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>Understanding the Performance Characteristics of Tcl</A
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>&#13;      Newcomers to Tcl, and detractors (read, <SPAN
CLASS="QUOTE"
>"proponents of other
      paradigms"</SPAN
>) often do not have a clear (newcomers) or truthful
      (detractors) perspective on Tcl performance.  In this section we try
      to convey a little orientation which may be helpful in working with
      the PLplot Tcl interface.
    </P
><P
>&#13;      <SPAN
CLASS="QUOTE"
>"Tcl is slow!"</SPAN
>  <SPAN
CLASS="QUOTE"
>"Yeah, so what?"</SPAN
>
    </P
><P
>&#13;      Debates of this form frequently completely miss the point.  Yes, Tcl
      is definitely slow.  It is fundamentally a string processing language,
      is interpreted, and must perform substitutions and so forth on a
      continual basis.  All of that takes time.  Think milliseconds instead
      of microseconds for comparing Tcl code to equivalent C code.  On the
      other hand, this does not have to be problematic, even for time
      critical (interactive) applications, if the division of labor is done
      correctly.  Even in an interactive program, you can use Tcl fairly
      extensively for high level control type operations, as long as you do
      the real work in a compiled Tcl command procedure.  If the high level
      control code is slow, so what?  So it takes 100 milliseconds over the
      life the process, as compared to the 100 microseconds it could have
      taken if it were in C.  Big deal.  On an absolute time scale, measured
      in units meaningful to humans, it's just not a lot of time.
    </P
><P
>&#13;      The problem comes when you try to do too much in Tcl.  For instance,
      an interactive process should not be trying to evaluate a
      mathematical expression inside a doubly nested loop structure, if
      performance is going to be a concern.
    </P
><P
>&#13;      Case in point:  Compare x16.tcl to x16c.c.  The code looks very
      similar, and the output looks very similar.  What is not so similar is
      the execution time.  The Tcl code, which sets up the data entirely in
      Tcl, takes a while to do so.  On the other hand, the actual plotting
      of the data proceeds at a rate which is effectively indistinguishable
      from that of the compiled example.  On human time scales, the
      difference is not meaningful.  Conclusion:  If the computation of the
      data arrays could be moved to compiled code, the two programs would
      have performance close enough to identical that it really wouldn't be
      an issue.  We left the Tcl demos coded in Tcl for two reasons.  First
      because they provide some examples and tests of the use of the Tcl
      Matrix extension, and secondly because they allow the Tcl demos to be
      coded entirely in Tcl, without requiring special customized extended
      shells for each one of them.  They are not, however, a good example of
      you should do things in practice.
    </P
><P
>&#13;      Now look at <TT
CLASS="filename"
>tk04</TT
> and <TT
CLASS="filename"
>xtk04.c</TT
>, you will see
      that if the data is computed in compiled code, and shuffled into the
      Tcl matrix and then plotted from Tcl, the performance is fine.  Almost
      all the time is spent in plshade, in compiled code.  The time taken to
      do the small amount of Tcl processing involved with plotting is
      dwarfed by the time spent doing the actual drawing in C.  So using Tcl
      cost almost nothing in this case.
    </P
><P
>&#13;      So, the point is, do your heavy numerics in a compiled language, and
      feel free to use Tcl for the plotting, if you want to.  You can of
      course mix it up so that some plotting is done from Tcl and some from
      a compiled language.  
    </P
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plplot-doc 5.9.9-5 / usr / share / doc / plplot-doc / html / tcl-understanding.html
