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Date: Tue, 2 Oct 2007 20:09:43 -0700
From: Alfred Perlstein <alfred@freebsd.org>
To: Daniel Eischen <deischen@freebsd.org>
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Cc: hackers@freebsd.org
Subject: Re: Critical Sections for userland.
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* 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.

> 
> Are you trying to prevent switching out of the thread
> amongst other threads of the same application, or all
> threads in the system?

All threads on that CPU.

It's basically, almost like a "soft spl" for userland.

Right?

-- 
- Alfred Perlstein