Python Thread Question

[Aahz]

[Mostly good answers from Jp, but one correction is needed]

In article <mailman.1050587609.6119.python-list at python.org>,
Jp Calderone  <exarkun at intarweb.us> wrote:
>On Thu, Apr 17, 2003 at 06:34:11AM -0700, Anand B Pillai wrote:
>> 
>>  4. Can I safely say that multithreading will improve my application
>>     performance if it has similar work to do on many resources at the
>>     same time ? (egs: a web parser/ spider/ a disk-to-disk file copier/
>>     directory synchronizer) Or does it depend upon the nature of the
>>     task at hand ?
>
>  Nope.  Threading costs you context switches.  A multi-threaded app
>using blocking IO will appear "faster" than a single-thread app doing
>the same, but both take longer to run than a single written app using
>non-blocking IO.
>
>  Context switches are usually relatively cheap compared to what your
>app is actually doing, though, so the difference between doing blocking
>IO in a multi-thread app and non-blocking IO in a single-thread app
>isn't always obvious.  (Other things associated with multi-threaded
>apps, such as deadlocks and race conditions are, though ;)

This isn't quite true on a multi-CPU box.  Because I/O releases the GIL,
the OS can schedule threads to run on all available CPUs.  So it's
certainly possible for a threaded app to outrun non-blocking I/O.
-- 
Aahz (aahz at pythoncraft.com)           <*>         http://www.pythoncraft.com/

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