Understanding core IDs

Question

Just to understand core IDs:

I have 4 CPUs:

$ cat /proc/cpu* | grep proc*
processor: 0
processor: 1
processor: 2
processor: 3

and the result of nproc is also 4.

But if I use cat /proc/cpu* | grep 'core id' I get the same twice

core id: 0
core id: 2
core id: 0
core id: 2

Why they are not numbered like the CPUs and how to distinguish the same core IDs?

The full contents of /proc/cpuinfo are as follows:

processor   : 0
vendor_id   : GenuineIntel
cpu family  : 6
model       : 37
model name  : Intel(R) Core(TM) i3 CPU       M 380  @ 2.53GHz
stepping    : 5
microcode   : 0x4
cpu MHz     : 1066.000
cache size  : 3072 KB
physical id : 0
siblings    : 4
core id     : 0
cpu cores   : 2
apicid      : 0
initial apicid  : 0
fdiv_bug    : no
f00f_bug    : no
coma_bug    : no
fpu     : yes
fpu_exception   : yes
cpuid level : 11
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 pbe nx rdtscp lm constant_tsc arch_perfmon pebs bts xtopology nonstop_tsc aperfmperf pni dtes64 monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr pdcm pcid sse4_1 sse4_2 popcnt lahf_lm tpr_shadow vnmi flexpriority ept vpid dtherm arat
bugs        :
bogomips    : 5053.71
clflush size    : 64
cache_alignment : 64
address sizes   : 36 bits physical, 48 bits virtual
power management:

processor   : 1
vendor_id   : GenuineIntel
cpu family  : 6
model       : 37
model name  : Intel(R) Core(TM) i3 CPU       M 380  @ 2.53GHz
stepping    : 5
microcode   : 0x4
cpu MHz     : 2533.000
cache size  : 3072 KB
physical id : 0
siblings    : 4
core id     : 2
cpu cores   : 2
apicid      : 4
initial apicid  : 4
fdiv_bug    : no
f00f_bug    : no
coma_bug    : no
fpu     : yes
fpu_exception   : yes
cpuid level : 11
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 pbe nx rdtscp lm constant_tsc arch_perfmon pebs bts xtopology nonstop_tsc aperfmperf pni dtes64 monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr pdcm pcid sse4_1 sse4_2 popcnt lahf_lm tpr_shadow vnmi flexpriority ept vpid dtherm arat
bugs        :
bogomips    : 5053.71
clflush size    : 64
cache_alignment : 64
address sizes   : 36 bits physical, 48 bits virtual
power management:

processor   : 2
vendor_id   : GenuineIntel
cpu family  : 6
model       : 37
model name  : Intel(R) Core(TM) i3 CPU       M 380  @ 2.53GHz
stepping    : 5
microcode   : 0x4
cpu MHz     : 1466.000
cache size  : 3072 KB
physical id : 0
siblings    : 4
core id     : 0
cpu cores   : 2
apicid      : 1
initial apicid  : 1
fdiv_bug    : no
f00f_bug    : no
coma_bug    : no
fpu     : yes
fpu_exception   : yes
cpuid level : 11
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 pbe nx rdtscp lm constant_tsc arch_perfmon pebs bts xtopology nonstop_tsc aperfmperf pni dtes64 monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr pdcm pcid sse4_1 sse4_2 popcnt lahf_lm tpr_shadow vnmi flexpriority ept vpid dtherm arat
bugs        :
bogomips    : 5053.71
clflush size    : 64
cache_alignment : 64
address sizes   : 36 bits physical, 48 bits virtual
power management:

processor   : 3
vendor_id   : GenuineIntel
cpu family  : 6
model       : 37
model name  : Intel(R) Core(TM) i3 CPU       M 380  @ 2.53GHz
stepping    : 5
microcode   : 0x4
cpu MHz     : 1199.000
cache size  : 3072 KB
physical id : 0
siblings    : 4
core id     : 2
cpu cores   : 2
apicid      : 5
initial apicid  : 5
fdiv_bug    : no
f00f_bug    : no
coma_bug    : no
fpu     : yes
fpu_exception   : yes
cpuid level : 11
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 pbe nx rdtscp lm constant_tsc arch_perfmon pebs bts xtopology nonstop_tsc aperfmperf pni dtes64 monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr pdcm pcid sse4_1 sse4_2 popcnt lahf_lm tpr_shadow vnmi flexpriority ept vpid dtherm arat
bugs        :
bogomips    : 5053.71
clflush size    : 64
cache_alignment : 64
address sizes   : 36 bits physical, 48 bits virtual
power management:

Answer 1

Your CPU has two cores and four threads, so seeing duplicated core identifiers is perfectly normal: each “processor” listed in /proc/cpuinfo is a logical processor, on a physical core, so you end up with two physical cores (ids 0 and 2), with four processors (ids 0, 1, 2, and 3).

The core numbering seems odd, but that’s up to your firmware. The kernel calculates core identifiers based on the APIC id, which is assigned by the firmware, and in your case the ids are 0, 1, 4, and 5, which results in core ids of 0 and 2.