grep speed on Opteron vs P4

[T. Horsnell]

I'm seeing huge discrepancies in grep speed between
my Opteron box and P4 boxes.

The Opteron box:
----------------

Dual Opteron 248, 2.2GHz, 1MB cache, 8GB memory, RHEL4
ls1_tsh> uname -a
Linux ls1.lmb.internal 2.6.9-22.0.1.ELsmp #1 SMP Tue Oct 18 18:39:02 EDT 2005 x86_64 x86_64 x86_64 GNU/Linux
ls1_tsh> grep -V
grep (GNU grep) 2.5.1


grepping thro an 800 MB file:

ls1_tsh> time grep rubbishrubbish /tshgrp0/tsh/junkjunk
41.618u 1.263s 0:43.30 99.0%    0+0k 0+0io 0pf+0w
ls1_tsh> time grep rubbishrubbish /tshgrp0/tsh/junkjunk
41.675u 1.477s 0:43.54 99.0%    0+0k 0+0io 0pf+0w

ls1_tsh> ls -l  /tshgrp0/tsh/junkjunk
-rw-r--r--  1 tsh tsh 869243540 Sep  5 13:13 /tshgrp0/tsh/junkjunk

I'm certain the file is all stored in cache during this, and
that no disk accesses are taking place, because:
1. 'top' shows grep at 99.9% cpu
2. iostat on the disk in question shows zero activity.

The P4 box:
-----------

Monocore P4, 3.4GHz, 1MB cache, 2GB memory, FC3.
fm22_tsh> uname -a
Linux fm22.lmb.internal 2.6.11-1.35_FC3 #1 Mon Jun 13 00:52:08 EDT 2005 i686 i686 i386 GNU/Linux
fm22_tsh> grep -V
grep (GNU grep) 2.5.1


fm22_tsh> time grep rubbishrubbish /scr0/tsh/junkjunk
0.272u 0.318s 0:00.59 98.3%     0+0k 0+0io 0pf+0w
fm22_tsh> time grep rubbishrubbish /scr0/tsh/junkjunk
0.311u 0.281s 0:00.59 100.0%    0+0k 0+0io 0pf+0w

fm22_tsh> ls -l /scr0/tsh/junkjunk
-rw-r--r--  1 tsh users 869243540 Sep  5 14:29 /scr0/tsh/junkjunk


This is indicating that grep on the Opteron is *forty* times
slower than on the P4. Any ideas why this might be?
64-bit nasties in grep? Has anyone else seen this?

Cheers,
Terry