hello:

I wrote a simple program which does simple math loop and I am testing
under dual core processor

systm1:
intel dual core laptop ; windows xp os
when I spawn of two threads in windows (both under visual c and cygwin
cc) the program behaves as expected.
in single thread mode the time is 2x and clearly one of hte one do of
the processor is being utilized

system2:
amd 4200x2 desktop ubuntu hardy 8.04
here actually the single thread gets slightly better performance. in
multi thereaded both the cpu's are 100%% utilized. but even in single
threaded both the cpu's are alternatively being used 50/100 %%!!

gurus:
any idea why the linux system (i am assuming the difference is due to
OS) is behaving differently?

here is the simple code

On May 16, 3:02 pm, kumar...@gmail.com wrote:

This question is definitely off topic here, but I managed to find one
topical aspect:
library functions (especially rand()) are not reentrant.
Do not use them without a synchronising mechanism. For better
performance, implement your own or use libc's reentrant
random number generators, where the internal information about
the next random number is stored in automatic variables, which
are on a thread local stack on your platforms.
Avoid calling the same library functions from the threads and

avoid the frequent access of the same memory (page) from
several threads.


Szabolcs

kumarchi@gmail.com writes:

[...]

[...]

Try asking in comp.programming.threads.

On 16 May 2008 at 14:02, kumarchi@gmail.com wrote:

I find dt = 4 running the two threads, versus dt = 7 for one thread
(Debian, 2.6.24 kernel), so I guess there's some problem with your
setup.

Are you running an SMP kernel? Does cat /proc/cpuinfo report both cores?
What was the system load when you ran the test?

On 16 May 2008 at 14:44, Szabolcs Borsanyi wrote:

I'm not sure what function you have in mind. The reentrant version of
rand provided by POSIX is rand_r(), which takes a pointer to a seed as
its argument - it's very unlikely that TLS will be needed or desirable.

On May 16, 6:31 pm, Antoninus Twink wrote:

You are right, rand_r() is posix, indeed. I did not mean TLS
as thread local storage for global or static variables, but the stack,
owned by the thread, that holds the object pointed to by the argument
of
rand_r(). And since rand_r takes just an unsigned*, its quality of
RNG is limited by design, this often rules out this function.

Szabolcs

On May 16, 1:09 pm, Antoninus Twink wrote:

yes my kernel is 2.6.24-17 and the procinfo reports the 2 cores

kumarchi@gmail.com writes:

[...]

Antoninus Twink is trying to disrupt this newsgroup by encouraging
off-topic posts. Please post any further questions to
comp.programming.threads or to a newsgroup that deals with your
operating system, where you will find experts who can actually answer
your questions. Thank you.

On May 16, 1:09 pm, Antoninus Twink wrote:

I compiled with optimization . mo i am getting dt=21(for 2 treads) vs
dt=27 ; some improvement. by the what ishte hardware you are using .
it is way way faster than mine 5x ; that is incredible

On 16 May 2008 at 23:36, kumarchi@gmail.com wrote:

Probably not - I reduced the number of outer loops :)