So if a child thread always spawns
another thread the stack size will grow infinitely.

Ruby's thread implementation isn't the best.  It is slow since
switching threads involves physically copying the stack memory.  And
you discovered another problem -- a new thread will always use the
stack memory of the thread that created it even though it will never
be used.

Despite the weaknesses, Ruby's threads are very portable since they
rely only on setjmp()/longjmp().  Maybe there are other reasons for
the design too?


-- 
matt