What is the meaning of this warn?

[Jason]

HI,ALL:

My freebsd server offer service to thousands of clients.
I found lots of warning in the /var/log/message like this :"Approaching
the limit on PV entries, consider increasing either the
vm.pmap.shpgperproc or the vm.pmap.pv_entry_max tunable"

My question is :
1.what is the meaning of PV?
2.Although there are lots of this warning,my server works still well.
Could someone explain the meaning of this warning ?