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The server, me and the conversation

We were moving a project from AWS to our co-located DC. We have setup KVMs scheduled by Cloudstack for each of the component in the architecture. The KVMs used local storage. The VMs are provisioned with more than required resources because we have the opinion that in our DC scaling during peak load and then downscaling doesn't offer much benefits financially as we are anyways paying for the hardware in advance and its also powered on. Its going to be idle if not used. Now we found something interesting our latency in co-located DC was 2 times more than in AWS. The time for first byte at our load balancer in aws was 60ms average and at our DC was 112ms. We started our debugging mission, Mission Conquer-AWS. All the servers are newer Dell hardwares. So the initially intuition was virtualisation is causing the issue.

Conversation with the Hypervisor

We started with CPU optimisation, we started using the host-passthrough mode of CPU in libvirt so VMs dont see QEMU emulated CPUs, they see the actual CPU so they understand the flags supported by the underlying hardware and can use instruction set accordingly. We did VCPU pinning so that latency due to NUMA is ruled out (we know this should cause nano second latency not in millisecond). We stopped Kernel shared pages completely so that CPU time is not spent in finding identical pages and use Copy on Write if shared pages are written to. We also disabled transparent huge pages to avoid CPU time being spent on defragmenting memory for allocating huge pages. We didn't see any of the above optimisations yield noticeable impact in latency.

During the debugging we noted libvirt is configured to use ide for disks and e1000 for network calls. We understood KVM is running full virtualisation with HVM instead of Paravirtualised HVM. This causes KVM to do VMEXIT on an interrupt, handle it and do VMRUN. We restarted all vms with Virtio driver so the PV mode is on. On an interrupt guest via PV driver will talk to emulated QEMU device and do the IO. This reduced our latency to 100ms



















We are out of optimisations at hypervisor level and we are still 40ms behind AWS. There are still options available but all of them will improve in nano second which is not going to help us when we are over-provisioned(here over provisioned means provisioned beyond requirement, not to be confused with CPU overprovisioning terminology in VM world)

The next obvious choice was to test at baremetal directly and see how much it is giving raw performance. The baremetal gave pretty much same latency. This was something a shocker to us as unless AWS has a magic sauce of their own, beating baremetal with a virtualisation technology with such a huge margin is unimaginable. We removed hypervisor from the possible suspect list.

Conversation with the server

We saw the ping latency to VM was sometimes higher than the ping latency to bare metal directly. This took us to the following 
response by Greg https://www.spinics.net/lists/kvm/msg138556.html
The IO thread of KVM may be scheduled at a processor in C6 state and the latency may be due to wake up latency of the processor to C0 state. So we made IOthread busy with iperf and tried ping, the responsiveness of ping increased and we saw an improvement of 0.2 ms in ping response. We decided to fix the C state to 0 at the grub so that CPU always runs hot. This needed reboot of the whole infra as host machines had to be restarted with desired max C state. During this literature survey we also hit at P state. C state controls the sleep cycle of processors for power efficiency. P states control the frequency of the processors for power efficiency. Intel supports two scaling governors perfomance and powersave. Powersave uses P states which are biased toward powersaving efficiency than performance and this is the default P state in Linux. /proc/cpuinfo showed the CPUs running at 1.2GHz which is 50% of their 2.4GHz. So the instruction cycle runs double the time than it can run when hot. We switched the governor to performance mode. (https://wiki.archlinux.org/index.php/CPU_frequency_scaling)
Performance mode selects P states and set frequency accordingly where performance is biased over power efficiency. The performance improved and the latency came down to 60ms.
In the hindsight this looks like the first thing to be checked. 

Overall the mission is accomplished and we have started migrating traffic from AWS. Lesson Learnt - Look at the /proc/cpuinfo cpu MHz before getting into further optimisation

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