Page fault performance

Hello, I was testing FreeBSD-13.2 page faulting performance on AMD64,
where a thread allocates a large segment of anonymous memory via mmap(2)
and then modifies every 4KiB page there. Transparent large pages were
deliberately disabled via vm.pmap.pg_ps_enabled flag. I only want to
test the base 4KiB page fault performance.

There is good scalability up to 4 concurrent threads, but after that
performance follows a somewhat flat line. Any idea if there is some
runtime setting to improve concurrency and decrease lock contention in
the virtual memory subsystem?

Thanks.