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    SubjectStrange performance behavior of 2.4.0-test9
    Hi,
    I found very odd performance behavior of 2.4.0-test9 on a large SMP
    server, and I want some clues to investigate it.

    The workload is WebBench,
    Machine is:
    8way PII-Xeon 450MHz 2MB cache with Profusion chip-set,
    1GB Main memory with no highmem,
    using 4 eepro100 networks.

    As shown in the table below, test8 is very preferable in both absolute
    performance and scalability.

    WebBench performance results.
    Req/sec(4cpu->8cpu)
    --------------------------
    2.4.0-test1 2816->3702 (31%up)
    2.4.0-test8 4006->5287 (63%up)
    2.4.0-test9 3669->2193 (40%down)
    (4cpu means two CPUs for each bus.)

    But the performance of test9 shows following problems:

    1) At the 8 cpu configuration, test9 shows extremely inferior
    performance.
    2) on test8, 8-cpu configuration shows about 2/3 performance of 4-cpu.
    Other experiment shows, as # of cpu increases from 4 to 8, the
    performance gradually decreases.
    CPUs Req/s
    4cpu 3660
    5cpu 3070
    6cpu 2730
    7cpu 2611
    8cpu 2210

    The reason of the performance degradation is increase of idle-time,
    and increase of context switches (2.2x @4cpu, 3.1x @8cpu).


    The following table shows one minute average of "vmstat 1".

    configuration Req/s | r b intr c-sw user os idle
    ------------------------+--------------------------------------------------
    2.4.0t1 4cpu 2816 | 60 0 28877 5103 18 82 0
    8cpu 3702 | 37 0 33495 16387 14 86 0
    2.4.0t8 4cpu 4066 | 57 0 38743 8674 27 73 0
    8cpu 5287 | 33 0 40755 30626 21 78 0
    2.4.0t9 4cpu 3669 | 53 3 35822 18898 25 73 2
    8cpu 2193 | 5 8 22114 94609 9 52 39

    In the experiments, enough free memory was keeped,
    no swap-in/out happened, almost no block-in/out occured.
    interrups is roughly propotional to the performance.

    Performance down in 2.4.0-t9 4cpu is not so prominent but actually some
    processes are blocked running by some reason.
    And as the number of CPUs increases the performance degrades.
    On 8cpu, only 1/10 of the processes are runnable.

    I've tried test10-pre5 but nothing improved.

    Does anybody tell me what I should do?
    --
    Computer Systems Laboratory, Fujitsu Labs.
    kumon@flab.fujitsu.co.jp
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