Some mmap observations compared to Linux 2.6/OpenBSD

[Terry Lambert]

Ted Unangst wrote:
> On Fri, 24 Oct 2003, Michel TALON wrote:
> > What is more interesting is to look at the actual benchmark results in
> > http://bulk.fefe.de/scalability/
> > in particular the section about mmap benchmarks, the only one where
> > OpenBSD shines. However as soon as touching pages is benchmarked
> > OpenBSD fails very much.
> 
> look closer.  openbsd's "touch page" times are identical to what you'd
> expect a disk access to be.  the pages aren't cached, they're read from
> disk.  so compared to systems that don't read from disk, it looks pretty
> bad.  a 5 line patch to fix the benchmark so that the file actually is
> cached on openbsd results in performance much in line with freebsd/linux.

Why does the benchmark need to be "fixed" for OpenBSD and not
for any other platform?

My point here is that a benchmark measures what it measures, and
if you don't like what it measures, making it measure something
else is not a fix for the problem that it was originally intended
to show.

Microbenchmarks are pretty dumb, in general, because they are not
representative of real world performance on a given fixed load,
and are totally useless for predicting performance under a mixed
load.

That said, if this microbenchmark bothers you, fix OpenBSD.

I know that Linux has some very good scores on LMBench, and that
optimiziing the code to produce good numbers on that test set has
pessimized performance in a number of areas under real world
conditions.

Unless there's an obvious win to be had without additional cost,
it's best to take the numbers with a grain of salt.

THAT said, it's probably a good idea for the other BSD's to use
the read/black code from OpenBSD as a guid for their own code.

-- Terry