vm.kmem_size_max and vm.kmem_size capped at 329853485875 (~307GB)

[Gezeala M. Bacuño II]

Hello fellow listers,

On a server with 512GB RAM it appears that vm.kmem_size_max is not
being auto-tuned to use >329853485875 (~307GB).

On this machine vm.kmem_size is equal to vm.kmem_size_max

# from sysctl
vm.kmem_size_max: 329853485875
vm.kmem_size: 329853485875

On a machine with 1GB of RAM, I have successfully set vm.kmem_size_max
to 330GB and vm.kmem_size automatically adjusts to 1GB even if I
manually set it in /boot/loader.conf.

But on the machine with 512GB of RAM it just resets. For the machine
to boot, we need to go to the loader prompt and issue:

OK set vm.kmem_size_max="300G"
OK boot

On all PCBSD (8,9) or FreeBSD (8.1,8.2,9) machines we have,
vm.kmem_size_max is always set to 329853485875.

How can I increase vm.kmem_size_max to use at least 500GB? And how is
329853485875 determined (formula)? I need to increase vm.kmem_size_max
and vm.kmem_size so I can set vfs.zfs.arc_max (ZFS ARC) to use say
490GB.

I'm browsing thru the source code at
http://fxr.watson.org/fxr/search?v=FREEBSD9&string=vm.kmem_size_max
and I'm still trying to make sense of how vm.kmem_size_max is
computed.

I have posted the same topic on forums.freebsd.org but I'm not getting
any recommendations.

Please see the link for additional details:
http://forums.freebsd.org/showthread.php?t=33977

TIA!

--

regards