IBM AIX Continuous Availability Features - IBM Redbooks

This option should be used with care because it can be expensive from a performance
standpoint. When large ranges are freed and deferred, all the pages in the range are
disclaimed. Presuming there is no error, all the memory will be faulted and zero filled the next
time it is referenced. “Read” references to freed memory are medium severity errors, while
“write” references always cause a system crash. If the disposition of medium severity errors is
set to cause a system crash, the system will crash on a “read” reference.
errctrl -c alloc.xmdbg deferred_free=
Redzones for large allocations
This option sets the frequency of redzone page construction. This option is specific for
allocations of a page or more. With default error disposition in effect any “read” references to
redzone pages will cause an error log event, and “write” references will cause a system
crash.
errctrl -c alloc.xmdbg redzone=
VMM page state checks
This option sets the frequency at which xmfree() will check page protection settings, storage
key bits and pin counts for memory being freed back to a heap. Not all errors in this area are
fatal. For instance, a page that has higher than expected pin count at free time will waste
pinned storage, but there are usually no fatal consequences of that.
When a page is returned that has a lower than expected pin count, has the wrong page
protection settings, or has the wrong hardware storage key associated with it, the system will
crash.
errctrl -c alloc.xmdbg vmmcheck=
3.9.5 XMDBG tunables not affected by error check level
Memory leak percentage
This tunable sets the percentage of heap memory that can be consumed before an error is
logged. This is specific to the heaps controlled by the component. Heaps that are controlled
by other components are not affected.
For example, alloc.heap0 is a separate component that controls the heap used by the loader,
and it uses a different percentage than the kernel_heap, which is controlled by alloc.xmdbg.
Component level heaps created by heap_create() can be registered separately, and can be
given different percentages. Refer to 3.9.7, “Heap registration for individual debug control” on
page 137 for information about the individual heap registration mechanism.
errctrl -c alloc.xmdbg memleak_pct=
Tip: This tunable requires the user to make a judgment about how much storage should be
consumed before a leak should be suspected. Users who do not have that information
should not use the command. The default percentage is 100% (1024/1024).
Memory leak errors are classified as LOW_SEVERITY errors and the default disposition is to
ignore them. The error disposition for low severity errors can be modified to log an error or to
cause a system crash. This tunable can be seen in KDB by using the xm -Q command. The
field Ratio of memory to declare a memory leak shows the memory leak percentage.
Memory leak count
This tunable sets an outstanding allocation limit for all the fragment sizes. This is intended as
an aid in catching memory leaks that are very slow-growing. If the total number of outstanding
allocations of any fragment size grows beyond this limit, an error is logged. For example, an
134 IBM AIX Continuous Availability Features