Date: Thu, 25 Aug 2005 20:12:08 +0200 From: Andre Oppermann <andre@freebsd.org> To: Robert Watson <rwatson@FreeBSD.org> Cc: "Justin R. Smith" <jsmith@drexel.edu>, freebsd-current@FreeBSD.org Subject: Re: Hyperthreading degrades performance? Message-ID: <430E09F8.8090001@freebsd.org> In-Reply-To: <20050825173440.O16967@fledge.watson.org> References: <430DF217.2020908@drexel.edu> <20050825173440.O16967@fledge.watson.org>
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Robert Watson wrote: > > On Thu, 25 Aug 2005, Justin R. Smith wrote: > >> This is in reply to the people who said this because of >> cache-contention. Has anyone benchmarked this? >> >> There's an article >> >> http://www.2cpu.com/articles/41_1.html >> >> that benchmarks hyperthreading in Linux and shows a modest (~29%) >> improvement in performance --- depending on applications (with java >> showing degradation of performance). Perhaps the linux sheduler does >> things differently... > > > In in the last couple of years, I've seen some changes in how we > interact with HTT. 2-3 years ago, when benchmarking MySQL with and > without HTT, I saw a 30%+ drop-off when HTT was enabled. Now, they come > out about the same. I previously also saw no improvement with > buildkernel, but recently I've seen credible reports of build > improvements when running with HTT. So I think that things have changed > a bit as a result of significant scheduler improvements in the last few > years, as well as reduced lock contention. A continued slight decrease > in performance for some benchmarks wouldn't surprise me, but seeing more > in the way of "break even" or even "improvement" strikes me as likely. > A thorough revisiting of the issue would be quite useful :-). Don't forget better PIV revisions with larger instruction decoder caches, better cache prefetching and branch prediction. I doubt much of the improvement is due to our SMP changes. A real test to find out whether it's our work or Intels would be to benchmark an old (pre Nacona) PIV running 5.3R and 7.0-current vs. a new one doing the same. -- Andre
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