From: Laurent Sansonetti <laurent.sansonetti@...>
Date: 2007-07-04T07:26:08+09:00
Subject: Re: Is it possible to detect thread switches?

Hi,

On 7/3/07, Charlie Savage <cfis@savagexi.com> wrote:
> Is there a way to detect a thread context switch? I'm interested, due to
> my work on ruby-prof.  Currently ruby-prof will report incorrect times
> for multi-threaded applications.  Take for instance this code:
>
> def test_thread_timings
>    RubyProf.start
>
>    sleep(2)
>
>    thread = Thread.new do
>      sleep(2)
>    end
>
>    thread.join
>
>    result = RubyProf.stop
> end
>
> ruby-prof will say the inner thread took 2 seconds, but that the outer
> thread took 4 (when in fact it was asleep doing nothing for 2 of those
> seconds).
>
>
> ruby-prof works by adding an event hook:
>
>      rb_add_event_hook(prof_event_hook,
>            RUBY_EVENT_CALL | RUBY_EVENT_RETURN |
>            RUBY_EVENT_C_CALL | RUBY_EVENT_C_RETURN
>            | RUBY_EVENT_LINE);
>
> This method gets invoked when a function is entered or left.  At that
> point ruby-prof knows the current method and thread.  But if a thread
> switch happens in the middle of the method (assumedly most of the time),
> ruby-prof doesn't know it and thus can't stop running the timer on the
> first thread.
>
> Is there anyway to detect this?

For RubyCocoa we wrote a patch for the interpreter to be callbacked
upon specific threading events, in order to properly save and restore
the NSThread contextual data.

http://rubycocoa.svn.sourceforge.net/viewvc/*checkout*/rubycocoa/trunk/src/misc/ruby_thread_hooks.diff?revision=1882

(This is still experimental.)

If you want to be callbacked upon save and restore thread context,
there are also the ruby_sandbox_save and ruby_sandbox_restore function
pointers that you can set, if they are available in your version of
Ruby.

Laurent