Question about Processor and HT (available resources)

[Tomek24VMWARE]

Hello,

I have R710 with 2x Intel X5680 3,3 Ghz

In ESXi I

enable HT in Configuration tab -> Processors

On Summary tab I see Resources:

12 x 3,324 Ghz = ~ 39,888 Mhz

OK

2 x CPU X5680 x 6 core each = 12

cores this is OK

Question where is HT??

On enable HT should be (right?):

2x CPU X5680 x 6 cores each x HT (2x) = 24 Logical

processors

Whay Capacity is ~39,8 Ghz and not 79,7 Ghz (enable HT)??

24

logical processors x 3,324 Ghz = ~79,7 Ghz.

When I disable HT

Capacity does not change.

Can someone explain to me how it works?

Tom

[J1mbo]

HT does not provide additional processing cores but rather allows two threads to be scheduled on one core (there are two sets of registers in one core essentially). Hence whilst the CPU core is stalled waiting for a fetch from memory, the core swiches to the thread waiting in the second set of registers. Hence HT provides a modest improvement in overall throughput by providing a mechanism to ustilise otherwise wasted CPU cycles.

http://blog.peacon.co.uk

Please award points to any useful answer.

[Varunsh]

Enabling HT depends on purpose of having it...it has its own Pros and cons

For example , Pros

Better co-scheduling of SMP vCPU

10-20 % application Performance improvement

Cons

Proccessor resource will be when you have hyperthreading enabled.

This means that the two threads running on the same processor compete for the same resources if they both have high demand for them. This can, in turn, degrade performance.

[Varunsh]

Enabling HT depends on purpose of having it...it has its own Pros and cons

For example , Pros

Better co-scheduling of SMP vCPU

10-20 % application Performance improvement

Cons

Proccessor resource will be when you have hyperthreading enabled.

This means that the two threads running on the same processor compete for the same resources if they both have high demand for them. This can, in turn, degrade performance.

[Tomek24VMWARE]

OK Thank you for response.

Then what is better?

enable HT and create VM box with 2 vcpu or 1 vcpu

or

disable HT and create VM box with 2 vcpu or 1 vcpu

?

[DSTAVERT]

First enable hyperthreading. It can and mostly does provide benefit. Second don't provide the virtual machine any more resources that it needs. Monitor the VM and if it requires more then increase as necessary. Overcommitment of resources is a sure way to decrease overall performance of the Host.

[Varunsh]

ESXi 4 do support hyperthreading.....you can try to test hyperthreading in a lab enviornment and see which option is good for production