HP OpenVMS Systems

Processes, Threads, and ASTs?

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The Question is:

 
Threads Vs  Asynchronous I/O, performance throughput
 
In Butenhofs book, Programming with POSIX Threads ... the author (who
i believe was heavily responsible for Pthreads on OpenVMS + TRU64),
 writes ...
 
>> Asynchronous I/O does have one advantage over threaded concurrency,
>> though. Just as a thread is "usually" cheaper (in execution time and
>> storage space) than a process, the context required for an Asynchronous
>> I/O is almost always cheaper than a thread. If you plan to have a lot
>> of Asynchronous I/O operations active at the same time, that might be
>> important enough to justify using the more complex programming model.
 
 
Q1) - By enlarge, do/does the wizard(s) agree with the above statement
           in respect of VMS ?
 
Q2) - Has anyone at ComPaq performed testing comparing the
          Threaded Vs  Asynchronous  I/O models.
 
 
      Based upon Butenhofs statement, it seems the best approach is to
      combine both Asynchronous I/O + Threads. Agreed, this is more
      complex, you are limited in what you can do within the AST (limited
use
      of Pthreads from an AST) but thats fine, get in, enqueue ptr to
      data to main, issue new i/o, and get out. The main line code can then
      decide where to route the request for processing, a thread, or
external
      process.
 
      Indeed for what i have in mind, an i/o intensive server (1000+
      connections)  where the received request usually results in a cpu
      instensive operation, being handled internally by a thread (one per
      cpu), or the request being  handed off to another external process,
      this methodology (although somewhat complex) seems a fairly
      intuitive way of keeping the cpu's busy doing useful work.
 
      Since a collegue disagrees with this methodology on the grounds
      of performance / throughput, and we do not have the luxury of
      writing multiple servers to compare the performance differences ...
 
Q3) - How you any comment in respect of the above model, is it fately
          flawed, have i missed something ?
 
merci
 
-Pierre
 

The Answer is :

 
  As for the quote -- as written, yes, that is entirely correct.
  Now as for whether or not that correctness really matters here...
 
  The OpenVMS Wizard does not believe you can or will receive an answer
  more specific than "it depends" -- it depends on the particular target
  system, growth, and headroom, on the amount of effort you are willing
  to expend for programming and debugging, on the number of systems you
  plan on using, on requirements for clustering, and on various other
  factors.  Threads, ASTs, and local or distributed processes are all
  potential approaches to solving this -- each has various advantages
  and disadvantages, rather obviously.
 
  Also please see topic (6099) for some details of threads-related
  programming.

  

     

     
answer written or last revised on ( 21-JUL-2003 )
     
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