DESCRIPTION
This value sizes the kernel dynamic memory resource map, an array of
address/length pairs that describe the free virtual space in the
kernel's dynamic address space.
This array has historically been statically-sized. That means
certain pathological workloads that fragment the kernel address space
will result in too many entries for the array.
Rather than panicking when this condition occurs, the system throws
away the last entry, which results in "leaked" kernel virtual address
space.
If this overflow happens often enough, the system eventually runs out of
virtual space and will panic with the
kalloc: out of virtual space
message.
By making the map size tunable, the system can automatically scale the map
size according to the system workload size and avoid this problem.
If the automatic scaling does not work, you can hand-tune to
fit a particular workload.
When the default value is overridden, the kernel may increase
the value beyond the your specification depending on the system size.
There are different tunables for 32- and 64-bit kernel because the
64-bit kernel has more virtual address space.
The
nsysmap
tunable controls 32-bit kernels and the
nsysmap64
tunable controls 64-bit kernels.
Who Is Expected to Change This Tunable?
Only those experiencing the resource map overflow will need to
modify this tunable.
Restrictions on Changing
Changes to this tunable take effect at the next reboot.
When Should the Value of This Tunable Be Raised?
The following message will appear on the console when the resource map
overflow occurs:
sysmap{32|64}: rmap ovflo, lost [X,Y).
If this happens only occasionally, no action is necessary.
If this happens with any sort of frequency, the tunable should be
increased, or the
kalloc: out of kernel virtual space
panic message may occur.
When increasing
nsysmap{32|64},
doubling the tunable value is a reasonable rule of thumb.
If the problem persists after doubling the tunable several times from
the default, there is likely to be a more serious kernel problem, and you
should contact HP support to investigate.
What Are the Side Effects of Raising the Value of This Tunable?
Kernel memory use increases very slightly.
Depending on the workload, if the tunable is quite large, the
performance of kernel memory allocation may be affected.
When Should the Value of This Tunable Be Lowered?
Lowering this tunable from the default is risky, in all probability
unnecessary, and should only be done after first consulting
HP support.
What Are the Side Effects of Lowering the Value of This Tunable?
A much higher probability of resource map overflows eventually leading
to a kernel panic.
What Other Tunables Should Be Changed at the Same Time?
None.
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