Pentium4 vs Xeon

Don Holmgren djholm at fnal.gov
Tue Oct 14 13:40:46 EDT 2003



On Tue, 14 Oct 2003, [iso-8859-1] José M. Pérez Sánchez wrote:

> Hi:
>
> We are going to buy a second machine! :-) It will be a diskless dual
> processor node. We are thinking about buying the same configuration:
> Xeon 2.4Ghz 533Mhz FSB, but since Xeon and the motherboards supporting
> them are so expensive, we have been thinking about dual normal Pentium4
> instead. We don't have now any P4 comparable processor to run some
> tests, and after looking at the Intel docs, the only difference we see
> between Xeon and P4 is Xeon having more cache. Does anyone has any
> idea about the relative performance of these processors, what about the
> price/performance ratio? Is it worth paying for more Xeon?
>
> The other point I wanna ask about is the "host bus speed" reported by
> the kernel at boot time, it reports 133Mhz, and our memories are
> supposed to run at 266Mhz, is it normal, is it just the double rate
> thing?
>
> Thanks in advance,
>
> Jose M. Perez.


The major difference between P4 and Xeon is that P4's are available with
up to 800 MHz FSB, and Xeon's with up to 533 MHz FSB.  If your code is
sensitive to memory bandwidth, a P4 can be a big win.  Otherwise they
are essentially equivalent.  P4 and standard Xeon both have 512K L2
caches.  Xeon's with larger L2 caches are available, but if I'm not
mistaken there's a big price difference.

Pricewise (YMMV), cheap desktop P4's can be had very roughly for half
the price of a comparable dual Xeon.  You may very well prefer to admin
half the number of boxes and so would prefer the Xeon.  If you are using
an expensive interconnect, you may also come out ahead with the dual
processor boxes, buying only half of the PCI adapters and half the
switch ports.

Currently P4 motherboards are only available (AFAIK) with 33MHz/32bit
PCI.  That can be a big bottleneck if your cluster application is
sensitive to I/O bandwidth.  Early in 2004, if the rumours are true,
there will be a P4 chipset supporting 66MHz/64bit PCI-X.  And in late
2004, PCI Express should be available on both P4 and Xeon motherboards,
providing a big increase in I/O bandwidth if one has a network which can
take advantage.

Xeon's and P4's do four transfers per clock - so, a 533MHz FSB is really
a 133MHz clock doing 4 transfers per cycle.  The kernel on my 800 MHz
FSB P4 reports a 200 MHz host bus speed.

Don Holmgren
Fermilab
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