On Wed, 19 Feb 2003, Ranjan Bagchi wrote:

> I've been reading LtU lately (lambda.weblogs.com) and
> came across this paper:
> http://www.sics.se/~joe/apachevsyaws.html detailing a
> high-performance web-server written in Erlang.
> 
> One point here is that the threads in Erlang uses a
> ton of "microthreads" which don't use OS Threads at
> all (and don't incur the overhead).  Stackless python
> (www.stackless.com) does this too, and is able to
> support massive concurrency.

Yep.  These are called "userspace" or "green" threads.  They're fairly 
easy to do- all you have to do is swap stacks.  Some experimentation on my 
part showed that on a P4, you could task switch between two such user 
space threads in 30-60 clock cycles on the x86- less expensive than a 
cache miss.

User space threads have some problems.  First off, they don't take 
advantage of multiple CPUs, or SMT CPUs.  Second, and more importantly, if 
one thread blocks- either by doing blocking I/O, or page faulting- all 
threads block.

A better approach is what's called the M-by-N approach.  You have M kernel 
level threads (so you can take advantage of M parallel CPUs) each 
executing threads from a pool of N user space threads.  This gives you the 
best of both worlds.

Brian

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