A dual core processor literally has two times as many function units as a single-core processor, and can really run two programs concurrently, with no competition for function units.

A dual core processor is built so that both cores share the same level 2 cache. A dual processor (separate physical cpus) system differs in that each cpu will have its own level 2 cache. This may sound like an advantage, and in some situations it can be but in many cases new research and testing shows that the shared cache can be faster when the cpus are sharing the same or very similar tasks.

In general Hyperthreading is considered older technology and is no longer supported in newer cpus. Hyperthreading can provide a marginal (10%) for some server workloads like mysql, but dual core technology has essentially replaced hyperthreading in newer systems.

A dual core cpu running at 3.0Ghz should be faster then a dual cpu (separate core) system running at 3.0Ghz due to the ability to share the cache at higher bus speeds.

The examples below details how we determine what kind of cpu(s) are present.

The kernel data Linux exposes in /proc/cpuinfo will show each logical cpu with a unique processor number. A logical cpu can be a hyperthreading sibling, a shared core in a dual or quad core, or a separate physical cpu. We must look at the siblings, cpu cores and core id to tell the difference.

If the number of cores = the number of siblings for a given physical processor, then hyperthreading is OFF.

/bin/cat /proc/cpuinfo | /bin/egrep ‘processor|model name|cache size|core|sibling|physical‘

Example 1: Single processor, 1 core, no Hyperthreading

processor	: 0model name	: AMD Duron(tm) processorcache size	: 64 KB

Example 2: Single processor, 1 core, Hyperthreading is enabled.

Notice how we have 2 siblings, but only 1 core. The physical cpu id is the same for both: 0.

processor	: 0model name	: Intel(R) Pentium(R) 4 CPU 2.80GHzcache size	: 1024 KBphysical id	: 0siblings	: 2core id		: 0cpu cores	: 1processor	: 1model name	: Intel(R) Pentium(R) 4 CPU 2.80GHzcache size	: 1024 KBphysical id	: 0siblings	: 2core id		: 0cpu cores	: 1

Example 3. Single socket Quad Core

Notice how each processor has its own core id. The number of siblings matches the number of cores so there are no Hyperthreading siblings. Also notice the huge l2 cache - 6 MB. That makes sense though, when considering 4 cores share that l2 cache.

processor	: 0model name	: Intel(R) Xeon(R) CPU           E5410  @ 2.33GHzcache size	: 6144 KBphysical id	: 0siblings	: 4core id		: 0cpu cores	: 4processor	: 1model name	: Intel(R) Xeon(R) CPU           E5410  @ 2.33GHzcache size	: 6144 KBphysical id	: 0siblings	: 4core id		: 1cpu cores	: 4processor	: 2model name	: Intel(R) Xeon(R) CPU           E5410  @ 2.33GHzcache size	: 6144 KBphysical id	: 0siblings	: 4core id		: 2cpu cores	: 4processor	: 3model name	: Intel(R) Xeon(R) CPU           E5410  @ 2.33GHzcache size	: 6144 KBphysical id	: 0siblings	: 4core id		: 3cpu cores	: 4

Example 3a. Single socket Dual Core

Again, each processor has its own core so this is a dual core system.

processor	: 0model name	: Intel(R) Pentium(R) D CPU 3.00GHzcache size	: 2048 KBphysical id	: 0siblings	: 2core id		: 0cpu cores	: 2processor	: 1model name	: Intel(R) Pentium(R) D CPU 3.00GHzcache size	: 2048 KBphysical id	: 0siblings	: 2core id		: 1cpu cores	: 2 

Example 4. Dual Single core CPU, Hyperthreading ENABLED

This example shows that processer 0 and 2 share the same physical cpu and 1 and 3 share the same physical cpu. The number of siblings is twice the number of cores, which is another clue that this is a system with hyperthreading enabled.

processor	: 0model name	: Intel(R) Xeon(TM) CPU 3.60GHzcache size	: 1024 KBphysical id	: 0siblings	: 2core id		: 0cpu cores	: 1processor	: 1model name	: Intel(R) Xeon(TM) CPU 3.60GHzcache size	: 1024 KBphysical id	: 3siblings	: 2core id		: 0cpu cores	: 1processor	: 2model name	: Intel(R) Xeon(TM) CPU 3.60GHzcache size	: 1024 KBphysical id	: 0siblings	: 2core id		: 0cpu cores	: 1processor	: 3model name	: Intel(R) Xeon(TM) CPU 3.60GHzcache size	: 1024 KBphysical id	: 3siblings	: 2core id		: 0cpu cores	: 1

Example 5. Dual CPU Dual Core No hyperthreading

Of the 5 examples this should be the most capable system processor-wise. There are a total of 4 cores; 2 cores in 2 separate socketed physical cpus. Each core shares the 4MB cache with its sibling core. The higher clock rate (3.0 Ghz vs 2.3Ghz) should offer slightly better performance than example 3.

processor	: 0model name	: Intel(R) Xeon(R) CPU            5160  @ 3.00GHzcache size	: 4096 KBphysical id	: 0siblings	: 2core id		: 0cpu cores	: 2processor	: 1model name	: Intel(R) Xeon(R) CPU            5160  @ 3.00GHzcache size	: 4096 KBphysical id	: 0siblings	: 2core id		: 1cpu cores	: 2processor	: 2model name	: Intel(R) Xeon(R) CPU            5160  @ 3.00GHzcache size	: 4096 KBphysical id	: 3siblings	: 2core id		: 0cpu cores	: 2processor	: 3model name	: Intel(R) Xeon(R) CPU            5160  @ 3.00GHzcache size	: 4096 KBphysical id	: 3siblings	: 2core id		: 1cpu cores	: 2