Some SP subsystems such as LoadLeveler, GPFS, and the Virtual Shared Disk component of PSSP, use switch adapter windows. You can also have locally developed applications use switch adapter windows. Any subsystem can reserve an adapter window for its use if it is configured to do so, such as GPFS can be. The maximum number of adapter windows available is reduced by each adapter window that is reserved. On the other hand, an application can be configured to use any available adapter window. For instance, a typical use of available adapter windows is for user space jobs managed by LoadLeveler. LoadLeveler will use one or more available windows until the user space job is done. As a result, LoadLever is capable of using all the adapter windows that are available.
For that reason, it is important to understand the type and amount of workload on your SP system, which and how many subsystems use adapter windows, how many are available, and the sequencing when starting up and shutting down subsystems. It is good practice, for instance, not to start LoadLeveler until after other subsystems that reserve adapter windows have been started. If your SP system runs out of available adapter windows, you need to know which applications you can manipulate or give you options to correct the situation.
Aside from the number of adapter windows, each adapter window uses some amount of device memory. Device memory is the total amount of system memory that can be used as interface network buffers. For large parallel user space jobs, the amount of device memory used can be significant. If the amount of device memory is too small, switch communication can be impeded. Before PSSP 3.2, the amount of this resource was not adjustable by customers. The switch communication subsystem simply allocated more and more as it needed to. In PSSP 3.2, the same amount of device memory as in earlier releases is set by default, but you also have the ability to change it if you find a need to. IBM provides default settings that are consistent with those of earlier releases which were not tunable. You do not have to change them unless you need to, based on your SP system's switch communication and workload demands. If you decide you need to consider making changes, see SP tuning information at the Web address http://www.rs6000.ibm.com/support/sp/
The chgcss command can be used to reserve, release, or query SP switch adapter windows and to change any of the following device memory characteristics:
For more information about the chgcss command, see the book PSSP: Command and Technical Reference.