Subj : Re: One thread on several processors in multiprocessor system?
To   : comp.programming.threads
From : Randy
Date : Tue Feb 01 2005 06:08 pm

Julius wrote:
> Hello,
> 
> I never thought befor deep about threads, but. I know that one thread
> can run on separate processor. but, can one thread runs on several
> processors? Say, simultaneously calculating sqrt(2), in one processor,
> and 2+2 on another?
> 
> For example in the code:
> 
> ...
> 
> double x = sqrt(2);
> int i = 2 + 2;
> ....

This sort of 'threading' happens routinely today on out-of-order CPUs, even
without the assistance of the compiler.  Operands are dispatched by Tomasulo's
algorithm (for example) from multiple reservation stations as soon as they
become available (arriving from memory or from earlier operations) and the
scheduler decides they can be executed independently.  In this case, even code
compiled without any optimization will probably execute concurrently on the CPU,
perhaps by employing two different floating point functional units.

Of course, no real threading is involved in this case, just asynchronicity.  But
threading does arise via something called Simultaneous MicroThreading (SMT), AKA
Hyperthreading (Intel's TM, as in the Pentium4).  SMT also occurs at the
discretion of the CPU, as it toggles among existing independent threads of
execution, mapping them onto and off of the CPU's resources as peripheral
resources (esp. operands to/from RAM) become available or unavailable, respectively.

There's been a lot of research into the potential for concurrent execution
throughout the average program.  I think the conclusion is that perhaps 80% of
the average program's instructions could execute concurrently with others,
assuming that 1) hardware resources were available, and 2) that the instructions
could be correctly identified on-the-fly as being independent (on-the-fly,
because branches play havoc with a program's control and data dependencies).

    Randy

-- 
Randy Crawford   http://www.ruf.rice.edu/~rand   rand AT rice DOT edu

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