TAL Programmer's Guide Data Alignment Addendum

TAL Programmer's Guide Data Alignment Addendum—524967-003
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2 Misalignment Tracing Facility
The misalignment tracing facility is enabled or disabled for all processors in the node.
By default, it is enabled (set to ON). It can be disabled (set to OFF) only by the persons
who configure the system, by means of the Subsystem Control Facility (SCF) attribute
MISALIGNLOG. Instructions are in the SCF Reference Manual for the Kernel
Subsystem.
When a misaligned address causes an exception that RVUs prior to G06.17 would
have rounded down, the tracing facility traces the exception.
While a process runs, the tracing facility:
•
Counts the number of misaligned-address exceptions that the process causes (the
exception count)
•
Records the program address and code-file name of the instruction that causes the
first misaligned-address exception
Because a process can run for a long time, the tracing facility samples the process
(that is, checks its exception data) approximately once an hour. If the process recorded
an exception since the previous sample, the tracing facility records an entry in the EMS
log. If the process ends, and any exception has occurred since the last sample, the
operating system produces a final Event Management Service (EMS) event.
The EMS event includes:
•
The process’s misalignment exception count
•
Details about one misaligned-address exception, including the program address,
data address, and relevant code-file names
Sampling is short and infrequent enough to avoid flooding the EMS log, even for a
continuous process with many misaligned-address exceptions. One sample logs a
maximum of 100 events, and at most one event is logged for any process.
If misaligned-address exceptions occur in different periods of a process, the operating
system produces multiple EMS events for the same process, and these EMS events
might have different program addresses.
For more information about EMS events or the EMS log, see the EMS Manual.
Note. HP recommends leaving the MISALIGNLOG attribute ON (its default setting) so that any
process that might round misaligned addresses will generate log entries, facilitating diagnosis
and code repair. Only if the volume of misalignment events becomes burdensome should this
attribute be turned OFF.
Note. The tracing facility does not count and trace every misaligned address, only those that
cause round-down exceptions. Other accesses that use misaligned addresses without
rounding them down do not cause exceptions and are not counted or traced. Also, only a
periodic sample of the counted exceptions are traced by means of their own EMS event
messages.