[RC5] RC5-Crunching speed

John Campbell jcampbel at lynn.ci-n.com
Sat Jun 1 09:57:07 EDT 2002

On Sat, 1 Jun 2002, Lars Rander wrote:

> At 11:02 01-06-02, you wrote:
> >I have a new Pentium4 at 1.6 GHz. It's cracking RC5 at a rate of 2.04
> >MKeys/s. I'm using the client v2.7106.436 GUI. This is the last -to my
> >knowledge- GUI client. Would upgrading to a newer client improve the key rate?
> I've got a 1000MHz AMD, and the benchmark says 3.4MKeys/sec - so yes, it
> will! :-)
	You can't compare the two. The K7 (Athlon, Duron) and the Pentium 4
are radically different designs. The P4 has a long pipeline and is designed
for massive ramping of clock speeds, while the K7 has a shorter pipeline
that doesn't ramp as well, but loses less ground in branch mispredicts. That
means that the K7 is a more MHz-efficient chip than the P4. My 1.6GHz
Athlons (XP 1900+) will outperform that 1.6GHz P4 by a fairly significant
margin across the board. The P4 makes up for it with much higher clock
speeds... Intel's fastest at the moment is a 2.53GHz chip, while AMD's
fastest is a 1.73GHz (XP 2100+).

	This is especially true of RC5... RC5 relies heavily on the ROTL
instruction, which is seldom used for other applications. Intel made the
design decision to use valuable die space that might've otherwise gone to
implementing a fast ROTL for other things, which means that the P4 is really
bad at the instruction that the RC5 algorithm uses most heavily. This is a
departure from previous x86 designs... the K7, the Pentium III, and other
x86 chips have fast hardware ROTLs, which means that even slower-clocked P3s
will outperform the P4 in RC5, and the K7s will utterly humiliate them (my
1.6GHz Athlons bench at 5.5Mk/s each... the P4 hasn't been made that can
touch them, even clocked 1000MHz faster).

	That said, I think the new clients *do* offer some improvements
that'll make the P4 a bit faster... don't expect it to come anywhere close
to matching AMD RC5 performance, though.

John Campbell
jcampbel at lynn.ci-n.com

QotD:  What this world needs is a good five-dollar plasma weapon.

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