Pierre-Luc Drouin <pldro...@pldrouin.net> wrote: > > Hi, > > Could someone explain me in which cases it is useful to enable > hyperthreading on a machine running FreeBSD 8.0 and in which other cases > it is not a good idea? Is that possible that hyperthreading is > disadvantageous unless the number of active (non-sleeping) threads is > really high? > > For example, if I have an i7 CPU with 4 physical cores and that I run > some multi-threaded code that has only 4 threads, it will run almost > always (twice) slower with hyperthreading enabled than when I disable it > in the BIOS. If I understand correctly, hyperthreading has the advantage > of being able to do CPU context switching faster than the OS, but it > does this context switching systematically instead of only when > requested, so it slows things down unless the number of running > (non-sleeping) threads is greater or equal to let say the number of > physical threads x 1.5-1.75.
I can't speak to the technical explanation, but I can give you my real- world experience. We asked this same question where I worked and had the time and ability to test it. What we found: * With hyperthreading on, workstations were more responsive to concurrent tasks. They weren't particularly faster at executing, but there were less incidents of a background task causing the UI to stall or stutter. * pgbench showed anywhere from 0% - 15% increased throughput. Kind of pathetic, but we never saw a workload on PostgreSQL that was hurt by turning hyperthreading on. -- Bill Moran http://www.potentialtech.com _______________________________________________ email@example.com mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to "freebsd-questions-unsubscr...@freebsd.org"