From: Andrey Melnikov (melnikov_at_[hidden])
Date: 2005-08-17 23:12:47
Oliver Kullmann wrote:
> Given two functions
>
> bool f1(X x);
> bool f2(X x);
>
> I want to compute the logical or (short-circuit!).
>
> Now boost::thread did not support thread cancellation or termination
> in version 1_32, and it seems that development of boost::thread has
> been cancelled itself.
> To cancel a thread is essential to gain the speed-up possible
> by the parallel short-circuit evaluation, and thus
> boost::thread seems not to be usable.
>
> Now how are you proceeding? In the FAQ's of boost::thread one can read
>
> ----
>
> 9. Why isn't thread cancellation or termination provided?
>
> There's a valid need for thread termination, so at some point
> probably will include it, but only after we can find a truly safe
> (and portable) mechanism for this concept.
>
> ----
>
> How do you cancel threads?
The explanation in the FAQ doesn't explain the problem in details.
Here's a brief example:
bool f1(X x)
{
X * y = new X;
// if f1() is aborted at this point
// *y won't be deallocated
delete y;
return false;
}
> Can one compute the short-circuit parallel or with your "futures" ?!
This is a common problem in the world of parallel computations. It's due
to the nature of parallel exectution and it's independent from
boost::thread, futures or any library.
Basically, the solution is always the same: f1() polls a flag
periodically, and if it is raised it exits.
To cancel the thread you just raise the flag and the thread terminates
itself:
class A
{
Flag cancel_f1;
bool f1(X x)
{
X * y = new X;
for (int i = 1; i < 1000; i++)
{
// some computations, e.g. one iteration of the algorithm
if (flag.is_raised())
{
delete y;
return false;
}
}
return actual_result;
}
}
...
A a;
a.run_f1_in_a_new_thread();
a.cancel_f1.raise();
> 3) How is access to common storage handled with you approach?!
> (I would appreciate a short explanation using not too much jargon
> --- yet I have only written one multi-threaded program (and that to explore
> the boost::thread library).)
It isn't handled at all. You should use boost::thread synchronization
primitives to handle it properly when you implement f1().
Andrey