pagezero_daemon_enabled(5)

NAME

pagezero_daemon_enabled — zeroing of free memory in the background is enabled

VALUES

Default

1

Allowed values

Minimum: 0

Maximum: 1

DESCRIPTION

HP-UX provides improved security by zeroing out any memory that is being assigned to user space. This ensures that no user can read what may have been written by some other user. Normally, the zeroing is done when the physical page is allocated to the user -- often when the application touches the page for the first time. Some system calls like mlock() also result in the zeroing of pages. The time taken for these kind of system calls and accesses depends upon the size of the memory being zeroed out. A 4G page may easily take many seconds to be allocated. A large database shared memory segment may take many minutes to be allocated. Allocation of a small page is generally unobservable.

The pagezero daemon is a performance enhancement that reduces the elapsed time for completing kernel operations like page faults, mlock(), and so on. The idea is to zero out large free pages (4MB and above in size) during times when the CPU is idle. The daemon has been specially designed to ensure that it only executes for brief times when the CPU is idle.

However, under certain conditions, where some resource (CPU, TLB or Memory bandwidth) is very highly utilized, it is possible that the operation of the daemon adversely impacts performance. For example, if the application is limited by the memory bandwidth, then it may be better to disable the daemon. This situation is expected to be rare. Most workloads will not need to disable the daemon.

This tunable allows system administrators to disable and enable the pagezero daemon. Once the daemon has been disabled, it will not zero out any more pages. Any pages already in the process of being zeroed will be zeroed out. When the daemon is enabled, it will zero out any unzeroed free pages of size 4MB and more.

Who is Expected to Change This Tunable?

Anyone.

Restrictions on Changing

Changes to this tunable take effect immediately.

When Should the Value of This Tunable Be Changed to 0?

This tunable should be changed only if the system is using large pages (i.e. vps_ceiling is set to 4MB or above). Changing it if the system is not using pages of size 4M and above will have no effect. It may be changed to 0 if the system is bottle-necked on some hardware resource. In particular, disabling the daemon may help in decreasing TLB misses and increasing the available CPU time and reducing memory latencies.

What Are the Side Effects of changing the value to 0?

Disabling the daemon will increase the time it takes to process page faults and to complete system calls that result in the allocation of memory (e.g. mlock()).

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.

Installation of optional kernel software, from HP or other vendors, may cause changes to tunable parameter values. After installation, some tunable parameters may no longer be at the default or recommended values. For information about the effects of installation on tunable values, consult the documentation for the kernel software being installed. For information about optional kernel software that was factory installed on your system, see HP-UX Release Notes at http://docs.hp.com.

AUTHOR

pagezero_daemon_enabled was developed by HP.