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Multi-core CPU: can I say I have a 3x2.1GHz=6.3GHz CPU?

I plan to buy latop with with i5-2410M processor that have 2 cores (according to spec it can also have 4 threads). I have had use Linode server that have 4 CPUs and one process (mysqld) on Debian (in top command) have 200% of CPU usage, will 2 cores have the same thing.

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    @DanielAndersson It's not duplicate it's different question, I use linode server that have 4 CPUs and on havy load it had about 200% of CPU usage for mysql, So my question was if I can have the same thing on 2 cores.
    – jcubic
    Commented Jul 31, 2012 at 9:37

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Linux supports multicore processors. If your processor can run 4 threads it means that it will have a speed of four processors in some cases. However if you plan to run single application (that don't support multicore) quickly than your processor needs to have big cache and great clock speed.

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  • Will Linux shedule processes to multicores, like one proccess to one core and second process to other one, or it's only per process? In my case most CPU usage (in my currect laptop) come from VLC or Totem and Google Chrome.
    – jcubic
    Commented Jul 31, 2012 at 5:28
  • Linux will schedule KSEs (Kernel Scheduling Entities) to cores. Each process will consist of at least one KSE, possibly more.
    – David Schwartz
    Commented Jul 31, 2012 at 6:57
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Your question asks us to compare an actual CPU to a hypothetical multi-CPU system. That's not really possible to do. Sure, it's possible in theory to imagine a system with two CPUs that will provide the same performance as your dual-core CPU. It's also possible to imagine two CPU systems that are faster or slower than an i5-2410M.

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As David Schwartz already pointed out, such a comparison is daunting, if possible at all.

Assuming that one talks of processors of the same generation and price class and operating at the same or a very similar clock frequency, a single dual-core processor (or 2*N core processor) is usually superior to two single-core processors (or two N core processors).
Obviously, you cannot compare a 2-core Conroe to two single-core Sandy Bridges (does that even exist? -- but you get the point) in a very meaninful way. Likewise, you obviously can't compare processors when one has three times the cache size or twice the frequency.

Making the assumption that the CPUs are sufficiently similar, several things are necessarily not as optimal between two CPUs as within a single multi-core one:

  • longer lanes both between the cores and to the RAM (millimeters/centimeters instead of micrometers)
  • more "extra logic" needed for memory access
  • more synchronization overhead for atomic operations
  • Level-2 cache not shared
  • chances are that there's NUMA

Thus, all in all, electric current, which moves at more or less fixed speed, has to go through a longer piece of wire, which takes time. Also, moving a thread to another idle core is not as trivial (it may be prohibitively expensive!) as it is on single a multi-core CPU. This limits the operating system's ability to utilize all available resources.

Therefore the answer to your question is: "No, not the same".

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  • By the same I mean if one process on 2 cores can have 200% of CPU usage in top program on linux. I'll update my question.
    – jcubic
    Commented Jul 31, 2012 at 9:39

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