Python threading?

[Aahz]

In article <T5Rj9.13058$6g7.39977 at news-server.bigpond.net.au>,
Mark Hammond  <mhammond at skippinet.com.au> wrote:
>Aahz wrote:
>> In article <f9Pi9.10275$Lg2.2191294 at news2.news.adelphia.net>,
>> Robert Oschler <Oschler at earthlink.net> wrote:
>>> 
>>>Also, what is a reasonable number of threads to expect to be able to run
>>>before context switching overhead becomes a problem (I'm using a PIII 500
>>>Mhz with 512MB ram if that helps).
>> 
>> It depends.  For pure Python-based computation, more than one thread
>> will cost you more than you gain.  If it's almost pure I/O (file or
>> socket stuff), you might be able to use as many as a couple of hundred
>> threads.  Mostly, you're probably best off somewhere between five and
>> thirty threads, depending on what you're doing.
>
>I would have thought that a brave call.  If you are performing lots of 
>IO and need many many connections, you are almost certainly better off 
>using an "asynchronous" approach, much like asyncore.  With a large 
>number of threads (where "large" can be as low as single figures) the OS 
>overhead of switching will be greater than your internal overhead of a 
>more complicated design.

I've got no practical experience with asyncore, but I do with threads.
While I accept the conventional wisdom that asyncore will frequently do
better than threads for performance, I stand by my claims for what will
work well with threads.

>A "one thread per connection" model is doomed from the start - 
>therefore, in most applications, I would consider 5 threads a large 
>number - particularly when all are actively doing something.  OTOH, if 
>you are willing to accept that your first version will accept a max of 
>~20 connections, the brain-power saved by using threads beats the 
>additional OS overhead by a mile ;)

Overall, yes, but a lot of I/O is bursty, and with a number of blocked
threads, up to a hundred or so can work well.  Note carefully that I
*am* assuming a thread pool to avoid startup costs.
-- 
Aahz (aahz at pythoncraft.com)           <*>         http://www.pythoncraft.com/

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