| Use multiple thread (de)compression in live migration |
| ===================================================== |
| Copyright (C) 2015 Intel Corporation |
| Author: Liang Li <liang.z.li@intel.com> |
| |
| This work is licensed under the terms of the GNU GPLv2 or later. See |
| the COPYING file in the top-level directory. |
| |
| Contents: |
| ========= |
| * Introduction |
| * When to use |
| * Performance |
| * Usage |
| * TODO |
| |
| Introduction |
| ============ |
| Instead of sending the guest memory directly, this solution will |
| compress the RAM page before sending; after receiving, the data will |
| be decompressed. Using compression in live migration can help |
| to reduce the data transferred about 60%, this is very useful when the |
| bandwidth is limited, and the total migration time can also be reduced |
| about 70% in a typical case. In addition to this, the VM downtime can be |
| reduced about 50%. The benefit depends on data's compressibility in VM. |
| |
| The process of compression will consume additional CPU cycles, and the |
| extra CPU cycles will increase the migration time. On the other hand, |
| the amount of data transferred will decrease; this factor can reduce |
| the total migration time. If the process of the compression is quick |
| enough, then the total migration time can be reduced, and multiple |
| thread compression can be used to accelerate the compression process. |
| |
| The decompression speed of Zlib is at least 4 times as quick as |
| compression, if the source and destination CPU have equal speed, |
| keeping the compression thread count 4 times the decompression |
| thread count can avoid resource waste. |
| |
| Compression level can be used to control the compression speed and the |
| compression ratio. High compression ratio will take more time, level 0 |
| stands for no compression, level 1 stands for the best compression |
| speed, and level 9 stands for the best compression ratio. Users can |
| select a level number between 0 and 9. |
| |
| |
| When to use the multiple thread compression in live migration |
| ============================================================= |
| Compression of data will consume extra CPU cycles; so in a system with |
| high overhead of CPU, avoid using this feature. When the network |
| bandwidth is very limited and the CPU resource is adequate, use of |
| multiple thread compression will be very helpful. If both the CPU and |
| the network bandwidth are adequate, use of multiple thread compression |
| can still help to reduce the migration time. |
| |
| Performance |
| =========== |
| Test environment: |
| |
| CPU: Intel(R) Xeon(R) CPU E5-2680 0 @ 2.70GHz |
| Socket Count: 2 |
| RAM: 128G |
| NIC: Intel I350 (10/100/1000Mbps) |
| Host OS: CentOS 7 64-bit |
| Guest OS: RHEL 6.5 64-bit |
| Parameter: qemu-system-x86_64 -accel kvm -smp 4 -m 4096 |
| /share/ia32e_rhel6u5.qcow -monitor stdio |
| |
| There is no additional application is running on the guest when doing |
| the test. |
| |
| |
| Speed limit: 1000Gb/s |
| --------------------------------------------------------------- |
| | original | compress thread: 8 |
| | way | decompress thread: 2 |
| | | compression level: 1 |
| --------------------------------------------------------------- |
| total time(msec): | 3333 | 1833 |
| --------------------------------------------------------------- |
| downtime(msec): | 100 | 27 |
| --------------------------------------------------------------- |
| transferred ram(kB):| 363536 | 107819 |
| --------------------------------------------------------------- |
| throughput(mbps): | 893.73 | 482.22 |
| --------------------------------------------------------------- |
| total ram(kB): | 4211524 | 4211524 |
| --------------------------------------------------------------- |
| |
| There is an application running on the guest which write random numbers |
| to RAM block areas periodically. |
| |
| Speed limit: 1000Gb/s |
| --------------------------------------------------------------- |
| | original | compress thread: 8 |
| | way | decompress thread: 2 |
| | | compression level: 1 |
| --------------------------------------------------------------- |
| total time(msec): | 37369 | 15989 |
| --------------------------------------------------------------- |
| downtime(msec): | 337 | 173 |
| --------------------------------------------------------------- |
| transferred ram(kB):| 4274143 | 1699824 |
| --------------------------------------------------------------- |
| throughput(mbps): | 936.99 | 870.95 |
| --------------------------------------------------------------- |
| total ram(kB): | 4211524 | 4211524 |
| --------------------------------------------------------------- |
| |
| Usage |
| ===== |
| 1. Verify both the source and destination QEMU are able |
| to support the multiple thread compression migration: |
| {qemu} info migrate_capabilities |
| {qemu} ... compress: off ... |
| |
| 2. Activate compression on the source: |
| {qemu} migrate_set_capability compress on |
| |
| 3. Set the compression thread count on source: |
| {qemu} migrate_set_parameter compress-threads 12 |
| |
| 4. Set the compression level on the source: |
| {qemu} migrate_set_parameter compress-level 1 |
| |
| 5. Set the decompression thread count on destination: |
| {qemu} migrate_set_parameter decompress-threads 3 |
| |
| 6. Start outgoing migration: |
| {qemu} migrate -d tcp:destination.host:4444 |
| {qemu} info migrate |
| Capabilities: ... compress: on |
| ... |
| |
| The following are the default settings: |
| compress: off |
| compress-threads: 8 |
| decompress-threads: 2 |
| compress-level: 1 (which means best speed) |
| |
| So, only the first two steps are required to use the multiple |
| thread compression in migration. You can do more if the default |
| settings are not appropriate. |
| |
| TODO |
| ==== |
| Some faster (de)compression method such as LZ4 and Quicklz can help |
| to reduce the CPU consumption when doing (de)compression. If using |
| these faster (de)compression method, less (de)compression threads |
| are needed when doing the migration. |
