kernel settings with x86 hyperthreading

[Nifty Hat Mitch]

On Sun, Dec 26, 2004 at 04:20:47PM +0100, Fabio wrote:
...
> I would like to know if hyperthreaded x86 computers need kernel SMP
> option activated in order to use that feature.

> Furthermore, in the above-mentioned case, I need to know if FC3 is
> provided with a default SMP kernel because I have to install the
> distribution on a hyperthreaded P4 box and only after a while I will
> have time for kernel recompiling.

As other indicate if this is enabled by the BIOS (acpi) then
the right thing happens on install.

> My last question is about the hyperthreading itself: Do we really
> get improvements by turning on that feature? The kernel has been
> written with supported multiple CPU operations, so I think it would
> benefit from hyper., but what about KDE, konqueror, kdevelop,
> qtdesigner and gcc (these are my most used applications) ?

Hyperthreading is an interesting hardware thing.

Many of the new processors have multiple functional blocks, dynamic
register naming and more.  The idea is that the hardware can do more,
and more work in parallel.  The hyperthreading folks seem to have
found a way to sneak up on the complexity problem and avoid some of
the tricky problems with instruction scheduling by adding a second
execution dispatch/unit that can contend for functional blocks and use
idle or duplicate blocks.

The result so far might be called aSMP (almost Symmetric Multi Processor).

Because the hardware is almost symmetric the symmetric expectations of
a Linux SMP kernel can sometimes queue a set of processes that contend
for resources rather than fly in parallel.

Since the processor guys seems to be on a true multi core design path
the advantage is that SMP software can be designed, developed and
debugged.  A critical path in this is device drivers.

A clever OS might also do a bit of aSMP activity and present a user
with a faster appearing machine (quicker feel).

Since Linux has a SMP expectation the net result depends on the specific
user load.  i.e. YMMV.

The next generation of hardware should find numerous OS and
applications that are MP safe.  Since I am a fan of SMP, this is good
IMO.


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