Re: To thread or not to thread ?

"Victor Bazarov" <>
19 Jan 2007 17:18:21 -0500
[ to the moderator: I apologize for dragging this less-than-topical
  discussion so long. this is my last posting in support of Peter
  Becker's original argument. ]

Sergey P. Derevyago wrote:

Victor Bazarov wrote:

But you _do_ choose the "1" (in your terms). Don't you actually see
it yourself?

Peter Becker wrote:
There are two different perspectives on multi-threaded
programming. On is "distribute the workload optimally among the
available processors." That's when you want just as many threads as
processor cores. The other is "logically divide the process into
tasks that can be run concurrently." That's when you want as many
threads as appropriate, and are willing to put up with some
thrashing and delays when you have more threads than cores.

The fact that you subdivide your processing into M tasks is
irrelevant because right after that you "map" them into 'N'
simultaneous threads based on the available hardware. So, the
number of actual threads running is governed by the hardware.

THAT'S the point.

       Not, really.
       It was stated that we have (only) two perspectives:
       (2). "Logically divide the process into tasks that can be run
and do YYY.
       (1). Do XXX.
       As you see, the (1) alternative doesn't mean that you can
"logically divide
the process into tasks that can be run concurrently".

Huh? Where do you get this YYY and XXX from? The quote from Peter
Becker is right there. Is the "XXX" you use the actual "distribute
the work load among the available processors"? What "YYY"?

You don't have to answer. I just mean to point out the weakness
of such method of "interpreting" other statements.

       What actually was stated is:

"Was stated" by whom? Is this your interpretation of somebody else's
words or is that your own thoughts? Are you referring to Peter's
statements or someone elses? Please clarify next time.

1. Count your processors but do NOT "divide the process into tasks
that can be run concurrently".

How can you NOT divide if the whole point is to come up with a set
of threads that perform some operations relatively independently?

2. "Divide the process into tasks that can be run concurrently" and
start as many threads as you can.

       I proposed the combination: Divide the process into jobs that
can be run concurrently and allow any user to set fixed number of
threads that will be used to process these jobs on his particular HW.
       Another user can (empirically) choose another number of
threads but the number of parallel tasks was already set by your
program design from the very beginning.

I understand that it can be difficult to admit a mistake. I admit my
mistake of getting into this "debate". It seems to have no resolution
in sight.

But here is one more, my last, attempt.

The splitting your program into threads has the goal that is _always_
"divide the process into tasks that can be run concurrently". There
can be no debate about this, it's the definition of a thread. A task
can be "to perform some percentage of calculations" or it can be "to
clean the database" or "to collect garbage". The difference is what
you use as the *defining* criterion for your splitting.

The "perform some portion of lengthy iterative work where iterations
don't depend on each other" variation, for example, falls under (1) and
the number of cores is the criterion. In that case the decision does
not involve the nature (so to speak) of the task, it only involves the
_amount of work_ to be done. This is what scientific simulations often
do on multi-processor systems.

"Collect this", "clean that", "refresh something", all fall under (2)
and the *defining* criterion is the nature (so to speak) of the task
to be performed. This is more common in complex systems where the
types of processing jobs is too different to identify "percentages".

In all cases there has to be relative independence of the tasks to
justify their running in parallel.

The main difference between (1) and (2) is that (1) has *hardware* as
the defining criterion and (2) does NOT. So, whenever you involve
hardware (whether your program decides how to split the work, or the
user) it falls under (1). Period. Whether the "all the work" does
actually consist of "'M' tasks to be performed independently" matters
not. As soon as you say "I have some amount of work to do, let's
start N threads because I have K cores", it's (1). If you happen to
say "I have L independent jobs to do, I'll start L threads and let
the system decide how to execute them optimally", it's (2).

That's all I have to say. Thank you for reading.

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