Monday, March 05, 2012

mbw MCBLOCK test results (used to be) bogus

Update: Andras updated mbw to fix this.

I was testing a machine's memory performance, and ran across the "mbw" package in Ubuntu lucid.

$ mbw -a -n 1 1024
0 Method: MEMCPY Elapsed: 0.23493 MiB: 1024.00000 Copy: 4358.801 MiB/s
0 Method: DUMB Elapsed: 0.16298 MiB: 1024.00000 Copy: 6282.864 MiB/s
0 Method: MCBLOCK Elapsed: 0.08543 MiB: 1024.00000 Copy: 11986.562 MiB/s

I was curious why the MCBLOCK numbers were so much higher than the others, and this led me to the relevant piece of code:

                for(t=0; t < array_bytes; t+=block_size) {
                if(t > array_bytes){

Which appears to be completely broken.  The mempcpy(b, a, block_size) call does exactly same thing every time.  All I can figure is that the author either was trying to demonstrate cache performance, or fumbled an attempt to do the memcpy() calls on blocks of block_size each.

Anyway, it's a good reminder of Caveat Emptor when using random benchmarks.

1 comment:

Alexandre Magno said...

What means the first test?
What means the second test?
What means the third test?