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Subject: Re: 3.06 Xeon Test Results

Author: Anthony Cozzie

Date: 21:39:24 04/09/03

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On April 09, 2003 at 23:00:58, Robert Hyatt wrote:

>On April 09, 2003 at 21:46:19, Anthony Cozzie wrote:
>
>>Crafty seems to be unique in that it gets a lot from hyperthreading - Fritz does
>>not, as Charles' benchmarks show.
>>
>>SMT is not a guaranteed win.   For SMT to accelerate a chess program, the
>>following inequality must be true:
>>
>>(1+S)*1.7/2 > 1
>>
>>=> S >= 17% (approximately: SMP speedup varies a lot over various positions)
>>
>>In other words, Crafty on a PIV will get 30% from hyperthreading and a positive
>>speedup, Fritz on a PIV will get 10% from hyperthreading and a relative slowdown
>>in search (even as NPS go down).
>>
>>anthony
>
>
>I do _not_ follow a discussion which talks about a slowdown.
>
>If a program produces _any_ speedup on two cpus, then it should produce a
>speedup using less than two cpus, but more than one cpu (SMT in other words).
>
>If your NPS goes up by 10%, then with a 1.7x multiplier on two real cpus,
>the program should run 1.07X faster using SMT.
>
>If the NPS goes _down_ with SMT on, something else is broken, either in the
>software or the hardware.

My understanding of SMT is as follows.  The processor divides its resources
(issue queue, functional units, cache, etc) among two threads.  Now, I *think*
that said threads are equal, that is, that they both get 50% of the CPU.
Otherwise, special OS code would have to be written to support SMT.

Therefore, suppose fritz on a PIV gets 1000 knps, while with SMT enabled it gets
1100 knps.  That would mean that each "virtual cpu" is creating 550 knps.
1.7x550 = 935 equivalent -> slower.  So while NPS goes up, time to solution goes
down.  The increased raw speed doesn't compensate for the SMP inefficiencies.
Now with *crafty*, the NPS increase is so big that it is worthwhile.

anthony



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