Which CPU features is most important? (Speed, cores, cache size..?)

[rickardnobel]

When looking at processors for servers to be used with ESX/ESXi, which features should be most important?

That is, from the wide range of CPUs available, what should best spent money for a customer?

Would it be the frequency of the cpu, or the amount of cores, the cache sizes, features like hardware virtualization support or any other things?

I am not looking for a specific CPU to buy at the moment, I am just interested in the topic and which considerations that could be made.

[AntonVZhbankov]

It depends on what are the applications customer wants to run on these CPUs.

But in general higher frequency, bigger cache and more cores are always better. If you meet situation when you have to choose between cache size and number of cores for ex, there is no universal answer. It depends.

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MCSA, MCTS Hyper-V, VCP 3/4, VMware vExpert

http://blog.vadmin.ru

[rickardnobel]

Thanks for your reply. The classic "it depends" then. ; )

Is there any general type of application/system that would often gain from larger CPU cache?

Could there be said anything regarding the value of speed (Ghz) vs amounts of cores?

[AntonVZhbankov]

More GHz is better in case of few VMs with high CPU demands, because each vCPU can run as fast as one physical core runs. So peak performance is limited by CPU speed.

But if you need to run MANY VMs, especially VMs with multiple vCPUs then CPUs with more core are the best choice.

More cache is better in any case, but it costs money and generates more heat. Problem with heat is real - for ex. in HP BL460c G1 you can install 64GB of RAM with 80W CPUs and only 48GB with 120W CPUs.

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MCSA, MCTS Hyper-V, VCP 3/4, VMware vExpert

http://blog.vadmin.ru

[rickardnobel]

More cache is better in any case, but it costs money and generates more heat. Problem with heat is real - for ex. in HP BL460c G1

you can install 64GB of RAM with 80W CPUs and only 48GB with 120W CPUs.

Is the increase of 40W above due just to cache size? Or frequency increase also?

But larger CPU cache should be good in a virtual enviroment? I would think that perhaps when running several VMs with many processes/threads the constant switching between many threads would make the value of CPU cache less?

[AntonVZhbankov]

>Is the increase of 40W above due just to cache size? Or frequency increase also?

It doesn't really matter. Just an example when there is some additional parameter that should be taken into account.

Yes, large cache is very good in virtual environment. But as I already said it costs money and additional heat, and there is some point of balance when performance is enough already and customer does not want to pay more to achieve top performance.

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MCSA, MCTS Hyper-V, VCP 3/4, VMware vExpert

http://blog.vadmin.ru

[AndreTheGiant]

Clock speed could be very important on CPU intensitive applications that are not scalable on the number of cores.

For example I've seen Terminal Servers and Citrix XenApp VMs with some performance issues on "low" speed CPU.

But usually the real bottlenecks are on storage or network, or in some cases on RAM...

Andre

[rickardnobel]

But usually the real bottlenecks are on storage or network, or in some cases on RAM...

The size of RAM or the speed / latency of the memory?

[AndreTheGiant]

RAM size define how many VMs can you run.

RAM speed could be important to reduce the latency of the RAM... In most cases you never notice it... but more VMs, less must be the latency...

Andre

[AntonVZhbankov]

Usually latency doesn't play any significant role, especially in case when all goes to swapping. More RAM is always better.

But in CPU-bound environments there is a border when more RAM doesn't change anything and you can't run more VMs because CPU is already loaded up to 100%.

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MCSA, MCTS Hyper-V, VCP 3/4, VMware vExpert

http://blog.vadmin.ru