freebsd stable - Jan 2015 - Question about sysctl's

I am curious about two particular sysctl's, they are described as

vm.stats.vm.v_wire_count: Wired pages
vm.max_wired: System-wide limit to wired page count

Now to me that sounds like v_wire_count should never get higher than
max_wired, yet today I have a v_wire_count that is ~40% above the max_wired.

vm.stats.vm.v_wire_count: 938417
vm.max_wired: 662820

So is this an error or do these two work differently than they appear?

FreeBSD leader.local 10.1-STABLE FreeBSD 10.1-STABLE #0 r276527: Fri Jan 
  2 21:55:56 ACDT 2015 
root at leader.local:/usr/obj/usr/src/sys/GENERIC  amd64


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Shane Ambler

On 1/13/15 8:37 AM, Shane Ambler wrote:
> 
> I am curious about two particular sysctl's, they are described as
> 
> vm.stats.vm.v_wire_count: Wired pages
> vm.max_wired: System-wide limit to wired page count
> 
> Now to me that sounds like v_wire_count should never get higher than
> max_wired, yet today I have a v_wire_count that is ~40% above the
> max_wired.
> 
> vm.stats.vm.v_wire_count: 938417
> vm.max_wired: 662820
> 
> So is this an error or do these two work differently than they appear?
Not all wired allocations are subject to the limit (which isn't obvious
from the description).  Anything that calls 'malloc()' in the kernel
will use wired memory for example, and those generally aren't subject to
the limit.  However, user-initiated operations (like a sysctl buffer or
user pages wired via mlock()) are.

-- 
John Baldwin