mirror of
https://github.com/xen-project/xen.git
synced 2026-10-29 10:40:03 +01:00
Dario Faggioli
025bc7a0b6
Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com> Acked-by: Wei Liu <wei.liu2@citrix.com>
98 lines
3.9 KiB
Plaintext
98 lines
3.9 KiB
Plaintext
% 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
|
|
---------- -------- -------- -------------------------------------------
|