PC164 mysteriously slow

I have two Alphas, an AlphaPC 164 and an AlphaServer 800.  Both are
21164A @ 500MHz machines with a 2117x core chipset.

I fired up the AS800 for the gcc4 stuff, and now that I have both
machines running in parallel I notice that the PC164 is slower than
the AS800.

A simple "md5 -ttt" shows:
PC164: 302 s
AS800: 210 s

In other words, the PC164 is running at 70% the speed of the AS800.
That is weird.

My first thought was, maybe the PC164 isn't really running at 500
MHz.  But it uses kern.timecounter.hardware=rpcc, and the RPCC
directly reflects the clock rate.  If that was wrong, the system
clock would diverge wildly, which it doesn't (ntpd.drift: 2.984988e-05).
I checked rpcc_timecounter.tc_frequency: 499989720.  500 MHz.  That
is calibrated against a time-of-year clock chip, I think, which has
a separate crystal.

So the CPU clock speed is okay.  What else is there?  md5 -t is a
very stupid test.  It runs a 10000-byte block of data through a
small loop; that shouldn't involve main memory or even the lower
cache levels.  Besides, I checked the board, and the CPU clock
divisor, Bcache size, and memory bus width jumpers are all in the
expected positions.

Any ideas?

-- 
Christian "naddy" Weisgerber                          [email protected]