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Andrew Cooper
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Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
123 lines
4.7 KiB
Plaintext
123 lines
4.7 KiB
Plaintext
% RTDS 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: **Experimental**
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Component: Hypervisor
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---------------- ----------------------------------------------------
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# Overview
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RTDS is one of the virtual CPU (vCPU) scheduler available in the Xen
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hypervisor.
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RTDS is a real-time scheduler, so its purpose is enabling
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**deterministic** scheduling of the virtual machine's vCPUs. It has
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been originally developed in the context of the RT-Xen project.
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# User details
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RTDS is not in use by default. In order to use it as the Xen scheduler
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the following parameter should be passed to the hypervisor at boot:
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`sched=rtds`
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Once the system is live, for creating a cpupool with RTDS 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=\"pool-rt\" sched=\"rtds\" cpus=[4,5,6,8]
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For checking or changing a VM's scheduling parameters from xl, do
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as follows:
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* `xl sched-rtds -d vm-rt -v all`
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* `xl sched-rtds -d vm-rt -v all -p 10000 -b 2500`
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It is possible, for a multiple vCPUs VM, to change the parameters of
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each vCPU individually:
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* `xl sched-rtds -d vm-rt -v 0 -p 20000 -b 10000 -e 1 -v 1 -p 45000 -b 12000 -e 0`
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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, RTDS code
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is all contained in `xen/common/sched_rtds.c`.
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In libxl, the availability of the RTDS scheduler is advertised by
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the presence of the LIBXL_HAVE_SCHED_RTDS symbol. The ability of
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specifying different scheduling parameters for each vcpu has been
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introduced later, and is available if the following symbols are defined:
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* LIBXL_HAVE_VCPU_SCHED_PARAMS,
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* LIBXL_HAVE_SCHED_RTDS_VCPU_PARAMS,
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* LIBXL_HAVE_SCHED_RTDS_VCPU_EXTRA.
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# Limitations
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RTDS is a special purpose scheduling. This is by design, and not at
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all a limitation, but it is certainly something to keep in mind when
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thinking about using it. The purpose of the scheduler is enabling
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deterministic and statically analyzable behavior (as per the
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real-time academic literature), according to the scheduling parameters
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assigned to each vCPU.
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Using RTDS a the Xen scheduler, and/or for general purpose workloads
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is definitely possible, but the vCPU scheduling parameters (of both
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Domain0 and of the various VMs) would probably require tweaking, with
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respect to their default values.
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# Testing
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Any change done in RTDS must be tested by doing the following:
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* create a cpupool with RTDS as its scheduler,
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* create a few virtual machines a move them in and out of the pool,
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* create a few virtual machines, directly inside the pool, and verify
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that they boot and can run some basic workload (e.g., login into them
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and run simple commands),
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* shutdown/reboot the virtual machines,
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The fact that the system boots fine when passing `sched=rtds` to Xen
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should also be verified.
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Finally, to check that the scheduler is working properly (although only
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at a macroscopic level), the following should be done:
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* create a VM with 1 vCPU and put it in the RTDS cpupool,
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* set the scheduling parameters such as it has a 50% reservation, with
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`xl sched-rtds -d vm -v all -p 100000 -b 50000`,
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* run a CPU-burning process inside the VM (e.g., `yes`),
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* check with `xentop` (in Domain0) that the VM is getting no more than
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50% pCPU time.
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# Areas for improvement
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* performance assessment, especially focusing on what level of real-time
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behavior the scheduler enables.
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# Known issues
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* OSSTest reports occasional failures on ARM.
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# References
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* "RT-Xen: Real-Time Virtualization" [XPDS14 Presentation](http://events.linuxfoundation.org/sites/events/files/slides/2014_Xen_Developer_Summit_0.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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* [RT-Xen Project](https://sites.google.com/site/realtimexen/)
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* [RTDS-Based-Scheduler](https://wiki.xenproject.org/wiki/RTDS-Based-Scheduler)
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* "The RTDS Scheduler" [on the Xen-Project wiki](https://wiki.xenproject.org/wiki/RTDS-Based-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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2017-08-31 2 Xen 4.10 Revise for work conserving feature
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---------- -------- -------- -------------------------------------------
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