6850 shows 16 CPUs?

[Sam Flory]

Brendan Heading wrote:
> Simon Gao wrote:
>> Check if hyper-threading is enabled in BIOS. That's probably why you see
>> double processors than you expect. You will want to turn
>> hypter-threading off since it only causes performance and other problems
>> with no much real benefit.
> 
> Wish I got a quid every time I heard that myth propagated. On my 2850 
> which is used as a build machine, I see an approx. 25% improvement on a 
> eight-way parallel build with it enabled. I'm willing to accept that it 
> probably depends a lot on your workload and how many tasks are going, 
> but it's wrong to say that it is of no real benefit in all cases.
> 

  It use to be true for most people as the kernel wasn't really good at
scheduling for ht systems.  This has been fixed in modern kernels.  Now
what you run into is that you are reducing your chance of cache hits.
(Basically you are in effect trying to cram twice as much in your L1-3
caches.)  Of course some work loads will still easily fit in cache,
others won't fit even in non-HT mode, and some by their nature don't get
many cache hits.  You can't be sure what the end result will be unless
you test it.  Personally I find HT is a net gain for most things, and
not a significant hit when it isn't.