A system running Compaq's Tru64 Unix, version 4.0D (with or
without patch-kits #1, #2 or #3) could hang due to a low
vm_page_free_count. This is particularly likely to happen
when the vm_page_free_count falls below 10.
When the system load is heavy, with a low amount of free memory
pages and a large number of processes contending for memory/RAM
resources, the system starts paging or at worst swapping to
recover some additional memory pages.
In the "Digital Unix - System Tuning and Performance Management"
guide for version 4.0D, it states that sometimes a system is unable
to recover any pages once the vm_page_free_count falls below 10
(the default setting for the kernel parameter vm-page-free-reserved).
This low memory condition could prevail for longer times thus
looking like a hung condition. Though the system may recover
eventually, we may never know how long this could be.
Often on a forced crash, we can look for such condition as follows:
dbx -k vmunix.# vmcore.#
(dbx) pd vm_page_free_count
9
(OR)
(dbx) pd vm_perfsum.vpf_freepages
9
The vm-page kernel parameters can be configured to prevent the
Unix 4.0D system from getting into such a low free memory condition
thus preventing a potential hang. Different systems with different
loads may need these parameters to be configured with higher values,
but the following parameters and suggested values can be used
as a starting point.
These kernel parameters belong to vm: sub-section of the
file /etc/sysconfigtab
vm:
vm-page-free-target = 512
vm-page-free-swap = 128
vm-page-free-min = 128
vm-page-free-reserved = 64
vm-page-free-optimal = 256
The default settings for these parameters are:
vm:
vm-page-free-target = 128
vm-page-free-swap = 74
vm-page-free-min = 20
vm-page-free-reserved = 10
vm-page-free-optimal = 74
Explanation and more information about these parameters can be
obtained from the System Tuning and Performance Management Guide
for Unix.
Most recently modified Fri Apr 30 11:15:24 EDT 1999 by linda.downing@compaq.com (117864)