Re: How to get number of physical CPU in linux from user space?

[Andrew Walrond]

I rebuilt 2.5.44 with the ACPI stuff enabled, and I see 2 cpus in 
/proc/cpuinfo:

    daedalus@hercules daedalus $ cat /proc/cpuinfo
    processor    : 0
    vendor_id    : GenuineIntel
    cpu family    : 15
    model        : 2
    model name    : Intel(R) XEON(TM) CPU 2.20GHz
    stepping    : 4
    cpu MHz        : 2200.469
    cache size    : 512 KB
    fdiv_bug    : no
    hlt_bug        : no
    f00f_bug    : no
    coma_bug    : no
    fpu        : yes
    fpu_exception    : yes
    cpuid level    : 2
    wp        : yes
    flags        : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge
    mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm
    bogomips    : 4341.76

    processor    : 1
    vendor_id    : GenuineIntel
    cpu family    : 15
    model        : 2
    model name    : Intel(R) XEON(TM) CPU 2.20GHz
    stepping    : 4
    cpu MHz        : 2200.469
    cache size    : 512 KB
    fdiv_bug    : no
    hlt_bug        : no
    f00f_bug    : no
    coma_bug    : no
    fpu        : yes
    fpu_exception    : yes
    cpuid level    : 2
    wp        : yes
    flags        : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge
    mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm
    bogomips    : 4390.91

Also dmesg says:

    daedalus@hercules daedalus $ dmesg
     1462.55 usecs.
    task migration cache decay timeout: 2 msecs.
    enabled ExtINT on CPU#0
    ESR value before enabling vector: 00000000
    ESR value after enabling vector: 00000000
    Booting processor 1/1 eip 2000
    Initializing CPU#1
    masked ExtINT on CPU#1
    ESR value before enabling vector: 00000000
    ESR value after enabling vector: 00000000
    Calibrating delay loop... 4390.91 BogoMIPS
    CPU: Before vendor init, caps: 3febfbff 00000000 00000000, vendor = 0
    CPU: Trace cache: 12K uops, L1 D cache: 8K
    CPU: L2 cache: 512K
    CPU: Physical Processor ID: 0
    CPU: After vendor init, caps: 3febfbff 00000000 00000000 00000000
    Intel machine check reporting enabled on CPU#1.
    CPU#1: Intel P4/Xeon Extended MCE MSRs (12) available
    CPU#1: Thermal monitoring enabled
    CPU:     After generic, caps: 3febfbff 00000000 00000000 00000000
    CPU:             Common caps: 3febfbff 00000000 00000000 00000000
    CPU1: Intel(R) XEON(TM) CPU 2.20GHz stepping 04
    Total of 2 processors activated (8732.67 BogoMIPS).

And a bit further down...

    checking TSC synchronization across 2 CPUs: passed.
    Starting migration thread for cpu 0
    Bringing up 1
    CPU 1 IS NOW UP!
    Starting migration thread for cpu 1
    CPUS done 4294967295

And down some more we seem to have 4 cpus...

    ACPI: Processor [CPU0] (supports C1)
    ACPI: Processor [CPU1] (supports C1)
    ACPI: Processor [CPU2] (supports C1)
    ACPI: Processor [CPU3] (supports C1)

I think with ACPI disabled I had 4 processors listed in /proc/cpuinfo, 
but I'll check.

[e-mail] wrote:

>It seems that /proc/cpuinfo will return the number of logical CPU.
>If the machine has Intel Hyper-Thread enabled, that number is bigger
>than physical CPU number. Usually twice as big.
>
>My question is, what is the reliable way for user space program
>to detect the number of physical CPU in the current machine?
>
>If in it is in the kernel, I can read from cpu_sibling_map[]
>or phys_cpu_id[]. But it seems not easy read that from
>user space.
>
>Of course I can do "gdb /proc/kcore" to get them. But is there
>any better way?
>
>Thanks in advance.
>
>Chris
>
>
>
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