User manual

Miscellaneous Notes
75
If the failure happens nevertheless, data items can be expected to be updated on the next
screen refresh cycle. (Probability of two or more failures in a row is exceedingly low.) Thus the
impact of the problem is that for a second or few seconds, depending on selected screen update
frequency, one or few data items can be displayed as question marks rather than valid values.
The failure cannot happen if VAX MP portable interlock mode is used (VSMP LOAD
INTERLOCK=PORTABLE, which is the default setting).
The underlying reason is that module [CLUTL.SRC]SHWCL_SNAP.MAR uses interlock queue
locking loop limited by LOCKRETRY with only 2 instructions in the loop, rather than usual 3 or
more as the rest of VMS does. VAX MP synchronization window size is tuned for 3+ instruction
loops, so 2-instruction loop can timeout under adverse host load conditions and return status
SS$_INTERLOCK. This status will then be picked by routine DISPLAY_QUEUE.
If this issue is deemed worth fixing, it should be addressed in subsequent release of VAX MP,
perhaps by statically patching SHWCLSTR.EXE to increase its busy-wait loop size or by
recognizing BBSSI-AOBLSS/SOBGTR loops inside VAX MP.