help: kernel panic: pfstatepl :page empty

[Sandglitter <gaolitao%gmail.com@localhost>]

Hello guys,
    I have a box running NetBSD with PF, when the traffic is high, it will down
every one hour throwing the panic message as follow:

//---
panic: pool_get: pfstatepl: page empty
Begin traceback...
pf_state_pl(0,0,c1a01000,486244e1,ecf19) at 0xc0975260
pf_state_pl(0,0,c1c68000,486244f0,749ef) at 0xc097526c
Bad frame pointer: 0xc17b1fdc
End traceback...
Stopped at      netbsd:cpu_Debugger+0x4:        popl    %ebp
db> 
//---

It seems like that there is not enough momery for pfstate,
Have you come to this problem? or do you have any idea about it?

How to configure pf in pf.conf to guarantee the total memory will 
not be used up?
Or to adjust kernel option to accommodate the demand of pf?

Thanks a lot for any help~