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Re: Linux and Intel's Hyperthreading



On Thu, Jan 02, 2003 at 07:35:41PM +0100, mess-mate wrote:
> On Tue, 31 Dec 2002 20:16:03 -0800 (PST)
> "nate" <debian-user@aphroland.org> wrote:
> 
> | nick lidakis said:
> | > I was looking to replace my 1Ghz P3 and motherboard with  a stable,  but
> | > fast mb/cpu combo that was fully supported by a recent linux kernel. I
> | > was looking a an Intel 845PE motherboard with a 3.0Ghz cpu. My question
> | > is, how is Hyperthreading supported under linux? Is it a matter of
> | > enabling SMP in the kernel? Anyone playing with one of these CPU's?
> | 
> | 
> | it is supported by default. the system will see double the cpus there
> | actually are. the kernel isn't tuned to fully take advantage of the
> | hyperthreading yet though. I think newer 2.5.x kernels can do it, perhaps
> | theres a patch for 2.4.x.. but last I read the current breed of stable
> | kernels are not optimized for it.
> | 
> | If I had a system with hyperthreading I would disable the hyperthreading
> | in the bios(one mailing list thread mentioned there is an option to do
> | so, at least on some systems). Because the kernel would get confused and
> | think there are 4 processors on a 2 processor system it might try to get
> | smart by loading stuff up on processor #2, not knowing its the same physical
> | processor as #1, before loading stuff on #3. I don't remember any benchmark
> | numbers but I seem to recall there being very little if any difference
> | in performance on hyperthreaded systems with hyperthreading on vs. off
> | on the stock kernels(performance can go way up on the newer 2.5.x which
> | are tuned to take advantage of it in some cases).
> | 
> | I've read one post on the redhat list recently, some guy was asking why
> | top showed 4 cpus on his dual p4, since it was a dual cpu system, a guy
> | responded because it was hyperthreading .......
> | 
> | so it should work, just not very optimal.
> | 
> | nate
> | 
> Well, ASUS suggest to compile with the Hyper-Threading compiler ???
> What about this compiler ?

I'd say they mean Intel's non-Free C/C++ compiler.  It costs a bucket in
general, but I think it's free (as in beer) for personal use.  Of
course, the C++ compiler's ABI is incompatible with every GCC release
thus far (not that GCC is even compatible with itself, across releases),
so you'll have a hell of a time getting it to integrate int your Debian
system, if you decide to use it.  Depending on what you do, it can
apparently lead to enormous speedups (mostly with numerical code
though, or so I hear), but GCC 3.x is closing the gap...

Anyhow, I can't imagine that a smart compiler could do *that* much to
help, since getting good performance on multi-processor systems is more
of a programming design issue, rather than compiler smarts.  No doubt
having two 'CPU's in the once chip, sharing registers and cache requires
some compiler intelligence though.

-rob

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