Signed-off-by: Dario Faggioli <dario.faggi...@citrix.com> --- Cc: George Dunlap <george.dun...@eu.citrix.com> Cc: Wei Liu <wei.l...@citrix.com> Cc: Lars Kurth <lars.ku...@citrix.com> Cc: Andrew Cooper <andrew.coop...@citrix.com> Cc: Ian Jackson <ian.jack...@eu.citrix.com> Cc: Jan Beulich <jbeul...@suse.com> Cc: Konrad Rzeszutek Wilk <konrad.w...@oracle.com> Cc: Stefano Stabellini <sstabell...@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 +---------- -------- -------- ------------------------------------------- _______________________________________________ Xen-devel mailing list Xen-devel@lists.xen.org https://lists.xen.org/xen-devel
