[Info-vax] Poulson at hot-chips 2011

[David Kanter]

On Aug 23, 10:36 am, JF Mezei <jfmezei.spam... at vaxination.ca> wrote:
> IanMiller wrote:
> > there is this
>
> >http://communities.intel.com/community/openportit/server/blog/2011/08...
>
> I am puzzled by the "instruction replay".  They say it is to try
> instructions again if an error is detected.
>
> Does this mean that there is a significant enough number of errors in
> cache to warrant a CPU having such logic to "retry" instructions ?
> Wouldn't it be easier to fix the cache so there are no errors ?
>
> Or is this just marketing gobbledeegook to make it look like they are
> adding new features even though they may be useless in practice ?

Hi, glad to see that some folks out there enjoyed my Poulson article.

I can clarify this pretty easily.

Replay is a technique to ensure correct execution of instructions in
an in-order CPU design.  It's also used in IBM's z10 CPU.

Basically, if the older Itanium design encountered an instruction that
it wasn't ready for (e.g. waiting for a load to get an input operand),
it would stall everything.  Basically, the entire CPU stops making any
forward progress until the instruction is ready (e.g. the load finally
comes back from cache or memory).  This is a problem since as soon as
you have a stall, you have to let every part of the chip know to stop.

With replay, it works differently.  The instructions are issued -
whether or not all the inputs are ready.  If there was a problem (e.g.
an input wasn't ready), the instruction basically gets sent back to
the front of the line for issue (or replayed).  This is nice, since
you can detect problems later in the pipeline - moreover, you don't
need to let every part of the CPU know there's a problem.

It's generally a much more power efficient approach than global stall
(i.e. McKinley/Montecito/Tukwila).

Either way, Poulson isn't really an out-of-order chip - at best it
might issue one or two instructions in slightly different order.
However, it's still very fundamentally in-order...just a much more
efficient approach.

David