Signed-off-by: Dario Faggioli <>
Cc: George Dunlap <>
Cc: Anshul Makkar <>
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:
* 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 
new file mode 100644
index 0000000..8609d9c
--- /dev/null
+++ b/docs/features/sched_credit2.pandoc
@@ -0,0 +1,118 @@
+% Credit2 Scheduler
+% Revision 1
+# 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
+# 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/` 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
+# Testing
+Any change done in Credit2 wants to be tested by doing at least the
+* 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
+* 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 
+* "Scheduling in Xen" [XPDS15 
+* "Scope and Performance of Credit-2 Scheduler" [XPDS16 
+* "The Credit2 Scheduler" [on the Xen-Project 
+* "Xen Project Schedulers" [on the Xen-Project 
+# History
+Date       Revision Version  Notes
+---------- -------- -------- -------------------------------------------
+2016-10-14 1        Xen 4.8  Document written
+---------- -------- -------- -------------------------------------------

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