LPAR failure when paging space is exhausted

Edit online

By default, when the free paging space on a logical partition (LPAR) reaches the paging-space warning level (npswarn), the AIX® operating system sends the SIGDANGER signal to its processes. If the free paging space becomes lower and reaches the paging-space kill level (npskill), the AIX operating system sends the SIGKILL signal to a process. The AIX operating system uses some heuristics, such as the age of the process, to determine a process to send the SIGKILL signal. When the AIX operating system kills a process due to low paging space, the process releases the used resources and free some paging space.

When the free paging space is low in an LPAR, killing a process will keep the LPAR running. However, killing a process does not ensure that the critical applications will run optimally. You can change this default behavior by using the npsassert vmo tunable parameter. The LPAR fails when the paging space is exhausted.

When the npsassert tunable parameter is enabled, processes are not killed in low paging space conditions. The LPAR fails when paging space is exhausted.

The npsassert tunable parameter is unrestricted, which means that you can change the value of the tunable parameter any time and the changes are effective immediately.

You can specify the following values for the npsassert vmo tunable parameter:
0
Retains the legacy behavior of the LPAR. The AIX operating system sends the SIGDANGER signal to processes when the npswarn threshold value is reached. The AIX operating system kills a process when the npskill threshold value is reached. The low_ps_handling vmo tunable parameter selects a process that can be killed. This is the default setting.
1
Does not kill processes when the npskill threshold value is reached. The LPAR fails when paging space is exhausted.

Changes in default behavior of LPARs

When you enable the npsassert vmo tunable parameter, the AIX operating system fails the LPAR when paging space is exhausted. When you enable the npsassert vmo tunable parameter, the LPAR shows the following changes in the default behavior:
  • When the free paging space reaches the npswarn levels, the AIX operating system does not send the SIGDANGER signal to the processes.
  • When the free paging space reaches the npskill threshold level, the AIX operating system does not kill processes. Paging space is allocated until the free paging space is exhausted.
  • By default, the AIX operating system does not fork new processes when free paging space reaches npswarn levels. When the npsassert vmo tunable parameter is enabled, forking of new processes is not stopped. Only those processes are not forked when free paging space reaches the threshold levels if the value of the PSALLOC environment variable is set to early.