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I'm trying to figure out how to interpret my /proc/cpuinfo. I've got an AMD FX-4100 quad-core and cpuinfo gives, as it should, 1 'physical id', 4 'core id's and 4 'siblings', but why does 'cpu cores' say 2 instead of 4? Also, the FX-4100 specs say 4MB L2 cache yet cpuinfo says 2MB.

Included below is the text for processor 0.

processor       : 0
vendor_id       : AuthenticAMD
cpu family      : 21
model           : 1
model name      : AMD FX(tm)-4100 Quad-Core Processor
stepping        : 2
microcode       : 0x600063d
cpu MHz         : 1400.000
cache size      : 2048 KB
physical id     : 0
siblings        : 4
core id         : 0
cpu cores       : 2
apicid          : 0
initial apicid  : 0
fpu             : yes
fpu_exception   : yes
cpuid level     : 13
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm constant_tsc $
bogomips        : 7248.64
TLB size        : 1536 4K pages
clflush size    : 64
cache_alignment : 64
address sizes   : 48 bits physical, 48 bits virtual
power management: ts ttp tm 100mhzsteps hwpstate cpb
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It says that you've 2 CPU cores because you have 2 CPU cores. The AMD FX-4100 has 2 cores and 2 modules.

From answers.yahoo.com

Your FX is closer to a dual core. To be precise, it's two modules. Bulldozer uses modules, and each module consists of one FP scheduler and 2 int schedulers.

This is AMD's answer to "hyperthreading" - it's cheaper to manufacture, can potentially result in faster overall performance at a marginal manufacturing cost, and it lets them get away with selling crappy cpus to people who don't understand or read benchmarks/reviews etc.

In the end, Bulldozer was a flop. The per-core performance is what matters and it loses to Intel by a significant margin. Even when all core are loaded, Bulldozer doesn't compare very well to Intel, but can pull ahead ever so slightly in some highly-threaded apps.

Finally, hyperthreading doesn't need to be enabled on the modern CPUs that do support it (yours does not).

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