Date: Tue, 2 Oct 2007 20:22:42 -0700 From: Alfred Perlstein <alfred@freebsd.org> To: Daniel Eischen <deischen@freebsd.org> Cc: hackers@freebsd.org Subject: Re: Critical Sections for userland. Message-ID: <20071003032242.GR31826@elvis.mu.org> In-Reply-To: <Pine.GSO.4.64.0710022311450.626@sea.ntplx.net> References: <20071003015231.GJ31826@elvis.mu.org> <Pine.GSO.4.64.0710022244250.626@sea.ntplx.net> <20071003025418.GN31826@elvis.mu.org> <Pine.GSO.4.64.0710022257340.626@sea.ntplx.net> <20071003030943.GQ31826@elvis.mu.org> <Pine.GSO.4.64.0710022311450.626@sea.ntplx.net>
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* Daniel Eischen <deischen@freebsd.org> [071002 20:16] wrote: > On Tue, 2 Oct 2007, Alfred Perlstein wrote: > > >* Daniel Eischen <deischen@freebsd.org> [071002 20:02] wrote: > >>On Tue, 2 Oct 2007, Alfred Perlstein wrote: > >> > >>>* Daniel Eischen <deischen@freebsd.org> [071002 19:46] wrote: > >>>>On Tue, 2 Oct 2007, Alfred Perlstein wrote: > >>>> > >>>>>Hi guys, we need critical sections for userland here. > >>>>> > >>>>>This is basically to avoid a process being switched out while holding > >>>>>a user level spinlock. > >>>> > >>>>Setting the scheduling class to real-time and using SCHED_FIFO > >>>>and adjusting the thread priority around the lock doesn't work? > >>> > >>>Too heavy weight, we want to basically have this sort of code > >>>in userland: > >> > >>Well, yeah, but are you _really_ sure that you aren't just > >>running something that should be real-time and have priority > >>over other applications? SCHED_FIFO means you will run until > >>you relinquish the CPU (you can only do this as root). If > >>all your threads are well behaved, would this work? Have > >>you tried it? > > > >No, because it wouldn't work. How do we know when to let go > >of the cpu? In my system, the kernel tells you without polling. > > You don't have to know when to "let go of the cpu" if your > threads are well behaved (meaning they block on some event, > or have periods when they wait). They will let go of the > CPU normally. When they're busy, they will not be switched > out (unless perhaps there is an interrupt thread that needs > to run -- I'm not sure how real-time threads get scheduled > against ithreads). > > If your threads are not well behaved (CPU hogs), then that > isn't going to work because they'll probably bog down the > system. Yes, I know how threading works. Unfortunately, your solution is not workable for us. thank you, -- - Alfred Perlstein
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