question about Intel P4 versus Alphas

Mikhail Kuzminsky kus at free.net
Tue Nov 9 01:00:01 PST 2010


According to Alan Scheinine
> In a previous message, Mikhail Kuzminksy spoke about multithreading
> and hyperthreading, and also superscalar microprocessors.  I would like
> to add a few remarks for the sake of greater precision.  Superscalar 
> and out-of-order execution has a high hardware cost because of the
> large amount of logic needed to organize the execution steps dynamically.
> The primary motivation for the Itanium was that this organizing of
> the work at a fine-grained level would be done by the compiler.
> Multithreading means that the processors gives time slices to various
> threads.  The state of the CPU for each thread is switched between
> thread.  Hyperthreading has several threads executing at the same time,
> so exceptions and condition codes may be for one or another thread
> at the same time.
>    For clusters, parallel execution generally uses message passing
> so the user does not write the code as a multithreaded program.
> As a consequence the application program would not be using hyperthreading.
  I'm sorry, one additional remark for clarification :-)
I beleive that if I have 2 logical CPUs as in the case
of HT, then I may use (for writing of my application) some
parallelization tools, not only OpenMP for shared memory,
but also MPI-  for doing paralelization. When I wrote
about "thanks" to Intel from beowulfers, I thought
about any kinds of parallelization, in particular MPI.
The questions "is in this case MPI better than OpenMP
or pthreads etc" or "is it reasonable for some application(s)
to use MPI for parallelization not only between the nodes,
but also inside the HT-P4-nodes ?" are another questions.

Mikhail Kuzminsky,
Zelinsky Institute of Organic Chemistry
Moscow



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