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diff --git a/docs/userguide/live_migration.userguide.rst b/docs/userguide/live_migration.userguide.rst new file mode 100644 index 000000000..9fa9b82fd --- /dev/null +++ b/docs/userguide/live_migration.userguide.rst @@ -0,0 +1,121 @@ +.. This work is licensed under a Creative Commons Attribution 4.0 International License. + +.. http://creativecommons.org/licenses/by/4.0 + +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 +------------------------- +a. 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. +b. 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%. +c. 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 + +.. figure:: lmnetwork.jpg + :align: center + :alt: live migration network connection + :figwidth: 80% + + +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. + +a. 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. + +.. figure:: lmtotaltime.jpg + :align: center + :alt: total live migration time + +b. 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. + +.. figure:: lmdowntime.jpg + :align: center + :alt: vm downtime + :figwidth: 80% |