[Info-vax] RealWorldTech on Poulson

[Johnny Billquist]

On 2011-07-03 18:21, John Wallace wrote:
> On Jul 3, 3:45 pm, JF Mezei<jfmezei.spam... at vaxination.ca>  wrote:
>> Neil Rieck wrote:
>>> As I understand the current debacle, EPIC relied on advances in
>>> complier technology which never occurred.
>>
>> How can a compiler predict how a data processing program will execute
>> since the compiler doesn't have any idea what data will be processed ?
>>
>> Compilers can do wonderful things when a program calculates pi to
>> infinite  precision since the logic is fixed and the copiler can really
>> optimise it. But for situations wherethe flow of the program depends on
>> the data being processed (data to which the compiler does not have
>> access), then EPIC can't rely on compiler advances.
>>
>> The Alpha presentations pre-June-25-2001 that showed why IA64 would not
>> work are still valid today.
>>
>> If you gave the 8086 as big a cache as IA64 gets, how much better would
>> the 8086 be compared to IA64 ?
>>
>> At the end of the day, IA64 gets palatable performance due to brute force.
>>
>>> Personal comment: I think Intel and HP are currently at a critical
>>> point where the next few decisions will allow the Itanium program to
>>> "take flight and dominate the enterprise  market" or crash.
>>
>> A rolling stone gathers no moss. Converting IA64 to a normal chip with
>> the advanced features found on competing platforms will liekly take a
>> couple of iterations during whic the platform will be in a state of
>> flux, with customers not too happy with performance unless they have to
>> recompile/recertify all their applications.
>>
>> And while Intel/HP are touting Poulson as the next best thing since
>> sliced bread, lets not forget that the 8086 is not be idle and by the
>> time Poulson comes out, the 8086 may have even widened the gap between
>> itself and IA64.
>>
>>> As far as HP is concerned, PA-RISC
>>> and Alpha are gone so the death of Itanium would hurt them.
>>
>> The plans set forth by LaCarly may contiue: move enterprise computing to
>> commodity hardware. HP will just need to move its OS to the 8086 and
>> focus on building a full range of industry standard servers intluding
>> big iron.
>>
>> When you compare the costs of developping IA64 and its compilers, it may
>> end up being much cheaper to port HP-UX/NSK and perhaps even VMS to the
>> 8086 and use existing compilers for that platform.
>
> "If you gave the 8086 as big a cache as IA64 gets, how much better
> would
> the 8086 be compared to IA64 ? "
>
> A very very fair question, to which we probably won't be told the
> answer. But IA64 chips are basically a big cache with a weird
> processor tacked on.
>
> I'll perhaps have more to say on this subject on a bit, in particular
> wrt the accuracy and relevance of "x86 microprocessors rely on snoop-
> based cache coherency;<snip>  In contrast, Tukwila and Poulson have a
> directory-based coherency protocol that scales much better ...
> <snip>  ... For a 4 or 16 socket system, the bandwidth savings are
> huge."

Let's make one thing clear here. The cache coherency strategy design is 
not inherent to the architecture. x86 can use a directory based cache 
coherency just as good as an Itanium. There is no actual performance 
advantage in Itanium itself in this.
It's just a question of current implementation.

Yes, compiler advances dreamt of by the Itanium team never materialized, 
and honestly it was a desperate dream to start with. Alpha made the 
right choice, but was killed for political reasons.
At this point, the x86 is a better platform to develop than Itanium, 
which made several bad design choices. It's like the Sparc, who decided 
to have a branch delay slot defined by the architecture. That has hurt 
them ever since.
Bad processor designs are very hard to recover from. And Itanium seem to 
have collected a lot of those. No clever cache snooping hardware is 
going to change that.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol