newbie SMP questions

Khairil Yusof kaeru at
Mon Jan 19 18:26:16 PST 2004

On Mon, 2004-01-19 at 15:32 +0000, Chris Stenton wrote:

> 1. I can't find a thread on the pro and cons of having HTT enabled.

For 5.x releases it should be better (less locking) but in my experience
lack of a scheduler that has virtual/physical processor affinity can
degrade your performance. This happened often on a client's 4 CPU XEON.

2 cpu intensive processes were often running on the same physical CPU
(linux), instead of different ones (ULE+KSE will help with regards for
this when it hopefully becomes default on 5.3).

You should do simple benchmarks of the apps you intent to run on your
server with HT on and off.

> 2. An old rule of thumb for a single processor machine was to have twice
> the amount of swap space to physical RAM. Is this still the case for a 4
> logical CPU machine? I never understood where that rule of thumb came
> from:-)

I've read in a few places that the swapping algorithms, are optimised
for multiples of 2 or 4. I think tuning(8) mentions it. But I don't
think the number of processors affects the swap ration you should have. 

It's more about how much physical memory, and how heavily loaded memory
wise do you think your server is going to be.

"You keep using that word. I do not think it means what you think it

FreeBSD 5.2-CURRENT i386 
9:52am up 1 day, 9:56, 4 users, load averages: 2.11, 1.50, 1.16

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