VCPUs vs Cores

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VCPUs vs Cores

admin-2
Hi,

on a server with two QuadCore Xeon CPUs I observe the following:

2 CPUs x 4 Cores x 2 threads result in 16 "cores" / VCPUs.
I now assign 12 of these to a windows server VM. In the
device manager I see 12 CPUs. Is every core (vcpu) passed
as a single CPU to my VM?

If so, is there any config file option to change this?
I am asking because one of the limitations of the free
SQL Server Express is that it is utilizing max 1 CPU / 4 cores.

If each thread / vcpu is passed as a independent CPU I
am lacking possible performance. Any hint on that?

Talking about Xen 4.4 here.

Regards,
Manuel

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Re: VCPUs vs Cores

Kun Cheng
Hi Manuel,

You can pin those vcpus to cpu cores in either your vm configurations or via xl command line.
See https://wiki.xenproject.org/wiki/Tuning_Xen_for_Performance or simply search xen vcpu pinning in Google.

For example, in your guest vm config file, use cpus="2-8" means pin vcpus to core 2-8, in your case you might want to pin them to 0-11 or 2-13.

Best,
Kenneth


On Wed, Oct 25, 2017 at 7:53 AM admin <[hidden email]> wrote:
Hi,

on a server with two QuadCore Xeon CPUs I observe the following:

2 CPUs x 4 Cores x 2 threads result in 16 "cores" / VCPUs.
I now assign 12 of these to a windows server VM. In the
device manager I see 12 CPUs. Is every core (vcpu) passed
as a single CPU to my VM?

If so, is there any config file option to change this?
I am asking because one of the limitations of the free
SQL Server Express is that it is utilizing max 1 CPU / 4 cores.

If each thread / vcpu is passed as a independent CPU I
am lacking possible performance. Any hint on that?

Talking about Xen 4.4 here.

Regards,
Manuel

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Re: VCPUs vs Cores

Simon Hobson-2
Kun Cheng <[hidden email]> wrote:

>> I am asking because one of the limitations of the free
>> SQL Server Express is that it is utilizing max 1 CPU / 4 cores.

> You can pin those vcpus to cpu cores in either your vm configurations or via xl command line.

I don't think that was the question he was asking ...
As I read it, the software he is using is restricted to ONE CPU but up to 4 cores in that one CPU. Running under Xen, passing multiple cores through results in a VM with multiple single core CPUs. So instead of being able to utilise 4 cores in the "real" CPU, it's limited to just one because of this abstraction.

Or put another way, due to licensing restrictions, he really needs a VM with 4-core virtual CPU(s) where each core is mapped to a real core.
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Re: VCPUs vs Cores

admin-2
Am Mittwoch, den 25.10.2017, 08:08 +0100 schrieb Simon Hobson:
> Kun Cheng <[hidden email]> wrote:
>
> > > I am asking because one of the limitations of the free
> > > SQL Server Express is that it is utilizing max 1 CPU / 4 cores.
> > You can pin those vcpus to cpu cores in either your vm
> > configurations or via xl command line.
>
> I don't think that was the question he was asking ...
exactly

> As I read it, the software he is using is restricted to ONE CPU but
> up to 4 cores in that one CPU. Running under Xen, passing multiple
> cores through results in a VM with multiple single core CPUs. So
> instead of being able to utilise 4 cores in the "real" CPU, it's
> limited to just one because of this abstraction.
that's correct

> Or put another way, due to licensing restrictions, he really needs a
> VM with 4-core virtual CPU(s) where each core is mapped to a real
> core.
This is the case if e.g. passing vcpus 4-7 to this VM while pinning
tp vcpus 4-7. In theory these are might be my second Xeon Core - but
how can I check how windows "sees this piece of hardware"?
Is it like 1 CPU with 4 cores, or 4 CPUs, each having 1 core?


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