[rc5] Identical results, two CPUs

Tom Wheeler tomw at intelligraphics.com
Sat Jul 26 16:48:10 EDT 1997


On Sat, 26 Jul 1997 15:10:34 -0500, root wrote:

>Tom Wheeler wrote:
>> 
>> On Sat, 26 Jul 1997 11:15:34 -0500, Mike Silbersack wrote:
>> 
>> >> Among other machines, I'm running the client on a P200/MMX and a
>> >> regular P200.  A coworker says there's "no way" these two machines can
>> >> be running at virtually identical speed, but they are - about 240Kk/s.
>> >> Would anyone care to take a crack (ha ha) at explaining why they are
>> >> the same so my coworker and I can end this silliness?
>> >>
>> >> Tom Wheeler
>> >
>> >Well, the Pentium MMX has an improved branch prediction unit and a larger
>> >internal cache, so it *should* be faster.
>
>Except that the ratio of ops to branch ops is _very_ large in the rc5
>algorithm, so the branch prediction will have little effect. Second,
>the BPU is optimized for some vague "normal" code mix. The rc5 is not
>anywhere near a "normal" everyday app as far as the instruction mix.
>
>>  However, the possibility also
>> >exists that the motherboards/bios settings differ on these systems to a
>> >great enough extent that the MMX is being slowed down to the speed of the
>> >Pentium Classic.  Maybe if you check out the BIOS settings on the two
>> >computers you will find that the Pentium Classic is using more aggressive
>> >timings, has a larger external cache, or something else.
>> 
>> I can see the better branch prediction making a small difference.  I
>> don't think the larger L1 cache is relevant - I would hazard to guess
>> that the client is small enough to fit in the P200's cache (or mostly,
>> anyway).
>
>Ah, think again, "mostly" means there is little benefit at all. The rc5
>is almost linear code that runs straight through from start to end. The

I think I'm missing something here.  When you say, "\"mostly\" means
there is little benefit at all", do you mean there is little benefit if
the client doesn't fit entirely in L1 (because of its being nearly
linear)?

Do you happen to know if the client fits in a P200's L1 cache?

>> As for BIOS settings, the machines unfortunately have the AMI "Let's
>> give the user exactly no control over his machine" BIOS, so there's not
>> much I can even look at there, much less change.  Nevertheless, the
>> client is massively compute-bound anyway, so I can't think of any
>> changes to the BIOS that would make a difference.
>
>You mean things like the level 2 cache wait states? Like a write-back,

(etc. etc.) Yeah, okay, point taken.  I was keying in on the word
"BIOS" when I should've been thinking system changes.

>MMX = More Money for X(intel). Not much else, really. Is a 5-15%
>increase in performance for _some_ benchmarks really worth a 50% price hike? 
>Face it, MMX is valuable to intel, intel's stockholders, the advertising

That I agree with entirely.  It does have a few applications outside
the "multimedia" space - look at what the instructions do and you can
see where - but the benefits in most cases are quite small.  Only
worthwhile when you really need that extra few percent.


Regards,
Tom Wheeler
tomw at intelligraphics.com


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