Date: Thu, 10 Jan 2002 15:36:01 -0700 From: Nate Williams <nate@yogotech.com> To: Kelly Yancey <kbyanc@posi.net> Cc: Alfred Perlstein <bright@mu.org>, Nate Williams <nate@yogotech.com>, Terry Lambert <tlambert2@mindspring.com>, Daniel Eischen <eischen@pcnet1.pcnet.com>, Dan Eischen <eischen@vigrid.com>, Peter Wemm <peter@wemm.org>, Archie Cobbs <archie@dellroad.org>, arch@FreeBSD.ORG Subject: Re: Request for review: getcontext, setcontext, etc Message-ID: <15422.5969.820708.433083@caddis.yogotech.com> In-Reply-To: <Pine.BSF.4.21.0201101401420.6961-100000@gateway.posi.net> References: <20020110135324.N7984@elvis.mu.org> <Pine.BSF.4.21.0201101401420.6961-100000@gateway.posi.net>
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> > > > I mean if we've got to go to the kernel to switch thread contexts, why > > > > not just have the kernel track all of the threads and restore context > > > > once, just for the current thread, rather than twice (once for the > > > > scheduler and another for the scheduler to switch to the current > > > > thread context)? > > > > > > For effeciency reasons... > > > > And flexibility as well, but I guess that can be lumped under > > effeciency. > > > > If the context switch overhead is the same (or worse) with a userland > scheduler, then what are the "effeciency reasons" for having it? *IF* it requires a system call/kernel help, then we have no choice. However, the reason we're spending some much time determining if this necessary is because of effeciency/flexibility reasons. If we can do it in userland, then we should have the option of doing it there. > Where does the userland scheduler reclaim it's lost ground? The only > things my limited understanding can produce are a number of trivial > data structures that can be moved from the kernel to userland. :/ There is debugging and flexibility to be gained by having it in userland as well. You can change the way a particular process wants it's threads scheduled much easier in a userland scheduler by changing the algorithm, since it doesn't require a kernel recompile/reboot. Plus, each application may want a specific threading algorithm used. > Of course, all this moot if userland context switches can be done properly > without entering the kernel and in a way the preserves flexibility. Right. > As an aside, I cannot find a reference now, but hasn't it been > rumoured that > Solaris has dropped it's userland scheduler? I hadn't heard that. However, I had heard that they were providing an alternative thread library that uses a one-one mapping for user/kernel threads for certain applications. In this case, there would be no reason for a userland thread scheduler. I don't believe this is the only model they support though... Nate To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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