🚀 go-pugleaf

In article <5699@amdahl.UUCP> chuck@amdahl.UUCP (Charles Simmons) writes:
>Seeing how two different people suggested that distributed memory is
>inappropriate for multiple users running independent tasks, maybe someone
>could tell me why.  I find it hard to imagine a problem which is more
>partitionable.  

  Someone else in this news group has already suggested that one problem
  that must be solved before a distributed memory parallel processor can 
  be used to support multiple users is access to I/O resources.  I agree 
  with this, but meeting this requirement is not sufficient.  Assuming 
  that the objective is to do the sort of work that is done on the average 
  UN*X system, a distributed memory parallel processors is a poor choice
  for supporting multiple users.

  Charles Simmons suggests in his article that one way to allow multiple
  users to make use of a distributed memory parallel processor is to 
  dedicate individual processors to the system's users.  I have seen 
  "home grown" systems that did just this.  A system that permanently 
  allocates a processor to a user until the user logs out makes very 
  poor use of the system hardware, since it is used only a fraction of 
  the time.  Such a system could not compete with shared memory systems 
  like those offered by Sequent or Encore.

  In a distributed memory system hardware utilization cannot be increased
  by dynamically moving a process between processors, since the overhead
  incurred is so large.  This option is only realistic for tasks that take
  so long that the overhead is a small part of the execution time.  This 
  is not true for many applications.

  Dynamic process "allocation" is practical on a distributed memory system
  that uses the Single Program Multiple Data Stream (SPMD).  This 
  model is used on the iNTEL cube.  SPMD downloads the same code to all
  processors.  To execute a process, a short message is sent to a
  processor telling it what to execute.  There is little overhead since 
  the code is already resident.  However this model introduces some 
  significant costs.  Consider a parallel processor that consists of 32 
  processing elements (PEs), where each one has 512K bytes of memory.  The 
  processor potentially has 16M bytes of memory available.  Under the SPMD 
  model, only 512K bytes is really available for use, since all 
  processors have the same code.

  Other problems with trying to use a distributed memory parallel processor
  to support multiple users include developing a distributed operating
  system and assuring deterministic system operation.  These are not
  trivial problems.


		     Ian Kaplan
		     Loral Dataflow Group
		     Loral Instrumentation
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