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    SubjectRe: [PATCH 4/5] x86: Add config variables for SMP_MAX

    * Mike Travis <travis@sgi.com> wrote:

    > >> +config THREAD_ORDER
    > >> + int "Kernel stack size (in page order)"
    > >> + range 1 3
    > >> + depends on X86_64_SMP
    > >> + default "3" if X86_SMP_MAX
    > >> + default "1"
    > >> + help
    > >> + Increases kernel stack size.
    > >> +
    > >
    > > Could you please elaborate, why this is needed and put more info
    > > about this requirement into this patch description?
    > >
    > > People worked hard to push data allocation from stack to heap to
    > > make THREAD_ORDER of 0 and 1 possible. So why increase it again and
    > > why does this help scalability?
    > >
    > > Many thanks and Best Regards
    > >
    > > Ingo Oeser, puzzled a bit :-)
    >
    >
    > The primary problem arises because of cpumask_t local variables.
    > Until I can deal with these, increasing NR_CPUS to a really large
    > value increases stack size dramatically.

    those should be fixed:

    > Here are the top stack consumers with NR_CPUS = 4k.
    >
    > 16392 isolated_cpu_setup
    > 10328 build_sched_domains
    > 8248 numa_initmem_init
    > 4664 cpu_attach_domain
    > 4104 show_shared_cpu_map
    > 3656 centrino_target
    > 3608 powernowk8_cpu_init
    > 3192 sched_domain_node_span
    > 3144 acpi_cpufreq_target
    > 2584 __svc_create_thread
    > 2568 cpu_idle_wait
    > 2136 netxen_nic_flash_print
    > 2104 powernowk8_target
    > 2088 _cpu_down
    > 2072 cache_add_dev
    > 2056 get_cur_freq
    > 0 acpi_processor_ffh_cstate_probe
    > 2056 microcode_write
    > 0 acpi_processor_get_throttling
    > 2048 check_supported_cpu

    (and most of that is performance-uncritical.)

    Ingo


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