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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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