TCP/IP variable SMC-R storage allocations

You can use Table 1 and Table 2 to estimate the minimal amount of real storage that is allocated for SMC-R communications, which is based on the number of IBM® 10GbE RoCE Express® interfaces and LAN connectivity. Another component to the amount of fixed SMC-R storage that is required for z/OS® Communications Server is the variable amount of storage that is allocated per stack, which is based on application workload. This value is not easily estimated and varies depending on several factors, such as the number of TCP connections, interactive versus streaming characteristics, and the volume of application traffic.

• The initial allocation of RMB storage for a particular SMC-R link group can accommodate as few as 3 TCP connections or more than 90 TCP connections, depending on the RCVBFRSIZE values for the TCP connections involved. If TCP connections are established with different RCVBFRSIZE settings, different RMB element sizes might be used. Because all RMB elements within a single RMB are the same size, allocation of more RMBs might be needed.
  Guideline: For estimating purposes, assume that the initial allocation of RMB and transmit elements accommodates 16 TCP connections.
• Each additional RMB that is allocated for a particular SMC-R link group can accommodate 4 – 32 more TCP connections, depending on the RCVBFRSIZE value of the TCP connections.
  Guideline: For estimating purposes, assume that an extra 1 MB of RMB storage is required for every eight more TCP connections.
• Additional staging buffers are allocated as the volume of application data that is sent increases.
• RDMA receive and transmit elements represent work elements that are used to manage requests to process received RDMA data or to send RDMA data to a peer. The number of work elements also increases as the volume of application data increases.
• RMBs, staging buffers, and RDMA receive and transmit elements are all eligible to be deallocated if the volume of application traffic decreases.

For example, in Example for calculating SMC-R storage requirements, assume stack A starts 100 TCP connections. According to the guidelines for estimating the TCP/IP workload storage, the first 16 TCP connections use the initial RMB storage, but the remaining 84 connections require more storage. Estimating an additional 1 MB per eight new TCP connections means that 11 (84/8 = 10+) new RMBs are required, so an additional 11 MB is allocated, which raises the overall total for stack A to at least 30 MB. More staging buffer and RDMA receive or transmit elements might also be allocated to handle the increased application data transfer.

After you estimate a minimum value for the fixed and variable amounts of storage for each TCP/IP stack, you can use that value as a basis for setting the FIXEDMEMORY subparameter of the SMCR parameter on the GLOBALCONFIG profile statement. You can use the DISPLAY TCPIP,,STOR command to display the current amount of storage that a particular TCP/IP stack uses for SMC-R. You can monitor this usage and adjust the setting for the FIXEDMEMORY subparameter based on the actual values for your systems.