Findings summary

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This paper demonstrates several findings that may be helpful to IBM® Z customers.

These findings are:

  1. KVM can be configured to leverage HiperSockets™ for better overall performance between logical partitions on the same system.
  2. HiperSockets can provide better performance across all workload types. However, for small transactional workloads, HiperSockets only outperforms OSA at small or medium concurrency levels.
  3. Better performance with HiperSockets typically means higher throughputs, faster transaction times, and greater CPU efficiency (meaning either more data transfer at the same processor consumption or the same data transfer with less processor consumption).
  4. Streaming reads perform better than streaming writes.
    Note: The reason for this was not determined at the time this paper was produced.
  5. Using the IP routing capabilities of the TCP/IP stack eliminates the requirement of binding a KVM guest interface to a single network interface in the KVM host, as is done when using a MacVTap/OSA configuration or a NAT-bridge/HiperSockets configuration. This allows the KVM guest the flexibility to communicate externally from the KVM host using any interface (or subnet) configured in the KVM host.
  6. KVM guests operate using the OSI model Layer 2 mode. IP routing provides a flexible and transparent conversion, allowing KVM guests to communicate with KVM host interfaces operating in Layer 3 mode. z/OS® logical partitions only operate using Layer 3. IP routing enables KVM guests to communicate with z/OS and z/VSE® logical partitions or z/VM® Layer 3 guests.
  7. An alternative method for KVM guests to connect to the Layer 3 network interface utilizes a NAT-bridge. However a NAT-bridge, like a MacVTap/OSA configuration binds to a single network interface. The NAT-bridge also prevents the KVM guests from running server applications because the NAT blocks unsolicited requests from an outside source from being delivered to the KVM guests. IP routing does not impose these restrictions and therefore supports server applications running in the KVM guest.
  8. HiperSockets are more CPU efficient than OSA for most of the tested workload types. However, higher performance typically requires higher total CPU consumption. If the workloads running in an LPAR are close to or reach the limits of the CPU resources, the additional CPU consumption could create quality of service impacts. In this case, using the OSA card may be better choice.

Other than some upfront setup time to enable and configure HiperSockets, there is very little reason not to leverage HiperSockets and IP routing in an IBM Z KVM configuration. If the destination is another logical partition on the same system with HiperSockets enabled, KVM (with a few added routing rules) can benefit from all the advantages that HiperSockets offer. If the destination has no HiperSockets configured, IP routing still allows KVM to use any other interface/subnet/routes available in the KVM host. There is very little downside to using IP routing, and much to be gained.