User manual

Interprocessor Synchronization Window

• Non-preemptable code trying to lock interlocked queue header with BBSSI instruction while

executing at IPL >= 3 must use BBSSI retry count yielding total number of instructions in retry

loop not less than 3 * min(900000, LOCKRETRY). If retry loop drops IPL below 3 in the interim,

retry count should be restarted.

Any application or component failing to meet these requirements can only be executed in

multiprocessor mode on VAX MP if portable interlock is selected, which is the default setting.

If an application or component not meeting listed requirements is executed on VAX MP with native

interlock mode enabled, described retry loops can timeout and cause component or application to fail.

They will not timeout when executed on VAX MP using portable interlock mode.

We are not aware at present time of any application or component that fails to meet these

requirements. Base OpenVMS system itself meets them. If you are sure all of your VAX software

executed under VAX MP meets them too, you may be able to increase system performance by enabling

native-mode interlock with the following VSMP LOAD options:

$ @SYS$SYSROOT:[VSMP]VSMP$LOAD INTERLOCK=NATIVE SYNCW=ILK

or:

$ @SYS$SYSROOT:[VSMP]VSMP$LOAD INTERLOCK=NATIVE SYNCW=ALL

Note that described limitations of native mode are not “failures” of synchronization window mechanism,

but rather reflect its design goals. Synchronization window is designed to provide highest possible

scalability of multiprocessor system and impose least constraining synchronization compatible with the

needs of operating system. Therefore it constrains only the patterns that OpenVMS is known to actually

use or that OpenVMS and well-written applications can reasonably be expected to use. Furthermore,

synchronization window constrains these patterns only to an extent (synchronization window size) they

need to be constrained to in order to assure their stable execution. Synchronization window by design

avoids over-constraining the system and hampering its scalability for the sake of supporting any and all

conceivable patterns.

If widest range of synchronized patterns is a desired goal, it is recommended to use portable interlock

mode, or better yet, portable interlock mode in conjunction with synchronization window. The latter

combination may produce slightly less scalable configuration, but will provide support for a wider set of

synchronization patterns.