Date: Sat, 28 Jun 2003 02:42:11 -0700 From: David Schultz <das@FreeBSD.ORG> To: Brooks Davis <brooks@one-eyed-alien.net> Cc: current@FreeBSD.ORG Subject: Re: Hyperthreading Message-ID: <20030628094211.GA30553@HAL9000.homeunix.com> In-Reply-To: <20030627235347.GC7217@Odin.AC.HMC.Edu> References: <20030627181836.GA81335@node1.cluster.srrc.usda.gov> <20030627154252.X5016@carver.gumbysoft.com> <20030627233912.GA90348@gforce.johnson.home> <20030627235347.GC7217@Odin.AC.HMC.Edu>
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On Fri, Jun 27, 2003, Brooks Davis wrote: > On Fri, Jun 27, 2003 at 06:39:12PM -0500, Glenn Johnson wrote: > > Thanks. I had read the smp manual page. I know _how_ to enable HTT; I > > was wondering whether I _should_ enable it. It seems the answer is that > > it is not beneficial in its current state because the scheduler does not > > yet differentiate between physical and logical processors. > > It's more complicated then that. For many users, it's true that HTT is > not useful due to the scheduling issues, but for some applications where > you keep all the CPUs busy, it does help. Somewhat suprisingly, > SETI@Home performs better with HTT enabled then without. The individual > workunits take longer to process, but the overall throughput is better > (4 workunits every 6hrs instead of 2 workunits every 4hrs). Hyperthreading will generally give you better thoughput because you get better utilization of the hardware; when one functional unit would normally be idle due to a pipeline bubble, the other logical CPU may be able to provide work for it. On the other hand, as you observe, latency is worse. In particular, if you're running a web browser on one processor, it's competing for resources with your SETI@Home client on the other processor, even though the SETI@Home client is niced.
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