Realtime capabilities?

[Aahz Maruch]

In article <mailman.1001219305.22703.python-list at python.org>,
Tim Peters <tim.one at home.com> wrote:
>[Aahz Maruch]
>>
>> In other words, if the original poster is doing more than a tiny amount
>> of I/O, standard Python/OS threads will probably beat Erlang processes
>> for fairness, but if an application is almost completely
>> compute-bound, Erlang will probably beat even Stackless Python.
>
>Why?  Scheduling fairness has nothing directly to do with speed, it has
>to do with whether a process ready to run is assured to get a chance to
>run "soon".  A process that needs to deliver a result within a given
>time delta needs to know (at least) that it can't be swapped out for
>longer than that time delta.  The speed of each process on its own is
>at best a second-order effect on that, compared to the granularity
>and fairness of the time-slicing policy, and especially when running
>thousands of processes (which Erlang is designed for).  I *expect*
>Erlang switches context more frequently than Stackless, so all else
>being equal (which I'm sure it's not), it would be slower overall (due
>to higher switching overhead, simply because switching more often) if
>so.  Real-time has to do with getting specific pieces of work done by
>specific times, not primarily with maximizing the amount of work done
>per unit time (if the latter is your goal-- "peak speed" --best to stay
>away from context switching entirely).

Right, my point was a condensation of what you said earlier about Python
only doing context-switching between bytecodes, assuming that "fairness"
means "CPU time gets divided evenly between threads".  I said, "even
Stackless" because it does context-switching more efficiently than
regular Python.

Oh, I see where the confusion is.  There was an implied "fairness"
clause in my comment about Stackless, too.
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