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



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.


Daniel
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