kernel - Another huge HUGE VM performance improvement for many-cores

kernel - Another huge HUGE VM performance improvement for many-cores

This requires a bit of explanation.  The last single-point spinlocks in the
VM system were the spinlocks for the inactive and active queue.  Even though
these two spinlocks are only held for a very short period of time they can
create a major point of contention when one has (e.g.) 48 cores all trying
to run a VM fault at the same time.  This is an issue with multi-socket/
many-cores systems and not so much an issue with single-socket systems.

On many cores systems the global VM fault rate was limited to around
~200-250K zfod faults per second prior to this commit on our 48-core
opteron test box.  Since any single compiler process can run ~35K zfod
faults per second the maximum concurrency topped out at around ~7 concurrent
processes.

With this commit the global VM fault rate was tested to almost 900K zfod
faults per second.  That's 900,000 page faults per second (about 3.5 GBytes
per second).  Typical operation was consistently above 750K zfod faults per
second.  Maximum concurrency at a 35K fault rate per process is thus
increased from 7 processes to over 25 processes, and is probably approaching
the physical memory bus limit considering that one also has to take into
account generic page-fault overhead above and beyond the memory impact on the
page itself.

I can't stress enough how important it is to avoid contention entirely when
possible on a many-cores system.  In this case even though the VM page queue
spinlocks are only held for a very short period of time, the convulsing of
the cache coherency management between physical cpu sockets when all the
cores need to use the spinlock still created an enormous bottleneck.  Fixing
this one spinlock easily doubled concurrent compiler performance on our
48-core opteron.

* Fan-out the PQ_INACTIVE and PQ_ACTIVE page queues from 1 queue to
  256 queues, each with its own spin lock.

* This removes the last major contention point in the VM system.

* -j48 buildkernel test on monster (48-core opteron) now runs in 55 seconds.
  It was originally 167 seconds, and 101 seconds just prior to this commit.

  Concurrent compiles are now three times faster (a +200% improvement) on
  a many-cores box, with virtually no contention at all.