multithreading concept

[Sergei Organov]

"sturlamolden" <sturlamolden at yahoo.no> writes:
> On Feb 7, 6:17 pm, John Nagle <n... at animats.com> wrote:
[...]
> MPI does not use threads on SMPs because it performs worse than using
> multiple processes.

I fail to see how threads in general could perform worse than
processes. I do understand that processes are inherently more
safe/secure, but when it comes to speed I really can't imagine why it
could happen that threads perform worse (poor threads implementation and
programming practices aside). Care to give some references?

-- Sergei.