Xen 4.2 Limits

To Do: This is a work in progress. Feel free to add requests for more to the talk page or add additional information here

This document contains information about limits of Xen 4.2.

CPUs & Memory

• Xen can scale up to 4,095 host CPUs with 5Tb of RAM.
• Using Para Virtualization (PV), Xen supports a maximum of 512 VCPUs with 512Gb RAM per guest.
• Using Hardware Virtualization (HVM), Xen supports a maximum of 128 VCPUs with 512Gb RAM per guest.

To Do: Should this be 256 for HVM?

Note that in theory Xen supports 8192 VCPUs with PV, but event channel limitations (4k per guest) make it impossible for a guest this huge to boot, as each vCPU is expected to minimally require 2 of them (timer vIRQ and IPIs). The guest limits stated above are limits that have been tested. However, it is possibly to exceed these in theory.

Other Limits

• Maximum of 32K interrupts and 2k PCI segments; this limits the number of devices that can be used in two ways:
  • the number of devices that can be attached to the system must fit into 2k PCI segments (i.e. 2k * 256 [buses] * 32 [devices] * 8 [functions]),
  • the number of hardware interrupts they may surface (one would expect at least one interrupt per device function, so this likely is the stricter of the two limitations).