Messages in this thread | | | Date | Thu, 12 Jun 2003 00:54:01 +0200 | From | "J.A. Magallon" <> | Subject | Re: SMP question |
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On 06.11, Artemio wrote: > Hello! > > > > How much performance will I loose this way? Is SMP *THAT* critical? > > > > You will lose about half your CPU power. > > Hmmm... So, you mean uni-processor Linux kernel can't see two processors as > one "big" processor? >
No, but it will work as if it does...explain below.
You have 2 processor packages, each one is HyperThreading capable. This means you have two 'CPUs' inside each package, so that sums up your 4 CPUs. But there is a flaw. The 2 'CPUs' inside each processor package are not full real CPUs, just two register sets that share cache, FP units, integer units and so on. So let's say your Xeon has 8 FP units, and you want to run a FPU intensive task with low or null disk IO. If you activate hyperthreading each of the 2 'cpus' has 4 FP units, so half the computation power. If you deactivate HT, you have 1 CPU with 8 FP units.
In short, for FP intensive tasks, hyperthreading is a big lie... You can't run 2 computations in parallel.
-- J.A. Magallon <jamagallon@able.es> \ Software is like sex: werewolf.able.es \ It's better when it's free Mandrake Linux release 9.2 (Cooker) for i586 Linux 2.4.21-rc7-jam1 (gcc 3.3 (Mandrake Linux 9.2 3.3-1mdk)) - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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