Date: Thu, 10 Jan 2002 14:20:45 -0800 (PST) From: Kelly Yancey <kbyanc@posi.net> To: Alfred Perlstein <bright@mu.org> Cc: 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: <Pine.BSF.4.21.0201101401420.6961-100000@gateway.posi.net> In-Reply-To: <20020110135324.N7984@elvis.mu.org>
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On Thu, 10 Jan 2002, Alfred Perlstein wrote: > * Nate Williams <nate@yogotech.com> [020110 13:52] wrote: > > > 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? 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. :/ It seems to me that if {get,set}context involve kernel calls, then any userland scheduler would, by definition, require N+1 context switches where N is the number of context switches required by a kernel-only scheduler. The extra 1 coming from invoking the scheduler context itself. The flexibility argument I can buy, given as a trade-off of performance versus flexibility. However, I do not see where having a mixed kernel/userland scheduler can be both more flexible and more efficient in the case that it requires a syscall to switch userland contexts. And it doesn't seem proper to keep the flexibility argument if {get,set}context only work with our specific implementation of libc_r (as Peter has already noted). Of course, all this moot if userland context switches can be done properly without entering the kernel and in a way the preserves flexibility. As an aside, I cannot find a reference now, but hasn't it been rumoured that Solaris has dropped it's userland scheduler? Kelly To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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