When compile from ports tree and which ports tree on NFS server.
I need to union mount /usr/ports(NFS mount directory).
mount -t nfs [nfs server's IP]:/usr/ports /usr/ports
mount -t unionfs -o noatime /var/tmp/ports /usr/ports
or use rpc_lockd
for example in /etc/rc.conf
rpc_lockd_enable="YES" # for both side(server/client)