Signed-off-by: Dario Faggioli
---
Cc: Meng Xu
Cc: George Dunlap
Cc: Wei Liu
Cc: Lars Kurth
Cc: Andrew Cooper
Cc: Ian Jackson
Cc: Jan Beulich
Cc: Konrad Rzeszutek Wilk
Cc: Stefano Stabellini
---
Changes from v1:
* file renamed from rtds.pandoc to sched_rtds.pandoc;
* feature name is now 'RTDS Scheduler';
* kill the 'e.g.'-s in the header (sorry!);
* removed 'Architecture' line from header, as this is arch independent;
* removed some text that was duplicated in other sched feature documents;
it's redundant wrt what's in the wiki about schedulers, and the page
is referenced.
---
docs/features/sched_rtds.pandoc | 121 +++
1 file changed, 121 insertions(+)
create mode 100644 docs/features/sched_rtds.pandoc
diff --git a/docs/features/sched_rtds.pandoc b/docs/features/sched_rtds.pandoc
new file mode 100644
index 000..2843d97
--- /dev/null
+++ b/docs/features/sched_rtds.pandoc
@@ -0,0 +1,121 @@
+% RTDS Scheduler
+% Revision 1
+
+\clearpage
+
+# Basics
+
+ Status: **Experimental**
+
+ Component: Hypervisor
+
+
+# Overview
+
+RTDS is one of the virtual CPU (vCPU) scheduler available in the Xen
+hypervisor.
+
+RTDS is a real--time scheduler, so its purpose is enabling
+**deterministic** scheduling of the virtual machine's vCPUs. It has
+been originally developed in the context of the RT-Xen project.
+
+# User details
+
+RTDS is not in use by default. In order to use it as the Xen scheduler
+the following parameter should be passed to the hypervisor at boot:
+
+`sched=rtds`
+
+Once the system is live, for creating a cpupool with RTDS as its
+scheduler, either compile a cpupool configuration file, as described
+in `docs/man/xlcpupool.cfg.pod.5` (and as exemplified in
+`tools/examples/cpupool`), or use just `xl` directly:
+
+xl cpupool-create name=\"pool-rt\" sched=\"rtds\" cpus=[4,5,6,8]
+
+For checking or changing a VM's scheduling parameters from xl, do
+as follows:
+* `xl sched-rtds -d vm-rt`
+* `xl sched-rtds -d vm-rt -t 1 -b 25000`
+
+It is possible, for a multiple vCPUs VM, to change the parameters of
+each vCPU individually:
+* `xl sched-rtds -d vm-rt -v 0 -p 2 -b 1 -v 1 -p 45000 -b 12000`
+
+# Technical details
+
+Implementation entirely lives in the hypervisor. Xen has a pluggable,
+hook based, architecture for schedulers. Thanks to this, RTDS code
+is all contained in `xen/common/sched_rtds.c`.
+
+In libxl, the availability of the RTDS scheduler is advertised by
+the presence of the LIBXL\_HAVE\_SCHED\_RTDS symbol. The ability of
+specifying different scheduling parameters for each vcpu has been
+introduced later, and is available if the following symbols are defined:
+* `LIBXL\_HAVE\_VCPU\_SCHED\_PARAMS`,
+* `LIBXL\_HAVE\_SCHED\_RTDS\_VCPU\_PARAMS`.
+
+# Limitations
+
+RTDS is a special purpose scheduling. This is by design, and not at
+all a limitation, but it is certainly something to keep in mind when
+thinking about using it. The purpose of the scheduler is enabling
+deterministic and statically analyzable behavior (as per the
+real-time academic literature), according to the scheduling parameters
+assigned to each vCPU.
+
+Using RTDS a the Xen scheduler, and/or for general purpose workloads
+is definitely possible, but the vCPU scheduling parameters (of both
+Domain0 and of the various VMs) would probably require tweaking, with
+respect to their default values.
+
+# Testing
+
+Any change done in RTDS must be tested by doing the following:
+
+* create a cpupool with RTDS as its scheduler,
+* create a few virtual machines a move them in and out of the pool,
+* create a few virtual machines, directly inside the pool, and verify
+ that they boot and can run some basic workload (e.g., login into them
+ and run simple commands),
+* shutdown/reboot the virtual machines,
+
+The fact that the system boots fine when passing `sched=rtds` to Xen
+should also be verified.
+
+Finally, to check that the scheduler is working properly (although only
+at a macroscopic level), the following should be done:
+
+* create a VM with 1 vCPU and put it in the RTDS cpupool,
+* set the scheduling parameters such as it has a 50% reservation, with
+ `xl sched-rtds -d vm -t 10 -b 5`,
+* run a CPU-burning process inside the VM (e.g., `yes`),
+* check with `xentop` (in Domain0) that the VM is getting no more than
+ 50% pCPU time.
+
+# Areas for improvement
+
+* Work-conserving mode to be added;
+* performance assessment, especially focusing on what level of real-time
+ behavior the scheduler enables.
+
+# Known issues
+
+* OSSTest reports occasional failures on ARM.
+
+# References
+
+* "RT-Xen: Real-Time Virtualization" [XPDS14
Presentation](http://events.linuxfoundation.org/sites/events/files/slides/2014_Xen_Developer_Summit_0.pdf)
+* "Scheduling in Xen" [XPDS15
Presentation
