Subject: Re: NetBSD and large pps
To: Mihai CHELARU <>
From: Thor Lancelot Simon <>
List: tech-kern
Date: 12/03/2004 17:17:31
On Sat, Dec 04, 2004 at 12:11:53AM +0200, Mihai CHELARU wrote:
> Yes, I beleive I need hyperthreading on this machine as concurency is 
> high about all the time (load average between 6 and 20).

You do understand that, because each CPU can already do out-of-order
execution whether hyperthreading is enabled or not, hyperthreading may
not actually expose any more real hardware concurrency than you get in
the non-hyperthreaded case?

Even when it does expose more hardware concurrency, the win is usually
*very* small.  And in a system with multiple physical processors, there
is a downside: our scheduler does not know which CPUs are virtual and
which are physical and may thus allocate processes in a way that actually
makes much *worse* use of the available executaion resources than if you
just turn hyperthreading off and let it see that there are really just
two physical CPUs.

You should make one change at a time, if you can, and do careful
benchmarks.  But my strong suspicion is that in fact hyperthreading is
not doing you any good.

 Thor Lancelot Simon	                            
 	Am I politic?  Am I subtle?  Am I a Machiavel?
							-William Shakespeare