Re: [Xen-users] CPU scheduling and allocated all VCPU.

[Austin S Hemmelgarn <ahferroin7@xxxxxxxxx>]

On 2014-07-01 08:07, Sophie wrote:
>  
> 
> Hi,
> 
>   We have lots of virtual machines running our Oracle Virtual Manager
> setup ( Oracle Linux with Xen) on x86.
> 
> 
> When I've setup new VMs I've always assigned CPUs to them instead of
> sharing them because all our VMs run RHEL and Oracle 11g.  In my opinion
> this ensures they have all CPU cycles dedicated without any chance
> they'll be starved.  Their combined SGA and PGA usually total 3Gb and
> I've allocated 4Gb of dedicated RAM.
> 
> Our DBA team, who were new to XEN and visualization seem to have a
> heightened interest in XEN and have asked me this:
> 
> ** Why don't we allocated 32 VCPUS to all virtual machines so that they
> can share all resources and when they need CPUs they can access those
> that were sitting idle ** Their logic was VCPUs could be better
> distributed like this.
> 
> 
> My question to you is what do you think?
> 
> Thanks, Sophie
> 
I think that they have failed to understand that when you don't bind
VCPUs to physical CPUs, xen automatically load-balances the VCPUs
dynamically.  Personally, I would keep the same number of VCPUs assigned
to each VM, but not pin them to specific physical CPUs (with the notable
exception of giving Domain-0 pinned VCPUs, and making sure that the VM's
don't run on those physical CPUs).  In my experience, this tends to get
better performance with I/O heavy workloads such as most database work.
 I would not, however, suggest over-provisioning VCPUs by more than
twice the number of physical CPUs.

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