Xen 4.2 Limits

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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.

Note that in theory Xen supports 8192 VCPUs, 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 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 many 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 is the stricter of the two limitations).