hyper threading.

[em1897 at aol.com]

I've never seen any "measurements". And most of your
"theories" are clearly incorrect for FreeBSD. So what good
is it?

You claim to have done measurements, so what do you have
to refute it? Being a fool is a choice. Its easily turned.

The problem is when you can't get more hardware. When you
are pushing the envelope, then you run out of choices. There
is also a price/performance consideration. You make a choice
to spend an extra 30% for certain hardware. But if you can
get the same performance using lesser hardware with different
settings or a different version of the OS, then you are wasting
your money. If you don't need much, or you are spending someone
else's money, then everything is moot. Just use whats cool.

-----Original Message-----
From: Anthony Atkielski <atkielski.anthony at wanadoo.fr>
To: freebsd-questions at freebsd.org
Sent: Sun, 27 Mar 2005 20:01:57 +0200
Subject: Re: hyper threading.

em1897 at aol.com writes:

> You know, you spout all of this wonderful theory without considering
> the quality of the implementation.

Somethings can be derived directly from theory.  If you know the design
of the hardware, you can predict that two processors will provide x%
increment of throughput over a single processor, even if you don't
actually measure them.

In my case, I cite both theory and my own experience in measuring actual
systems.  The general principles of behavior of multiprocessor systems
are well understood, although specific implementations vary.  It is
clear, based even on design data alone, that hyperthreading will
generally improve throughput and should never diminish it (disregarding
OS overhead).  It is equally clear that the gain won't be as great as
having physically independent processors, but the idea of putting more
of the idle processor logic to work is a good one.

> And a key point that you consistently overlook is that FreeBSD 5.x is
> a particularly poor implementation of SMP. Linux and Dragonfly get 80%
> improvement in performance with a 2nd processor, and FreeBSD doesn't.

I'd need to see measurements to substantiate this.

In general, when it comes to optimization, it's best not to fret too
much over how many percentage points of processor power or throughput
you gain or lose with specific configuration or implementation choices.
If your system is running so close to the wire that five percent makes
the difference between 100% busy and less than 100% busy, you need more
hardware in any case.

> The concept that the kernel is poorly implemented by userland is well
> done is just not an assumption that you can make.

Actually, it's not something that I spend a lot of time thinking about.
Right now, my production system is never more than 0.4% busy.  And if it
were 99% busy, I'd be looking at faster hardware, no matter what OS or
HT/MP options I might have implemented.

--
Anthony


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