Date: Sat, 25 Oct 2003 11:25:11 -0700 From: Terry Lambert <tlambert2@mindspring.com> To: Ted Unangst <tedu@stanford.edu> Cc: Michel TALON <talon@lpthe.jussieu.fr> Subject: Re: Some mmap observations compared to Linux 2.6/OpenBSD Message-ID: <3F9AC007.DB48DE9D@mindspring.com> References: <Pine.GSO.4.44.0310241406390.6538-100000@saga0.Stanford.EDU>
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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
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