Fast Live Migration

The NFV project requires fast live migration. The specific requirement is total live migration time < 2Sec, while keeping the VM down time < 10ms when running DPDK L2 forwarding workload.

We measured the baseline data of migrating an idle 8GiB guest running a DPDK L2 forwarding work load and observed that the total live migration time was 2271ms while the VM downtime was 26ms. Both of these two indicators failed to satisfy the requirements.

Current Challenges

The following 4 features have been developed over the years to make the live migration process faster.

  • XBZRLE:

    Helps to reduce the network traffic by just sending the compressed data.

  • RDMA:

    Uses a specific NIC to increase the efficiency of data transmission.

  • Multi thread compression:

    Compresses the data before transmission.

  • Auto convergence:

    Reduces the data rate of dirty pages.

Tests show none of the above features can satisfy the requirement of NFV. XBZRLE and Multi thread compression do the compression entirely in software and they are not fast enough in a 10Gbps network environment. RDMA is not flexible because it has to transport all the guest memory to the destination without zero page optimization. Auto convergence is not appropriate for NFV because it will impact guest’s performance.

So we need to find other ways for optimization.

Optimizations

  1. Delay non-emergency operations By profiling, it was discovered that some of the cleanup operations during the stop and copy stage are the main reason for the long VM down time. The cleanup operation includes stopping the dirty page logging, which is a time consuming operation. By deferring these operations until the data transmission is completed the VM down time is reduced to about 5-7ms.
  2. Optimize zero page checking Currently QEMU uses the SSE2 instruction to optimize the zero pages checking. The SSE2 instruction can process 16 bytes per instruction. By using the AVX2 instruction, we can process 32 bytes per instruction. Testing shows that using AVX2 can speed up the zero pages checking process by about 25%.
  3. Remove unnecessary context synchronization. The CPU context was being synchronized twice during live migration. Removing this unnecessary synchronization shortened the VM downtime by about 100us.

Test Environment

The source and destination host have the same hardware and OS: :: Host: HSW-EP CPU: Intel(R) Xeon(R) CPU E5-2699 v3 @ 2.30GHz RAM: 64G OS: RHEL 7.1 Kernel: 4.2 QEMU v2.4.0

Ethernet controller: Intel Corporation Ethernet Controller 10-Gigabit X540-AT2 (rev 01) QEMU parameters: :: ${qemu} -smp ${guest_cpus} -monitor unix:${qmp_sock},server,nowait -daemonize -cpu host,migratable=off,+invtsc,+tsc-deadline,pmu=off -realtime mlock=on -mem-prealloc -enable-kvm -m 1G -mem-path /mnt/hugetlbfs-1g -drive file=/root/minimal-centos1.qcow2,cache=none,aio=threads -netdev user,id=guest0,hostfwd=tcp:5555-:22 -device virtio-net-pci,netdev=guest0 -nographic -serial /dev/null -parallel /dev/null

Network connection

live migration network connection

Test Result

The down time is set to 10ms when doing the test. We use pktgen to send the packages to guest, the package size is 64 bytes, and the line rate is 2013 Mbps.

  1. Total live migration time

    The total live migration time before and after optimization is shown in the chart below. For an idle guest, we can reduce the total live migration time from 2070ms to 401ms. For a guest running the DPDK L2 forwarding workload, the total live migration time is reduced from 2271ms to 654ms.

total live migration time

  1. VM downtime

    The VM down time before and after optimization is shown in the chart below. For an idle guest, we can reduce the VM down time from 29ms to 9ms. For a guest running the DPDK L2 forwarding workload, the VM down time is reduced from 26ms to 5ms.

vm downtime