Optimized latency mode

One way to improve the performance of an OSA-Express®3 or later feature in QDIO mode for processing workloads with demanding latency requirements is to configure the OSA-Express feature to operate in optimized latency mode. Optimized latency mode optimizes interrupt processing for both inbound and outbound data, which decreases latency and can provide significant increases in throughput, particularly for high volume, interactive, non-streaming workloads.

To configure an OSA-Express3 or later feature to operate in optimized latency mode, use the INTERFACE statement with the OLM parameter. Because optimized latency mode affects both inbound and outbound interrupts, it supersedes other inbound performance settings that the INBPERF parameter sets. For more information about optimized latency mode and the OLM and INBPERF parameters on the INTERFACE statement for IPAQENET and IPAQENET6, see z/OS Communications Server: IP Configuration Reference.

Because of the operating characteristics of optimized latency mode, two other configuration changes might be required.

• For outbound traffic to gain the benefit of optimized latency mode, direct traffic to priority queues 1, 2, or 3 by using the WLMPRIORITYQ parameter on the GLOBALCONFIG statement, or by using Policy Agent and configuring a policy with the SetSubnetPrioTosMask statement.

  Although an OSA-Express feature supports multiple outbound write priority queues, outbound optimized latency mode occurs only for traffic on priority queue 1 (priority level 1). The TCP/IP stack combines all the traffic that is directed to priority queues 1, 2, and 3 into priority queue 1 for any OSA-Express3 or later feature that is operating in optimized latency mode.

  For more information about directing traffic to outbound OSA-Express priority queues by using the WLMPRIORITYQ parameter on the GLOBALCONFIG statement or using the SetSubnetPrioTosMask statement, see z/OS Communications Server: IP Configuration Reference.

  Guideline: Configure the WLMPRIORITYQ parameter with no subparameters, which assigns a default mapping of service class importance levels to OSA-Express outbound priority queues. This default mapping directs traffic that is assigned to the higher priority service class importance levels 1–4 to queues that operate in optimized latency mode, and enables the appropriate types of traffic to benefit from optimized latency mode.
  Result: If neither the WLMPRIORITYQ or SetSubnetPrioTosMask statements are specified, any packet that has a type of service (ToS) byte with the first 3 bits being 000 or 001 is directed to queue 4 and does not benefit from optimized latency mode.
• To achieve optimal latency for one or more network interfaces that are operating in optimized latency mode, limit the number of network interfaces that can concurrently share an OSA-Express3 or later feature.

  When at least one network interface is operating in optimized latency mode, ensure that no more than four concurrent network interfaces are sharing an OSA-Express3 or later port, and no more than eight concurrent network interfaces are sharing an OSA-Express3 or later channel path identifier (CHPID). The following configurations can result in multiple users that are sharing an OSA-Express3 or later feature.

  • Multiple LPARs sharing the OSA-Express3 or later feature
  • Multiple stacks on the same LPAR sharing the OSA-Express3 or later feature
  • Multiple VLAN interfaces to the OSA-Express3 or later feature
  • Both an IPv4 and an IPv6 active interface to the OSA-Express3 or later feature
  • A TCP/IP stack that is enabling the OSA-Express network traffic analyzer (OSAENTA) for the OSA-Express3 or later feature

Optimized latency mode is intended for high volume, interactive workloads. Although optimized latency mode can compensate for some mixing of workloads, an excessive amount of high volume, streaming workloads, such as bulk data or file transfer, can result in higher processor consumption.