The test was done by Marc Lehmann and he showed his results at the german perl workshop this year. Sadly his talk is not available online and I had to cite from memory when I answered. I have it before me now and can translate some points for you:

1) Perls interpreter-threads are a sort of backport of ActiveStates implementation of forks through windows threads. The whole perl interpreter gets copied with all variables. Every function internally gets an additional parameter that tells perl where to find the variables (I guess he means for synchronising). This makes perl slower even if you don't use threads, makes it instable and doesn't work well will XS modules. There is no common address space, so you don't get any of the advantages of threads and have to pay the price of the synchronisation

2) Threads don't work well in multi-core systems because every cpu has its own MMU and Cache. Because threads use resources together, all MMUs and caches have to be synchronized often. For example if a thread allocates memory, every cpu has to be halted and their state synchronized. Perls thread implementation doesn't do that (see above), but pays with the additional indirection on every variable access which costs 15 to 200% compared to a perl without thread support (even when not using threads).

3) Marc did tests with a matrix multiplication (selected because it uses much variable sharing). Slowest was the one with 4 interpreter-threads on a quad-core machine. 20 times faster was the same 4 interpreter threads on a single core(!). 300 times faster than the interpreter threads was an implementation of cooperative/non-preemptive threads (Coro, written by Marc Lehmann) on a single core.

To answer your question 4 now, perls interpreter threads seem not to work well on multi-cores in those cases where they actually make extensive use of the sharing of their variables (that is if Marcs results are not fake, fabricated or erroneous(sp?)). Some of his points you can read in his documentation to Coro if you are interested

In reply to Re^3: If I am tied to a db and I join a thread, program chrashes by jethro
in thread If I am tied to a db and I join a thread, program chrashes by lance0r

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