Re: Selecting target CPU for thread
Patricia Shanahan wrote:
Kenneth P. Turvey wrote:
On Mon, 28 Jan 2008 15:59:50 -0800, Todd wrote:
On Jan 28, 1:55 pm, Eric Sosman <esos...@ieee-dot-org.invalid> wrote:
[Snip]
I am sorry that I was unclear - is there any way using Java to get the
cpu id for the current thread? if the answer is yes, how is it done?
The answer is no, not without native code. But the real answer is that
you don't need it. You should probably be using the id of the thread, not
the CPU. This you can get.
I don't agree with "you don't need it".
Consider the following basic question: "Are threads moving around too
much?". There is a non-zero cost to moving a thread, because each
processor accumulates cache contents and other history for the threads
it is running. Excessive thread movement is a possible hypothesis if a
thread has an unexpectedly large cache miss rate. On the other hand, it
is also undesirable to leave an imbalance too long.
How would one investigate this sort of question without asking about
mappings between threads and processors?
Certainly not from within the application. All you can possibly hope to do is
find out what CPU/core the current thread is executing on at any given instant
in time. You can't find out which CPU it was running on before you asked and
you can only guess what it will do afterwards. This is a prime example of
Heisenberg's uncertainty principle. The only way to get more information is to
ask more often (and therefore spend less time doing useful work, and you still
don't know if any of that useful work was carried out on the CPU on which you
asked the question). Ultimately, the only way to know all the time on which CPU
your thread is executing is to do nothing other than ask "on what CPU am I
running?". Hence the futility of the exercise.
--
Nigel Wade, System Administrator, Space Plasma Physics Group,
University of Leicester, Leicester, LE1 7RH, UK
E-mail : nmw@ion.le.ac.uk
Phone : +44 (0)116 2523548, Fax : +44 (0)116 2523555