NAME
ncallout — size of the kernel callout table
VALUES
Failsafe
1024
The worst case requires one callout entry for every active thread, and
some number of callout entries for use by different kernel subsystems and
device drivers. A value of 1024 should be failsafe when booting the system
to make system repairs.
Default
nkthread+64
(in HP-UX 11i)
HP-UX 11.0 systems have their default value derived from the
nproc
tunable.
These systems may panic with the
Callout Table Overflow
panic message if
nproc
is smaller than
nkthread
and the system callout usage exceeds the available callout entries.
The default value should be changed as above on these systems.
Allowed values
Any positive value is
valid
for this tunable, although in practice
a very small value will likely cause a panic during boot (or soon after),
and a very large one will consume unnecessary kernel memory resources.
Changing the tunable from the default value should not be required.
Recommended values
Use the default value as derived based on
nkthread
tunable.
DESCRIPTION
The
ncallout
tunable defines the size of the callout table allocated during
system initialization.
When a timeout service is requested by kernel subsystems
either for their internal usage or due to a user initiated timer request, an
entry is used from the callout table.
When the callout table runs out of available free entries, the system will
panic with the
Callout Table Overflow
message.
HP-UX 11.0 systems have no mechanism for dynamic expansion of the callout table,
so the system will panic when it runs out of free entries in the callout table.
HP-UX 11.11 systems have a partial dynamic expansion mechanism in place.
If there is no available callout entry to service the
request, the kernel will try to dynamically allocate new callout entries
and service the request.
If the kernel cannot allocate memory (in tight memory situations), the system
will eventually panic.
Who Is Expected to Change This Tunable?
Only the 11.0 systems whose default is not set correctly.
Restrictions on Changing
Changes to this tunable take effect at the next reboot.
When Should the Value of This Tunable Be Raised?
If the system panics with the
Callout Table Overflow
panic message, the tunable value needs to be raised.
What Are the Side Effects of Raising the Value of This Tunable?
The tunable value defines how much memory the kernel allocates, so a large
value will result in additional physical memory consumption thus reducing
the memory available for applications and other uses.
When Should the Value of This Tunable Be Lowered?
There is no need for it.
The
nkthread
tunable is expected to define the system workload, the
ncallout
tunable is derived from there.
What Are the Side Effects of Lowering the Value of This Tunable?
The lowering of the
ncallout
tunable will increase the chances of hitting the callout table overflow
panic in high workload conditions.
What Other Tunables Should Be Changed at the Same Time?
None.
WARNINGS
All HP-UX kernel tunable parameters are release specific.
This parameter may be removed or have its meaning changed in
future releases of HP-UX.
AUTHOR
ncallout
was developed by HP.
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