Standard Threads vs Weightless Threads
spam.csubich+block at block.subich.spam.com
Mon Aug 1 15:46:16 CEST 2005
> 1)What is the difference (in terms of performance, scalability,[insert
> relevant metric here]) between microthreads and "system" threads?
System-level threads are relatively heavyweight. They come with a full
call stack, and they take up some level of kernel resources [generally
less than a process]. In exchange, they're scheduled by the OS, with
the primary benefit (on uniprocessor systems) that if one thread
executes a blocking task (like IO writes) another thread will receive
The primary disadvantage is that they're scheduled by the CPU. This
leads to the concurrency nightmare, where the developer needs to keep
track of what blocks of code (and data) need locks to prevent deadlock
and race conditions.
> 2)If microthreads really are superior then why aren't they the standard
> Python implementation (or at least within the standard library)? (where
> my assumption is that they are not the standard implementation and are
> not contained within the standard library).
Microthreads are very different; they're entirely internal to the Python
process, and they're not seen at all by the operating system.
Scheduling is done explicitly by the microthread implementation --
multitasking is not preemptive, as with system threads.
They're not in the standard library because implementing microthreads
has thus far required a very large rewrite of the CPython architecture
-- see Stackless Python.
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