Athlon memory speed asymmetry
mathog at mendel.bio.caltech.edu
Tue Feb 25 11:27:13 EST 2003
I'm observing an odd asymmetry in memory utilization on
Athlons. Trivial array operations (read then write)
are up to 30% faster going up through the array then
down through the array.
This was true with both gcc and Intel C compilers
on Linux and with the Intel C compiler on Windows XP. It
was true on Athlon 2000MP, 2200MP and 2000XP. My one
Sparc machine gave symmetric memory speeds (to within a
percent or two). There is a known prefetch issue on
Athlon's but my understanding of that was that it affected
only self modifying code.
I didn't go looking for this odd effect - it fell out
when benchmarking a combsort variant which swapped
directions on every pass - and anomolously ran
much slower than the original which only went up through
Here's a tiny example program (97 lines, mostly comments):
# gcc -O3 -o memasymmetry memasymmetry.c
# ( time ./memasymmetry 10000000 ) 2>&1 | grep real
# ( time ./memasymmetry -10000000 ) 2>&1 | grep real
AMD support has been contacted but has not got back to me on
Anybody else seen this before?
Who knows what causes it?
Is there a way around it on these Athlons?
mathog at caltech.edu
Manager, Sequence Analysis Facility, Biology Division, Caltech
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