1 Basics

2 Overview

Credit2 is the default 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.

3 User details

Xen supports multiple schedulers. As said, Credit2 is the default, so it is used automatically, unless the sched=$SCHED (with $SCHED different than credit2) parameter is passed to Xen via the bootloader.

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

4 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.

5 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.

6 Areas for improvement

• 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)

7 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.

8 References

• “Scheduler development update”, XenSummit Asia 2009 whitepaper
• “Scheduling in Xen” XPDS15 Presentation
• “Scope and Performance of Credit-2 Scheduler” XPDS16 Presentation
• “The Credit2 Scheduler” on the Xen-Project wiki
• “Xen Project Schedulers” on the Xen-Project wiki

9 History