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Poor TCP/IP Performance Over HiperSockets

Troubleshooting


Problem

TCP/IP performance between z/OS and z/Linux systems using the HiperSockets (iQDIO) interfaces is not as good as expected.

Cause

The default allocation of interface buffers (SBAL) on z/Linux is 16. While sufficient for light workloads, this is typically inadequate for high volumes of IP traffic. This is especially true if the LPAR (or z/VM guest) has a lower allocation of CPU resources.

For z/OS systems, this allocation defaults to the maximum supported (126) but can be altered with the VTAM IQDIOSTG start option or overridden by the READSTORAGE keyword on the TCPIP LINK or INTERFACE statement.

Another item that can affect HiperSocket performance for all platforms is the Maximum Frame Size specified in the IOCP for this CHPID. For high volumes of traffic, this should be set to the maximum (OS=C0). See the IP Configuration Guide for more information on this setting.

Resolving The Problem

Increase the specification of buffers for all HiperSockets interfaces on the z/Linux systems. Ideally, this should be set to the maximum (128), although this does increase the real storage demands for the system. To change this specification:
  • Edit the /etc/sysconfig/hardware/hwcfg-qeth-bus-ccw-device_bus_id file.
  • Change or add the line QETH_OPTIONS="buffer_count=num"

To have this change take effect immediately (without an IPL), perform the following after making the above update:

   ifdown qeth-bus-ccw-device_bus_id


   hwdown qeth-bus-ccw-device_bus_id
   hwup qeth-bus-ccw-device_bus_id
   ifup qeth-bus-ccw-device_bus_id
 

The CHPID's Maximum Frame Size can be updated in the Hardware Configuration Definition. See the section on Dynamically changing channel paths in the HCD Planning manual for more information on the steps needed.

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Document Information

Modified date:
06 May 2020

UID

swg21299070

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