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SubjectRe: Possible bug from kernel 2.6.22 and above
Simon Holm Thøgersen wrote:
> ons, 21 11 2007 kl. 20:52 -0500, skrev Jie Chen:

> There is a backport of the CFS scheduler to 2.6.21, see
> http://lkml.org/lkml/2007/11/19/127
>
Hi, Simon:

I will try that after the thanksgiving holiday to find out whether the
odd behavior will show up using 2.6.21 with back ported CFS.

>>>> Kernel 2.6.21
>>>> Number of Threads 2 4 6 8
>>>> SpinLock (Time micro second) 10.5618 10.58538 10.5915 10.643
>>>> (Overhead) 0.073 0.05746 0.102805 0.154563
>>>> Barrier (Time micro second) 11.020410 11.678125 11.9889 12.38002
>>>> (Overhead) 0.531660 1.1502 1.500112 1.891617
>>>>
>>>> Each thread is bound to a particular core using pthread_setaffinity_np.
>>>>
>>>> Kernel 2.6.23.8
>>>> Number of Threads 2 4 6 8
>>>> SpinLock (Time micro second) 14.849915 17.117603 14.4496 10.5990
>>>> (Overhead) 4.345417 6.617207 3.949435 0.110985
>>>> Barrier (Time micro second) 19.462255 20.285117 16.19395 12.37662
>>>> (Overhead) 8.957755 9.784722 5.699590 1.869518
>>>>

>
>
> Simon Holm Thøgersen
>
>
I just ran a simple test to prove that the problem may be related to
load balance of the scheduler. I first started 6 processes using
"taskset -c 2 donothing&; taskset -c 3 donothing&; ..., taskset -c 7
donothing". These 6 processes will run on core 2 to 7. Then I started my
test program using two threads bound to core 0 and 1. Here is the result:

Two threads on Kernel 2.6.23.8:
SpinLock (Time micro second) 10.558255
(Overhead) 0.068965
Barrier (Time micro second) 10.865520
(Overhead) 0.376230

Similarly, I started 4 donothing processes on core 4, 5, 6 and 7, and
ran the test program. I have the following result:

Four threads on Kernel 2.6.23.8:
SpinLock (Time micro second) 10.579413
(Overhead) 0.090023
Barrier (Time micro second) 11.363193
(Overhead) 0.873803

Finally, here is the result for 6 threads with two donothing processes
running on core 6 and 7:

Six threads on Kernel 2.6.23.8:
SpinLock (Time micro second) 10.590030
(Overhead) 0.100940
Barrier (Time micro second) 11.977548
(Overhead) 1.488458

Now the above results are very much similar to the results obtained for
the kernel 2.6.21. I hope this helps you guys in some ways. Thank you.

--
#############################################################################
# Jie Chen
# Scientific Computing Group
# Thomas Jefferson National Accelerator Facility
# Newport News, VA 23606
#
# chen@jlab.org
# (757)269-5046 (office)
# (757)269-6248 (fax)
#############################################################################
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