On Sat, Jul 16, 2011 at 12:27:14PM -0400, erik quanstrom wrote:
> > The Itanium story, as guessed early by Hennessy and Patterson in
> > "Computer Architecture", shows that efficiency relying on too
> > complex knowledge, asking too much to the programmers and the
> > compilers, is likely to fail.
>
> another way of looking at itanium is that it's like a multicore
> processor that is programed with a single instruction stream.
> given a general-purpose workload, it stands to reason that
> independent threads are going to be scheduled more
> efficiently and independent threads can be added at will without
> changing the architechtural model. so it's also easier to scale.
That's probably a legitimate view since the gains from pipelining in
current processors were finished and engineers were searching gains
elsewhere. But from what I remember when reading
the description of the aims of the architecture---in CAQA---, since
there was no panacea and no great gain to be easily obtained,
optimizations had to rely on special cases and great knowledge of low
level details by programmers, and some knowledge of higher level for
compilers to do "the right thing(TM)", and that seemed unlikely to work
without a lot of pain.
If RISC has succeeded, this is precisely because the elements were
simple enough to be implemented in hardware, and this simplicity allowed
to work reliably on optimizations.
There is an english expression, IIRC: penny wise and pound fool.
Having the basis right is the main gain. One can compare Plan9,
that can be viewed as achieving what MACH was aiming to achieve,
while Plan9 is really a micro-kernel (to start with by the size of
code), while the MACH like microkernels seem to have survived only
in assembly since it was the only mean to get a decent efficiency.
But people continued to publish thesis and papers about it---some
paragraph in the plan9 presentation paper is about this, if my
english is not totally at fault...---, refusing to conclude that
the results were showing there was definitively something wrong to
start with. But in what was called "science", there is now fashions
too. Story telling everywhere...
--
Thierry Laronde <tlaronde +AT+ polynum +dot+ com>
http://www.kergis.com/
Key fingerprint = 0FF7 E906 FBAF FE95 FD89 250D 52B1 AE95 6006 F40C
