Date: Tue, 02 Nov 2004 10:12:05 -0500 From: Stephan Uphoff <ups@tree.com> To: Julian Elischer <julian@elischer.org> Cc: FreeBSD Current <freebsd-current@freebsd.org> Subject: Re: wakeup/sleep handoff. Message-ID: <1099408325.88989.6.camel@palm.tree.com> In-Reply-To: <4176C94E.3000700@elischer.org> References: <41759681.1060700@elischer.org> <4176C94E.3000700@elischer.org>
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On Wed, 2004-10-20 at 16:23, Julian Elischer wrote: > Stephan Uphoff wrote: > > >On Tue, 2004-10-19 at 18:34, Julian Elischer wrote: > > > > > >>Is there a need to be able to somehow implement a 'wakeup_one()' that > >>as part of its semantic is that the woken thread will run immediatly, > >>(as in preemprion), > >>and the old thread will sleep? With preemption, the old thread is left > >>in the run queue, > >>and after the other thread has completed, it will > >>run again and probably go away and sleep for some reason.. (or at least > >>go do some work that isn't > >>necessarily required..) > >> > >>Something like handover(wakeupchan, sleepchan, msleep_args...). > >> sort of an atomic wakeup/msleep. > >> > >>This would be used in places where work used to be done by the same > >>thread, but is now done > >>by a server thread.. > >> > >>An example would be kicking off a geom thread, when in the past we would > >>have gone all > >>the way down to the hardware ourself. we want to get as close to acting > >>like we are still > >>going all the way done as we can (performance wise). We may get some > >>efficiency by > >>letting the sleep system, and scheduler know what we are trying to do. > >>Possibly with some > >>priority inherritance implications.. (if we have a high priority, we > >>probably want to ensure that the > >>worker thread is run with at least that priority.) > >> > >> > > > >Why not just give the geom thread a high priority? > >This, full preemption and changing a few functions to guaranty that the > >highest priority thread will always run should do what you want. > >( And maybe always raising the priority of threads working in the > >kernel) > >Actually this is relatively high on my to do list and I should have some > >patches to try out in a week or two. > > > > yessss but after the preemption (which is invisible to the caller of > setrunqueue/wakeup) > that thread continues on to do it's "check for completion/sleep".. > > it would be more efficient in my book to have an official way to hand > over to a designated worker > all in one hit.. You could then optimise such cases.. They are often in > required fast-paths. OK - I finally got it. Maybe sections that temporarily disable preemption would do the trick. Spinning on an adaptive mutex or blocking/sleeping should automatically re-enable preemption. On a related topic: I don't like the way condition variables and msleep wait threads will be scheduled on a wakeup - just to block again on trying to acquire a mutex. However I don't see any way to avoid this that does not involve a lot of work. Any idea beside not protecting the wakeup by a mutex? Stephan
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