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

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.