[libvirt] [PATCH RESEND RFC v4 1/6] Introduce the function virCgroupForVcpu

Anthony Liguori aliguori at us.ibm.com
Thu Jul 21 14:24:10 UTC 2011

On 07/21/2011 09:21 AM, Daniel P. Berrange wrote:
> On Thu, Jul 21, 2011 at 08:44:35AM -0500, Anthony Liguori wrote:
>> On 07/21/2011 08:34 AM, Daniel P. Berrange wrote:
>>> On Thu, Jul 21, 2011 at 07:54:05AM -0500, Adam Litke wrote:
>>>> Added Anthony to give him the opportunity to address the finer points of
>>>> this one especially with respect to the qemu IO thread(s).
>>>> This feature is really about capping the compute performance of a VM
>>>> such that we get consistent top end performance.  Yes, qemu has non-VCPU
>>>> threads that this patch set doesn't govern, but that's the point.  We
>>>> are not attempting to throttle IO or device emulation with this feature.
>>>>   It's true that an IO-intensive guest may consume more host resources
>>>> than a compute intensive guest, but they should still have equal top-end
>>>> CPU performance when viewed from the guest's perspective.
>>> I could be mis-understanding, what you're trying to achieve,
>>> here, so perhaps we should consider an example.
>>>   - A machine has 4 physical CPUs
>>>   - There are 4 guests on the machine
>>>   - Each guest has 2 virtual CPUs
>>> So we've overcommit the host CPU resources x2 here.
>>> Lets say that we want to use this feature to ensure consistent
>>> top end performance of every guest, splitting the host pCPUs
>>> resources evenly across all guests, so each guest is ensured
>>> 1 pCPU worth of CPU time overall.
>>> This patch lets you do this by assigning caps per VCPU. So
>>> in this example, each VCPU cgroup would have to be configured
>>> to cap the VCPUs at 50% of a single pCPU.
>>> This leaves the other QEMU threads uncapped / unaccounted
>>> for. If any one guest causes non-trivial compute load in
>>> a non-VCPU thread, this can/will impact the top-end compute
>>> performance of all the other guests on the machine.
>> But this is not undesirable behavior.  You're mixing up consistency
>> and top end performance.  They are totally different things and I
>> think most consumers of capping really only care about the later.
>>> If we did caps per VM, then you could set the VM cgroup
>>> such that the VM as a whole had 100% of a single pCPU.
>> Consistent performance is very hard to achieve.  The desire is to
>> cap performance not just within a box, but also across multiple
>> different boxes with potentially different versions of KVM.  The I/O
>> threads are basically hypervisor overhead.  That's going to change
>> over time.
>>> If a guest is 100% compute bound, it can use its full
>>> 100% of a pCPU allocation in vCPU threads. If any other
>>> guest is causing CPU time in a non-VCPU thread, it cannot
>>> impact the top end compute performance of VCPU threads in
>>> the other guests.
>>> A per-VM cap would, however, mean a guest with 2 vCPUs
>>> could have unequal scheduling, where one vCPU claimed 75%
>>> of the pCPU and the othe vCPU got left with only 25%.
>>> So AFAICT, per-VM cgroups is better for ensuring top
>>> end compute performance of a guest as a whole, but
>>> per-VCPU cgroups can ensure consistent top end performance
>>> across vCPUs within a guest.
>> And the later is the primary use case as far as I can tell.
>> If I'm a user, I'll be very confused if I have a 4 VCPU guest, run 1
>> instance of specint, and get X as the result.  I then run 4
>> instances of specint and get X as the result.  My expectation is to
>> get 4X as the result because I'm running 4 instances.
> I don't see why doing limits at per-VM vs per-VM has an impact
> on performance when adding extra guests. It would certainly
> change behaviour if adding extra vCPUs to a guest though.

My use of "instance" was referring to a copy of specint.  IOW,

4-VCPU guest:

$ ./specint
score - 490

$ ./specint --threads=4
score - 490

That would confuse a user.  The expectation is:

$ ./specint --threads=4
score - 1960


Anthony Liguori

> Daniel

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