Interesting analysis of linux kernel threading by IBM

[Michael Werner]

Alan Cox writes:

That is the least of your worry. 1000 threads is 8Mb of kernel stacks, and 
enough switching of tasks to be sure you might as well turn most of your 
cache off. A computer is a state machine. Threads are for people who cant
program state machines.



It is interesting to note that some hardware guys have decided that large
numbers of threads are indeed useful and can perform significantly better on
real problems. The hardware is nothing like an x86.
Lets face it, embarassingly parallel problems are rather trivial to get
linear speedup on. It takes just a few commands in MPI to do it. But for
scientific users there are a large class of problems which are hard to
parallelise and traditional SMP suck at -- Cray T90 (cached memory,SMP) for
example. 
A highly threaded machine which is available is the MTA machine from TERA
(SMP). There is an 8 processor MTA at SDSC. Take a look at www.npaci.edu
Its processors have 128 threads which can switch every clock cycle.
There is probably no cache.
And the EV8 Alpha processor is said to have 4 threads
in hardware but thats still O(1). Linux runs on EV67 now.

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