z/VMA TCP/IP Performance Presentations

TCP/IP Performance Tips

The following are miscellaneous performance tips for VM/ESA TCP/IP. Your mileage may vary.

  • There are a number of applications that may read the TCPIP DATA file. If these applications are frequently used, remove unneeded comments from the file.
  • If using RSCS and LPR, a separate IUCV link is required for each LPR link. Therefore, make sure CCBPOOLSIZE and the MAXCONN directory statements are sufficient.
  • CCBPOOLSIZE describes the pool for CCBs, or Client Control blocks which are associated with each client the stack is working with (Telnet is considered to be an internal client). To see what the stack considers in the CCB pool, use the NETSTAT CLIENT command (this can generate lots of console output). A single VM userid may have multiple clients. Some CCBs are considered as permanent, such as those for userids in the OBEY and NOTIFY. Garbage collection will be performed on the pool when the warning message level is reached for the CCBPoolSize.
  • Prior to FL320, the SCB pool could be easily exhausted when using FTP MGET or MPUT commands with a large number of files. A separate connection is acquired for each file. According to the RFCs, after a transfer is complete TCP/IP needs to wait 2 minutes before freeing up the connection. However, there is relief in FL320 that mitigates, or possibly avoids, the problem. In FL320, the FTP client and servers were changed to release their internal connections when the connection closes. The stack still watches for the 2 minute mark.