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Dario Faggioli
92f0d4392e
As soft-affinity and caps will be available in Xen 4.10. Signed-off-by: Dario Faggioli <raistlin@linux.it> Reviewed-by: George Dunlap <george.dunlap@citrix.com>
110 lines
4.1 KiB
Plaintext
110 lines
4.1 KiB
Plaintext
% Credit2 Scheduler
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% Revision 2
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\clearpage
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# Basics
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---------------- ----------------------------------------------------
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Status: **Supported**
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Component: Hypervisor
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---------------- ----------------------------------------------------
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# Overview
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Credit2 is one of the virtual CPU (vCPU) scheduler available in the
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Xen hypervisor.
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Credit2 was designed as a general purpose scheduler, with particular
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focus on improving handling of mixed workloads, scalability and
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support for low latency applications inside VMs, with respect to
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Credit1.
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# User details
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Credit2 is not in use by default. In order to use it as the Xen
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scheduler the following parameter should be passed to the hypervisor
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at boot:
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`sched=credit2`
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Other parameters are available for tuning the behavior of Credit2
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(see `docs/misc/xen-command-line.markdown` for a complete list and
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for their meaning).
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Once the system is live, for creating a cpupool with Credit2 as
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its scheduler, either compile a cpupool configuration file, as
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described in `docs/man/xlcpupool.cfg.pod.5` (and as exemplified
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in `tools/examples/cpupool`), or use just `xl` directly:
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xl cpupool-create name=\"pool1\" sched=\"credit2\" cpus=[1,2]
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Two kind of interactions with the scheduler are possible:
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* checking or changing the global parameters, via, e.g.:
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* `xl sched-credit2 -s`
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* `xl sched-credit2 -s -p pool1`
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* `xl sched-credit2 -s -r 100`
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* checking or changing a VM scheduling parameters, via, e.g.:
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* `xl sched-credit2 -d vm1`
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* `xl sched-credit2 -d vm1 -w 1024`
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# Technical details
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Implementation entirely lives in the hypervisor. Xen has a pluggable,
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hook based, architecture for schedulers. Thanks to this, Credit2 code
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is all contained in `xen/common/sched_credit2.c`.
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Global scheduling parameters, such as context switching rate
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limiting, is only available from Xen 4.8 onward. In libxl, the
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`LIBXL\_HAVE\_SCHED\_CREDIT2\_PARAMS` symbol is introduced to
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indicate their availability.
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# Testing
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Any change done in Credit2 wants to be tested by doing at least the
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following:
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* boot the system with `sched=credit2`,
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* create a few virtual machine and verify that they boot and can
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run some basic workload (e.g., login into them and run simple commands),
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* shutdown/reboot the virtual machines,
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* shutdown/reboot the system.
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Ideally, all the above steps should **also** be performed in a configuration
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where Credit2 is used as the scheduler of a cpupool, and by also doing the
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following:
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* move a virtual machine inside and outside a Credit2 cpupool.
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# Areas for improvement
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* vCPUs' reservations (similar to caps, but providing a vCPU with guarantees
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about some pCPU time it will always be able to execute for);
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* benchmarking for assessing the best combination of values for the various
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parameters (`sched\_credit2\_migrate\_resist`, `credit2\_balance\_over`,
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`credit2\_balance\_under`)
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# Known issues
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* I/O oriented benchmarks (like network and disk throughput) have given
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contradictory and non-conclusive results so far. Need to run more of
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those.
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# References
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* "Scheduler development update", XenSummit Asia 2009 [whitepaper](http://www-archive.xenproject.org/files/xensummit_intel09/George_Dunlap.pdf)
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* "Scheduling in Xen" [XPDS15 Presentation](http://events.linuxfoundation.org/sites/events/files/slides/Faggioli_XenSummit.pdf)
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* "Scope and Performance of Credit-2 Scheduler" [XPDS16 Presentation](http://www.slideshare.net/xen_com_mgr/xpds16-scope-and-performance-of-credit2-scheduler-anshul-makkar-ctirix-systems-uk-ltd)
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* "The Credit2 Scheduler" [on the Xen-Project wiki](https://wiki.xenproject.org/wiki/Credit2_Scheduler_Development)
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* "Xen Project Schedulers" [on the Xen-Project wiki](https://wiki.xenproject.org/wiki/Xen_Project_Schedulers)
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# History
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------------------------------------------------------------------------
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Date Revision Version Notes
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---------- -------- -------- -------------------------------------------
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2016-10-14 1 Xen 4.8 Document written
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2017-11-6 2 Xen 4.10 Soft-affinity and caps implemented
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---------- -------- -------- -------------------------------------------
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