Does my computer have 1 or 2 CPUs? (cpuinfo and lscpu differ)

Question

When I run cat /proc/cpuinfo I get the following output (only the relevant line included):

cpu cores: 1

However, lscpu gives me the following output:

CPU(s): 2

Which of the two is correct, and more importantly, why are they giving me different results?

---

The full output of cat /proc/cpuinfo is:

processor   : 0
vendor_id   : GenuineIntel
cpu family  : 15
model       : 3
model name  : Intel(R) Pentium(R) 4 CPU 2.80GHz
stepping    : 4
microcode   : 0xe
cpu MHz     : 2800.135
cache size  : 1024 KB
physical id : 0
siblings    : 2
core id     : 0
cpu cores   : 1
apicid      : 0
initial apicid  : 0
fdiv_bug    : no
f00f_bug    : no
coma_bug    : no
fpu     : yes
fpu_exception   : yes
cpuid level : 5
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 constant_tsc pebs bts pni dtes64 monitor ds_cpl cid xtpr
bogomips    : 5600.27
clflush size    : 64
cache_alignment : 128
address sizes   : 36 bits physical, 32 bits virtual
power management:

processor   : 1
vendor_id   : GenuineIntel
cpu family  : 15
model       : 3
model name  : Intel(R) Pentium(R) 4 CPU 2.80GHz
stepping    : 4
microcode   : 0xe
cpu MHz     : 2800.135
cache size  : 1024 KB
physical id : 0
siblings    : 2
core id     : 0
cpu cores   : 1
apicid      : 1
initial apicid  : 1
fdiv_bug    : no
f00f_bug    : no
coma_bug    : no
fpu     : yes
fpu_exception   : yes
cpuid level : 5
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 constant_tsc pebs bts pni dtes64 monitor ds_cpl cid xtpr
bogomips    : 5600.27
clflush size    : 64
cache_alignment : 128
address sizes   : 36 bits physical, 32 bits virtual
power management:

The full output of lscpu is:

Architecture:          i686
CPU op-mode(s):        32-bit
Byte Order:            Little Endian
CPU(s):                2
On-line CPU(s) list:   0,1
Thread(s) per core:    2
Core(s) per socket:    1
Socket(s):             1
Vendor ID:             GenuineIntel
CPU family:            15
Model:                 3
Stepping:              4
CPU MHz:               2800.135
BogoMIPS:              5600.27
L1d cache:             16K
L2 cache:              1024K

(This can also be viewed at https://gist.github.com/IQAndreas/f3f9139b8968987d3716.)

Solution

This is consistent with having a single processor containing a single core which contains two execution threads through hyperthreading. Thus you have two logical processors, but they share most of their resources (instruction decoding, arithmetic, etc.). This allows some parallelism — one thread can progress while the other one is blocked (e.g. waiting for a memory access) while keeping the hardware cost down (fewer components than two independent cores).

Look at the rest of the output from lscpu and the content of /proc/cpuinfo. See So what are logical cpu cores (as opposed to physical cpu cores)? for an example.

Comments

[David]

Really it's only the out-of-order execution core that is shared in a processor with hyperthreading. Each virtual CPU has instruction decoding.