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George Dunlap
9db777dda3
Signed-off-by: George Dunlap <george.dunlap@citrix.com> Acked-by: Jan Beulich <jbeulich@suse.com> Release-acked-by: Juergen Gross <jgross@suse.com>
101 lines
3.9 KiB
Plaintext
101 lines
3.9 KiB
Plaintext
% Credit Scheduler
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% Revision 1
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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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Credit (also known as Credit1) is the old virtual CPU (vCPU) scheduler
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of the Xen hypervisor.
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It is a general purpose, weighted fair-share scheduler.
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# User details
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Xen supports multiple schedulers. Credit is no longer the default. In
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order to use it as the Xen scheduler the following parameter should be
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passed to the hypervisor at boot:
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`sched=credit`
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Once the system is live, for creating a cpupool with Credit as its
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scheduler, either compile a cpupool configuration file, as described
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in `docs/man/xlcpupool.cfg.pod.5` (and as exemplified in
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`tools/examples/cpupool`), or use just `xl` directly:
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xl cpupool-create name=\"pool1\" sched=\"credit\" cpus=[4,8]
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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-credit -s`
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* `xl sched-credit -s -p pool1`
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* `xl sched-credit -s -t 20`
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* checking or changing a VM's scheduling parameters, via, e.g.:
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* `xl sched-credit -d vm1`
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* `xl sched-credit -d vm1 -w 512`
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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, Credit code
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is all contained in `xen/common/sched_credit.c`.
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# Limitations
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In Credit, a vCPU has a priority, a status (i.e., active or inactive),
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a weight and some credits... and all these things interact in a rather
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involved way. Also, with years of use, things have gotten even more
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complex (due to, e.g., the introduction of boosting, caps and vCPU
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soft-affinity).
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Dealing with such complexity is starting to be an issue. Odd behavior
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or subtle scheduling anomalies, that is not always possible to act upon,
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have been identified already. [1][2][3]
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A certain lack of scalability and difficulties and weakness in dealing
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with mixed workloads and VMs with low latency requirements are other
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known problems. [4] For all these reasons, effort is ongoing to have
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Credit2 become the new default scheduler.
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# Testing
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Any change to Credit code must to be tested by doing at least the following:
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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 the system.
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Ideally, all the above steps should **also** be performed in a configuration
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that includes cpupools, better if with pools using different schedulers, and
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by also doing the following:
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* move the virtual machines between cpupools.
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# References
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* [potential non-ideal behavior on hyperthreaded systems](https://lists.xenproject.org/archives/html/xen-devel/2014-07/msg01848.html) [1]
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* [long standing BOOST vs. migration bug](https://lists.xen.org/archives/html/xen-devel/2015-10/msg02851.html) [2]
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* [priority handling issues](https://lists.xenproject.org/archives/html/xen-devel/2016-05/msg01362.html) [3]
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* "Scheduler development update", XenSummit Asia 2009 [whitepaper](http://www-archive.xenproject.org/files/xensummit_intel09/George_Dunlap.pdf) [4]
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* "Scheduling in Xen" [XPDS15 Presentation](http://events.linuxfoundation.org/sites/events/files/slides/Faggioli_XenSummit.pdf)
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* "The Credit Scheduler" [on the Xen-Project wiki](https://wiki.xenproject.org/wiki/Credit_Scheduler)
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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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2019-02-7 3 Xen 4.12 No longer default scheduler
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---------- -------- -------- -------------------------------------------
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