Date: Thu, 5 Sep 2002 22:14:43 +0200 From: Bernd Walter <ticso@cicely9.cicely.de> To: Andrew Gallatin <gallatin@cs.duke.edu> Cc: John Baldwin <jhb@FreeBSD.ORG>, freebsd-alpha@FreeBSD.ORG Subject: Re: ithread preemption Message-ID: <20020905201443.GD13050@cicely9.cicely.de> In-Reply-To: <15735.47204.905352.900631@grasshopper.cs.duke.edu> References: <15735.44660.835003.901974@grasshopper.cs.duke.edu> <XFMail.20020905153533.jhb@FreeBSD.org> <15735.47204.905352.900631@grasshopper.cs.duke.edu>
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On Thu, Sep 05, 2002 at 04:02:44PM -0400, Andrew Gallatin wrote: > > John Baldwin writes: > > > > On 05-Sep-2002 Andrew Gallatin wrote: > > > > > > John Baldwin writes: > > > > > > > > On 05-Sep-2002 Andrew Gallatin wrote: > > > > > > > > > > I've forgotten -- What are the symptoms of ithread preemption causing > > > > > troubles on alpha? > > > > > > > > Hangs on SMP under load. > > > > > > > > > I have one (probably dumb) idea: Is the ithread preemption code > > > > > guaranteed to switch back to the preempted thread when the ithread > > > > > completes or blocks? And continue through to the end of the interrupt > > > > > dispatch code, returning back to the palcode? > > > > > > > > It is not guaranteed to do that. > > > > > > What keeps you from (eventually) running out of kernel stack space > > > then, as the interrupts keep coming in? > > > > The thread that received the interrupt stays at the high IPL until it > > returns. When you switch to another thread you are on another stack > > and you can take an interrupt ok. When we switch back to an interrupted > > thread, it executes at the raised IPL until it returns back to the PAL > > code. > > OK, so the interrupted thread will (eventually) return back to PAL. OK - I have some basic understandig problems here. Why should ithreads ever return to PAL? Why is IPL raised while an ithread is running? From what I understood before the interrupt handler, which is called from PAL, just triggers the ithread, block the intline and returns. > But, theoritically, under heavy load we could have lots of threads > preempted. And lots of interrupts pending which never returned to > PAL. Are we certain that this doesn't somehow violate assumptions made > by pal? Does any other OS work like this? > > Perhaps we should only allow a small number of preemptions at any one > time. -- B.Walter COSMO-Project http://www.cosmo-project.de ticso@cicely.de Usergroup info@cosmo-project.de To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-alpha" in the body of the message
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