On 08/21/2014 02:11 PM, Bruce Momjian wrote: > On Thu, Aug 21, 2014 at 02:02:26PM -0700, Josh Berkus wrote: >> On 08/20/2014 07:40 PM, Bruce Momjian wrote: >>> On Wed, Aug 20, 2014 at 12:13:50PM -0700, Josh Berkus wrote: >>>> On a read-write test, it's 10% faster with HT off as well. >>>> >>>> Further, from their production machine we've seen that having HT on >>>> causes the machine to slow down by 5X whenever you get more than 40 >>>> cores (as in 100% of real cores or 50% of HT cores) worth of activity. >>>> >>>> So we're definitely back to "If you're using PostgreSQL, turn off >>>> Hyperthreading". >>> >>> Not sure how you can make such a blanket statement when so many people >>> have tested and shown the benefits of hyper-threading. >> >> Actually, I don't know that anyone has posted the benefits of HT. Link? >> I want to compare results so that we can figure out what's different >> between my case and theirs. Also, it makes a big difference if there is >> an advantage to turning HT on for some workloads. > > I had Greg Smith test my system when it was installed, tested it, and > recommended hyper-threading. The system is Debian Squeeze > (2.6.32-5-amd64), CPUs are dual Xeon E5620, 8 cores, 16 virtual cores. Can you post some numerical results? I'm serious. It's obviously easier for our users if we can blanket recommend turning HT off; that's a LOT easier for them than "you might want to turn HT off if these conditions ...". So I want to establish that HT is a benefit sometimes if it is. I personally have never seen HT be a benefit. I've seen it be harmless (most of the time) but never beneficial. -- Josh Berkus PostgreSQL Experts Inc. http://pgexperts.com -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
