[Beowulf] Opteron downrates DDR400 to DDR333 performance?

[Vaclav Hanzl]

> do any of you know if the AMD Opteron downrates the PC3200 400 MHz DIMMS
> (DDR400) to 333 MHz (DDR333) performance  when the system has more that 2
> Gbytes of memory installed?

Our ASUS A8N-SLI DELUXE mainboard downrates them as described above.

However we ran various tests to determine how sad we should be and
concluded that the performance drop is much smaller than the naive
400/333 expectation.

With dual core Opteron at 2.2GHz, we measured (among many other
numbers) using the same 1G RAMs rated DDR400:

                                    2x1GB,400        4x1GB,333
 tar xjf linux-2.6.14.3.tar.bz2      16.37 seconds    17.82 seconds
 compile kernel, make -j5           855.4  seconds   865.6  seconds

The 'tar xjf' test was by far the most sensitive to various memory
configuration changes (here 17.82/16.37 = 1.089), most codes we really
want to run were similar to kernel compile result (865.6/855.4 =
1.012).

I think the performance drop is well worth the sustained reliability
when going up to 4GB.

Furthermore, all tests were done using Knoppix in RAM. Any harddisk or
network involved will make the difference even smaller.

Of course, YMMV, somebody else might have seen 20% difference for
his/her code. But it would be far from usual.

(By the way, dual-core kernel compile is really twice faster than
using just one core with 'make'. Well done, AMD.)

Bye

Vaclav