System Freezes after a few hours


I installed FreeBSD 11 RELEASE with zfs on my HP Pavilion dm3-1130us notebook. When I ran VirtualBox, it was running really slow (it took more than 5 mins for guest servers to boot up), so I upgraded my memory to 8GB. After this, VB guest servers were running smoothly, but it occasionally started to freeze up the entire system after a few hours. I figured that zfs and VB was eating up the memory, so after googling, I put below settings in /boot/loader.conf


After rebooting, the problem didn't go away. Once it freezes, mouse, keyboard, and network become nonresponding. All I can do is cold reboot.

Is there anyway to find what actually causes this freeze up? Any logs or something to see what happened right before it froze?

Thank you for your inputs in advance.
Du you understand what you put in /boot/loader.conf?
If not better remove the lines.

I'm curious with what explanation you come up for this:
Config is normal, but wrong symbol, yes, don't see this... what any more say, is a little bug in config, you change ?
I gave you the recommendations as a whole.
With memory problems often arise, plus the system does not fall and does not even have a dump.
Therefore recommended to check the memory, perhaps in the way that's the problem, you say I have a memory, a new check