Date: Thu, 13 Jul 2000 09:22:22 -0700 (PDT) From: "Rodney W. Grimes" <freebsd@gndrsh.dnsmgr.net> To: bandix@looksharp.net (Brandon D. Valentine) Cc: mckay@thehub.com.au (Stephen McKay), se@FreeBSD.ORG (Stefan Esser), wpaul@FreeBSD.ORG (Bill Paul), freebsd-current@FreeBSD.ORG Subject: Re: dc driver and underruns (was: Strangeness with 4.0-S) Message-ID: <200007131622.JAA12738@gndrsh.dnsmgr.net> In-Reply-To: <Pine.BSF.4.21.0007131131141.26547-100000@turtle.looksharp.net> from "Brandon D. Valentine" at "Jul 13, 2000 11:34:14 am"
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> On Thu, 13 Jul 2000, Stephen McKay wrote: > > >>Guess it will show up if you measure latencies (or your application is > >>doing lots of RPCs). But as soon as there is a cheap 100baseT switch in > >>the path to the destination, there will be store-and-forward at work ;-) > > > >Does anyone here actually measure these latencies? I know for a fact > >that nothing I've ever done would or could be affected by extra latencies > >that are as small as the ones we are discussing. Does anybody at all > >depend on the start-transmitting-before-DMA-completed feature we are > >discussing? > > I don't like the idea of removing that feature. Perhaps it should be a > sysctl or ifconfig option, but it should definitely remain available. > Those minute latencies are critical to those of us who use MPI for > complex parallel calculations. I have to agree here. The store and forward adds an approximate 11uS (by theory under ideal conditions 1500bytes@132MB/s = 11uS, practice actually makes this worse as typical PCI does something less than 100MB/s or 15uS) to a 120uS packet time on the wire (again, ideal, but here given that switches, and infact often cut-through switches, are used for these types of things, ideal and practice are very close.) I don't think these folks, nor myself, are wanting^H^H^H^H^H^H^Hilling to give up 12.5%. -- Rod Grimes - KD7CAX @ CN85sl - (RWG25) rgrimes@gndrsh.dnsmgr.net To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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