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Getting Better Network Performance for your zCX Containers

Learn how to optimize the network communication to and from your zCX instance.

Getting Better Network Performance for your zCX Containers

Most of you are probably already familiar with the power of container technology on Linux. Deploying containers allows for better application isolation, improved developer productivity by simplifying the configuration and startup of test environments, and better code quality because the container environment that is used for testing the application will be the same environment used for production. This technology is now available on z/OS with Containers Extensions, or zCX.

In a prior Hot Topics article, Rapid Containers: Improving zCX Runtime Performance, you learned how the hardware page frame size used can impact the performance of your zCX runtime. In this article, you will learn how to optimize the network communication to and from your zCX instance.

zCX Networking

Let’s first examine how zCX leverages the existing z/OS TCP/IP networking stack. Because zCX takes advantage of the virtio Linux interfaces, the Linux network that is used by zCX can be virtualized such that it can access the existing network interfaces managed and provided by z/OS.

Figure 1

Each provisioned zCX address space is assigned its own dynamic virtual IP address (VIPA) that is managed and advertised by z/OS TCP/IP stack. Access to any Docker container running within a zCX instance is accomplished by using this dynamic VIPA. The dynamic VIPA in the example in Figure 1 is Because zCX uses a dynamic VIPA, if the z/OS system fails, the zCX instance can be restarted on any other z/OS system in the sysplex, providing a highly available container environment.

All communication between a Docker container running in zCX and an external endpoint occurs over the z/OS TCP/IP stack. There are two network types that are used for communicating with Docker containers. For endpoints that are colocated in the same z/OS system as the container running in zCX, a high speed, cross-memory virtual network is used for communication, indicated by the brown arrow in Figure 1.
For an endpoint that resides on a different z/OS system or different platform altogether, an external network interface is selected, shown by the green arrow in Figure 1. This selection is based on what the best available route to this endpoint is from the z/OS TCP/IP stack’s perspective – in Figure 1, this route is over the network interface with IP address

zCX Co-location

When considering whether to take advantage of the Linux Docker container environment provided by zCX for your application tier, you should aim to keep the container colocated with the z/OS database management system (DBMS), if possible.

Figure 2

As shown in Figure 2, by leveraging the cross-memory virtual network for zCX, you can reduce the overall network latency between the application and data tiers that can negatively impact the response times for your transactions. Comparing a containerized application tier on zCX colocated with the data tier versus a containerized application tier deployed on a different system from the data tier, benchmarking shows that network latency reductions of 50% or more is possible. Colocation of the application and data tiers can have a dramatic improvement on transaction rates for OLTP workloads.

For scenarios where there are large transaction payloads between the application and data tiers, an additional step can be performed to provide even more optimizations in a colocated zCX environment. Because the cross-memory virtual network is used for communication when zCX instances are colocated with their endpoints, there is no physical limit imposed on the size of a packet that can flow over this network. By configuring a larger MTU for zCX when deploying a zCX instance, the amount of processing to segment and transmit the larger payloads is reduced. This method translates into reduced CPU utilization and substantial improvements in the throughput can be achieved.

zCX External Communication

There can be scenarios or conditions where a Docker container running in zCX cannot be colocated with the z/OS data tier. In these scenarios, optimizations can still be made to make the communication over the external network as efficient as possible.

Inbound Workload Queueing, or IWQ, is an OSA Express feature that allows inbound packets to be separated based on workload traffic type. A new enhancement to enable IWQ for zCX workloads is available with TCP/IP apar PH16581 and VTAM apar OA58300.

Figure 3

When configuring IWQ and using zCX, as shown in Figure 3, any inbound packets destined to a Docker container in zCX would be separated from packets targeting other applications running on z/OS. This separation helps to preserve the ordering of packets being delivered to zCX, thus reducing unnecessary retransmits whenever any packets arrive out of order. This traffic separation also enables the z/OS processing of these inbound zCX packets to be scheduled to run on zIIPs.

Comparing environments with and without IWQ configured, benchmarking shows that network latency reductions of 25% or more can be achieved. And with IWQ enabled for workloads using zCX, benchmarking results confirmed that over one-third of this networking processing can be scheduled to run on zIIPs.


zCX is an exciting new capability for running workloads in a z/OS environment. Network communication is an important piece to get right. When deploying zCX instances, where the zCX instance for the application is deployed in relation to the backend z/OS DBMS the application accesses must be considered. Likewise, taking advantage of the latest z/OS Communications Server enhancements to enable IWQ for zCX workloads can result in significant improvements in transaction rates and a reduction in processing costs on general-purpose CPs to run these workloads.

About the author

Mike Fitzpatrick is an architect with the z/OS Communications Server group, focusing on resiliency and networking performance.

Anne Romanowski contributed to the editorial review of this article.