[PATCH v2 0/3 for 4.8] docs: feature documents for the schedulers

[PATCH v2 0/3 for 4.8] docs: feature documents for the schedulers

[Dario Faggioli]

Hi,

Here's v2.

The differences are:
 - review comments taken into account.
 - I'm *NOT* claiming 'SuppOrted' status for Credit2. I'll do that in a
   separate patch.

v1 is here:

 https://lists.xen.org/archives/html/xen-devel/2016-10/msg00857.html

And there is a git branch for this new version:

 git://xenbits.xen.org/people/dariof/xen.git  rel/sched/feature-docs
 http://xenbits.xen.org/gitweb/?p=people/dariof/xen.git;a=shortlog;h=refs/heads/rel/sched/feature-docs

Thanks and Regards,
Dario
---
Dario Faggioli (3):
      docs: Credit1 feature document.
      docs: Credit2 feature document.
      docs: RTDS feature document.

 docs/features/sched_credit.pandoc  |   97 +++++++++++++++++++++++++++++
 docs/features/sched_credit2.pandoc |  118 +++++++++++++++++++++++++++++++++++
 docs/features/sched_rtds.pandoc    |  121 ++++++++++++++++++++++++++++++++++++
 3 files changed, 336 insertions(+)
 create mode 100644 docs/features/sched_credit.pandoc
 create mode 100644 docs/features/sched_credit2.pandoc
 create mode 100644 docs/features/sched_rtds.pandoc
--
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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[PATCH v2 1/3] docs: Credit1 feature document.

[Dario Faggioli]

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
---
Cc: George Dunlap <George.Dunlap@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Lars Kurth <lars.kurth@citrix.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
---
Changes from v1:
* file renamed from credit.pandoc to sched_credit.pandoc;
* feature name is now 'Credit 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_credit.pandoc |   97 +++++++++++++++++++++++++++++++++++++
 1 file changed, 97 insertions(+)
 create mode 100644 docs/features/sched_credit.pandoc

diff --git a/docs/features/sched_credit.pandoc b/docs/features/sched_credit.pandoc
new file mode 100644
index 0000000..79587d8
--- /dev/null
+++ b/docs/features/sched_credit.pandoc
@@ -0,0 +1,97 @@
+% Credit Scheduler
+% Revision 1
+
+\clearpage
+
+# Basics
+---------------- ----------------------------------------------------
+         Status: **Supported**
+
+      Component: Hypervisor
+---------------- ----------------------------------------------------
+
+# Overview
+
+Credit (also known as Credit1) is the default virtual CPU (vCPU) scheduler
+of the Xen hypervisor.
+
+It is a general purpose, weighted fair-share scheduler.
+
+# User details
+
+Xen supports multiple schedulers. As said, Credit is the default, so it
+is used automatically, unless the `sched=$SCHED` (with `$SCHED` different
+than `credit`) parameter is passed to Xen via the bootloader.
+
+Once the system is live, for creating a cpupool with Credit 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=\"pool1\" sched=\"credit\" cpus=[4,8]
+
+Two kind of interactions with the scheduler are possible:
+
+* checking or changing the global parameters, via, e.g.:
+    * `xl sched-credit -s`
+    * `xl sched-credit -s -p pool1`
+    * `xl sched-credit -s -t 20`
+* checking or changing a VM's scheduling parameters, via, e.g.:
+    * `xl sched-credit -d vm1`
+    * `xl sched-credit -d vm1 -w 512`
+
+# Technical details
+
+Implementation entirely lives in the hypervisor. Xen has a pluggable,
+hook based, architecture for schedulers. Thanks to this, Credit code
+is all contained in `xen/common/sched_credit.c`.
+
+# Limitations
+
+In Credit, a vCPU has a priority, a status (i.e., active or inactive),
+a weight and some credits... and all these things interact in a rather
+involved way. Also, with years of use, things have gotten even more
+complex (due to, e.g., the introduction of boosting, caps and vCPU
+soft-affinity).
+
+Dealing with such complexity is starting to be an issue. Odd behavior
+or subtle scheduling anomalies, that is not always possible to act upon,
+have been identified already. [1][2][3]
+
+A certain lack of scalability and difficulties and weakness in dealing
+with mixed workloads and VMs with low latency requirements are other
+known problems. [4] For all these reasons, effort is ongoing to have
+Credit2 become the new default scheduler.
+
+# Testing
+
+Any change to Credit code must to be tested by doing at least the following:
+
+* create a few virtual machine 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,
+* shutdown the system.
+
+Ideally, all the above steps should **also** be performed in a configuration
+that includes cpupools, better if with pools using different schedulers, and
+by also doing the following:
+
+* move the virtual machines between cpupools.
+
+# References
+
+* [potential non-ideal behavior on hyperthreaded systems](https://lists.xenproject.org/archives/html/xen-devel/2014-07/msg01848.html) [1]
+* [long standing BOOST vs. migration bug](https://lists.xen.org/archives/html/xen-devel/2015-10/msg02851.html) [2]
+* [priority handling issues](https://lists.xenproject.org/archives/html/xen-devel/2016-05/msg01362.html) [3]
+* "Scheduler development update", XenSummit Asia 2009 [whitepaper](http://www-archive.xenproject.org/files/xensummit_intel09/George_Dunlap.pdf) [4]
+* "Scheduling in Xen" [XPDS15 Presentation](http://events.linuxfoundation.org/sites/events/files/slides/Faggioli_XenSummit.pdf)
+* "The Credit Scheduler" [on the Xen-Project wiki](https://wiki.xenproject.org/wiki/Credit_Scheduler)
+* "Xen Project Schedulers" [on the Xen-Project wiki](https://wiki.xenproject.org/wiki/Xen_Project_Schedulers)
+
+# History
+
+------------------------------------------------------------------------
+Date       Revision Version  Notes
+---------- -------- -------- -------------------------------------------
+2016-10-14 1        Xen 4.8  Document written
+---------- -------- -------- -------------------------------------------


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[PATCH v2 2/3] docs: Credit2 feature document.

[Dario Faggioli]

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
---
Cc: George Dunlap <George.Dunlap@eu.citrix.com>
Cc: Anshul Makkar <anshul.makkar@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Lars Kurth <lars.kurth@citrix.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
---
Changes from v1:
* don't claim 'Supported' status;
* file renamed from credit2.pandoc to sched_credit2.pandoc;
* feature name is now 'Credit2 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_credit2.pandoc |  118 ++++++++++++++++++++++++++++++++++++
 1 file changed, 118 insertions(+)
 create mode 100644 docs/features/sched_credit2.pandoc

diff --git a/docs/features/sched_credit2.pandoc b/docs/features/sched_credit2.pandoc
new file mode 100644
index 0000000..8609d9c
--- /dev/null
+++ b/docs/features/sched_credit2.pandoc
@@ -0,0 +1,118 @@
+% Credit2 Scheduler
+% Revision 1
+
+\clearpage
+
+# Basics
+---------------- ----------------------------------------------------
+         Status: **Experimental**
+
+      Component: Hypervisor
+---------------- ----------------------------------------------------
+
+# Overview
+
+Credit2 is one of the virtual CPU (vCPU) scheduler available in the
+Xen hypervisor.
+
+Credit2 was designed as a general purpose scheduler, with particular
+focus on improving handling of mixed workloads, scalability and
+support for low latency applications inside VMs, with respect to
+Credit1.
+
+# User details
+
+Credit2 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=credit2`
+
+Other parameters are available for tuning the behavior of Credit2
+(see `docs/misc/xen-command-line.markdown` for a complete list and
+for their meaning).
+
+Once the system is live, for creating a cpupool with Credit2 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=\"pool1\" sched=\"credit2\" cpus=[1,2]
+
+Two kind of interactions with the scheduler are possible:
+
+* checking or changing the global parameters, via, e.g.:
+    * `xl sched-credit2 -s`
+    * `xl sched-credit2 -s -p pool1`
+    * `xl sched-credit2 -s -r 100`
+* checking or changing a VM scheduling parameters, via, e.g.:
+    * `xl sched-credit2 -d vm1`
+    * `xl sched-credit2 -d vm1 -w 1024`
+
+# Technical details
+
+Implementation entirely lives in the hypervisor. Xen has a pluggable,
+hook based, architecture for schedulers. Thanks to this, Credit2 code
+is all contained in `xen/common/sched_credit2.c`.
+
+Global scheduling parameters, such as context switching rate
+limiting, is only available from Xen 4.8 onward. In libxl, the
+`LIBXL\_HAVE\_SCHED\_CREDIT2\_PARAMS` symbol is introduced to
+indicate their availability.
+
+# Limitations
+
+The Credit1 scheduler comes with vCPU hard-affinity, vCPU soft-affinity
+and caps (see `docs/man/xl.pod.1.in` for more details). In Credit2,
+vCPU hard affinity is supported starting from Xen 4.8, while soft-affinity
+and caps, while being worked on, are not yet available in any released
+hypervisor.
+
+# Testing
+
+Any change done in Credit2 wants to be tested by doing at least the
+following:
+
+* boot the system with `sched=credit2`,
+* create a few virtual machine 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,
+* shutdown/reboot the system.
+
+Ideally, all the above steps should **also** be performed in a configuration
+where Credit2 is used as the scheduler of a cpupool, and by also doing the
+following:
+
+* move a virtual machine inside and outside a Credit2 cpupool.
+
+# Areas for improvement
+
+* Close the feature gap with Credit1 (i.e., finishing implementing vCPU
+  soft-affinity and caps);
+* vCPUs' reservations (similar to caps, but providing a vCPU with guarantees
+  about some pCPU time it will always be able to execute for);
+* benchmarking for assessing the best combination of values for the various
+  parameters (`sched\_credit2\_migrate\_resist`, `credit2\_balance\_over`,
+  `credit2\_balance\_under`)
+
+# Known issues
+
+* I/O oriented benchmarks (like network and disk throughput) have given
+  contradictory and non-conclusive results so far. Need to run more of
+  those.
+
+# References
+
+* "Scheduler development update", XenSummit Asia 2009 [whitepaper](http://www-archive.xenproject.org/files/xensummit_intel09/George_Dunlap.pdf)
+* "Scheduling in Xen" [XPDS15 Presentation](http://events.linuxfoundation.org/sites/events/files/slides/Faggioli_XenSummit.pdf)
+* "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)
+* "The Credit2 Scheduler" [on the Xen-Project wiki](https://wiki.xenproject.org/wiki/Credit2_Scheduler_Development)
+* "Xen Project Schedulers" [on the Xen-Project wiki](https://wiki.xenproject.org/wiki/Xen_Project_Schedulers)
+
+# History
+
+------------------------------------------------------------------------
+Date       Revision Version  Notes
+---------- -------- -------- -------------------------------------------
+2016-10-14 1        Xen 4.8  Document written
+---------- -------- -------- -------------------------------------------


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[PATCH v2 3/3] docs: RTDS feature document.

[Dario Faggioli]

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
---
Cc: Meng Xu <mengxu@cis.upenn.edu>
Cc: George Dunlap <George.Dunlap@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Lars Kurth <lars.kurth@citrix.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
---
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 0000000..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 10000 -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 20000 -b 10000 -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 100000 -b 50000`,
+* 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](http://events.linuxfoundation.org/sites/events/files/slides/Faggioli_XenSummit.pdf)
+* [RT-Xen Project](https://sites.google.com/site/realtimexen/)
+* [RTDS-Based-Scheduler](https://wiki.xenproject.org/wiki/RTDS-Based-Scheduler)
+* "The RTDS Scheduler" [on the Xen-Project wiki](https://wiki.xenproject.org/wiki/RTDS-Based-Scheduler)
+* "Xen Project Schedulers" [on the Xen-Project wiki](https://wiki.xenproject.org/wiki/Xen_Project_Schedulers)
+
+# History
+
+------------------------------------------------------------------------
+Date       Revision Version  Notes
+---------- -------- -------- -------------------------------------------
+2016-10-14 1        Xen 4.8  Document written
+---------- -------- -------- -------------------------------------------


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Re: [PATCH v2 0/3 for 4.8] docs: feature documents for the schedulers

[Wei Liu]

On Fri, Oct 14, 2016 at 10:59:55AM +0100, Dario Faggioli wrote:
> Hi,
> 
> Here's v2.
> 
> The differences are:
>  - review comments taken into account.
>  - I'm *NOT* claiming 'SuppOrted' status for Credit2. I'll do that in a
>    separate patch.
> 
> v1 is here:
> 
>  https://lists.xen.org/archives/html/xen-devel/2016-10/msg00857.html
> 
> And there is a git branch for this new version:
> 
>  git://xenbits.xen.org/people/dariof/xen.git  rel/sched/feature-docs
>  http://xenbits.xen.org/gitweb/?p=people/dariof/xen.git;a=shortlog;h=refs/heads/rel/sched/feature-docs
> 
> Thanks and Regards,
> Dario
> ---
> Dario Faggioli (3):
>       docs: Credit1 feature document.
>       docs: Credit2 feature document.
>       docs: RTDS feature document.
> 

All three:

Acked-by: Wei Liu <wei.liu2@citrix.com>

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Re: [PATCH v2 0/3 for 4.8] docs: feature documents for the schedulers

[Andrew Cooper]

On 14/10/16 14:24, Wei Liu wrote:
> On Fri, Oct 14, 2016 at 10:59:55AM +0100, Dario Faggioli wrote:
>> Hi,
>>
>> Here's v2.
>>
>> The differences are:
>>  - review comments taken into account.
>>  - I'm *NOT* claiming 'SuppOrted' status for Credit2. I'll do that in a
>>    separate patch.
>>
>> v1 is here:
>>
>>  https://lists.xen.org/archives/html/xen-devel/2016-10/msg00857.html
>>
>> And there is a git branch for this new version:
>>
>>  git://xenbits.xen.org/people/dariof/xen.git  rel/sched/feature-docs
>>  http://xenbits.xen.org/gitweb/?p=people/dariof/xen.git;a=shortlog;h=refs/heads/rel/sched/feature-docs
>>
>> Thanks and Regards,
>> Dario
>> ---
>> Dario Faggioli (3):
>>       docs: Credit1 feature document.
>>       docs: Credit2 feature document.
>>       docs: RTDS feature document.
>>
> All three:
>
> Acked-by: Wei Liu <wei.liu2@citrix.com>

All LGTM as well.

~Andrew

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Re: [PATCH v2 0/3 for 4.8] docs: feature documents for the schedulers

[Wei Liu]

On Fri, Oct 14, 2016 at 04:03:18PM +0100, Andrew Cooper wrote:
> On 14/10/16 14:24, Wei Liu wrote:
> > On Fri, Oct 14, 2016 at 10:59:55AM +0100, Dario Faggioli wrote:
> >> Hi,
> >>
> >> Here's v2.
> >>
> >> The differences are:
> >>  - review comments taken into account.
> >>  - I'm *NOT* claiming 'SuppOrted' status for Credit2. I'll do that in a
> >>    separate patch.
> >>
> >> v1 is here:
> >>
> >>  https://lists.xen.org/archives/html/xen-devel/2016-10/msg00857.html
> >>
> >> And there is a git branch for this new version:
> >>
> >>  git://xenbits.xen.org/people/dariof/xen.git  rel/sched/feature-docs
> >>  http://xenbits.xen.org/gitweb/?p=people/dariof/xen.git;a=shortlog;h=refs/heads/rel/sched/feature-docs
> >>
> >> Thanks and Regards,
> >> Dario
> >> ---
> >> Dario Faggioli (3):
> >>       docs: Credit1 feature document.
> >>       docs: Credit2 feature document.
> >>       docs: RTDS feature document.
> >>
> > All three:
> >
> > Acked-by: Wei Liu <wei.liu2@citrix.com>
> 
> All LGTM as well.
> 

Series pushed.

> ~Andrew

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Re: [PATCH v2 3/3] docs: RTDS feature document.

[Meng Xu]

Hi Dario,

Thank you very much for writing this nice document!
Sorry for the late reply...

I found two typos in the command example.
I saw the patch has been committed. Maybe I (or you) should send a
patch to correct the typo?
>
>
> +
> +For checking or changing a VM's scheduling parameters from xl, do
> +as follows:
> +    * `xl sched-rtds -d vm-rt`


It should be
xl sched-rtds -d vm-rt -v all


>
> +    * `xl sched-rtds -d vm-rt -t 10000 -b 25000`


There are three typos:
1) We don't support "-t" option.
2) We must specify -v in the latest Xen. (Once the per-vcpu parameter
is supported, we won't support setting a VM's parameter like this. The
toolstack will reports error.)
3) The budget 25000 is larger than the period.

It should be:
sudo xl sched-rtds -d vm-rt -v all -p 10000 -b 2500



>
> +
> +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 20000 -b 10000 -v 1 -p 45000 -b 12000`


This is correct.

>
> +
> +# 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 100000 -b 50000`,


-t here should be -p.
The -v all should be specified here.

Thanks,

Meng


-----------
Meng Xu
PhD Student in Computer and Information Science
University of Pennsylvania
http://www.cis.upenn.edu/~mengxu/

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Re: [PATCH v2 3/3] docs: RTDS feature document.

[Wei Liu]

Hi Meng,

I've committed this patch. Feel free to send update when you and
Dario have all the details hashed out.

Wei.

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Re: [PATCH v2 3/3] docs: RTDS feature document.

[Dario Faggioli]

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[Trimming the Cc-list]

On Tue, 2016-10-18 at 13:49 -0400, Meng Xu wrote:
> > +For checking or changing a VM's scheduling parameters from xl, do
> > +as follows:
> > +    * `xl sched-rtds -d vm-rt`
> 
> 
> It should be
> xl sched-rtds -d vm-rt -v all
> 
Yeah, you're right.

I wrote this basing on my memory, then went there and check, but I
apparently have forgotten to update all the examples.

All you say in this mail makes sense. So, yes, do put together a patch
and send it here. :-)

Thanks and Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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