Re: To thread or not to thread ?
JohnQ wrote:
"Peter Dimov" <pdimov@gmail.com> wrote in message
news:1168567134.727505.269630@v45g2000cwv.googlegroups.com...
JQP wrote:
Carlos Moreno wrote:
JohnQ wrote:
Why have more threads than the number of processor cores?
I'm baffled by this question ...
The above question is exactly equivalent to "why would we want
multithreading?"
Not at all. I was just wondering why/when that "rule" should be
used. Is it whenever you want two or more tasks to APPEAR to
be running at the same time rather wanting to get the most
work done per core? For example: GUI prog, use as many as
your heart desires; Database server: limit to the number of cores.
Pretty much, except that you want the number of _running_ threads to be
(approximately) equal to the number of cores.
That's all that CAN run (#of RUNNING threads = # of cores). Limiting the
number of ACTIVE threads to the number of cores should increase efficiency
by eliminating thread switching. Probably what you meant (?).
My bad, "running" already has an established meaning. So yes, I meant
the number of "runnable" threads in Java terminology, or the number of
threads that are either "running" or "ready" in non-Java terminology.
It's clear from your other posts that you understood what I meant
despite my sloppy wording. :-) The gist of it is that if you expect 40%
of your threads to be blocked at any given moment, you need to spawn
(1.7 * cores) threads (kernel-provided thread pools can do better since
they know which threads are blocked).
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