On Tue, Mar 06, 2001 at 10:12:17PM -0500, Jeff Dike wrote:
> [EMAIL PROTECTED] said:
> > If you're a UP system, it never makes sense to spin in userland, since
> > you'll just burn up a timeslice and prevent the lock holder from
> > running. I haven't looked, but assume that their code only uses
> > spinlocks on SMP. If you're an SMP system, then you shouldn't be using
> > a spinlock unless the critical section is "short", in which case the
> > waiters should simply spin in userland rather than making system calls
> > which is simply overhead.
>
> This is a problem that UML is going to have when I turn SMP back on.
> Emulating a multiprocessor box on a UP host with the existing locking
> primitives is going to result in exactly this problem.
>
Yes. On a uniprocessor system, a simple fallback is to just use a semaphore
instead of a spinlock, since you can guarantee that there's no point in
scheduling the current task until the holder of the "lock" releases it.
Otherwise, the spin calling sched_yield() each iteration isn't too horrible.
> > Actually, what's really needed here is an efficient form of
> > dynamically marking a process as non-preemptible so that when
> > acquiring a spinlock the process can ensure that it exits the critical
> > section as fast as possible, when it would relinquish its
> > non-preemptible privilege.
>
> That sounds like a pretty fundamental (and abusable) mechanism.
>
It would be if it were generally available. The implementation on DYNIX/ptx
requires a privilege (PRIV_SCHED IIRC), to be able to use it. It was added
for a database to prevent preemption during critical sections.
> I had a suggestion from an IBM guy at ALS last year to make UML "spin"-locks
> actually sleep in the host (this doesn't make them sleep locks in userspace
> because they don't call schedule), which sounds reasonable. This gives the
> lock-holder an opportunity to run immediately. It's unclear to me what the
> wake-up mechanism would be, though.
>
Hmmm.. depends what you mean by sleep i.e sleep(3) vs. making a system call
that sleeps. I would have thought the latter, and use semaphores again.
> Another thought I had was to raise the priority of a thread holding a
> spinlock. This would reduce the chance that it would be preempted by a thread
> that will waste a timeslice spinning on that lock. I don't know whether this
> is a good idea either.
>
That's basically a weaker version of the no-preempt. Not a bad idea, but
less than optimal :-)
Regards,
Tim
--
Tim Wright - [EMAIL PROTECTED] or [EMAIL PROTECTED] or [EMAIL PROTECTED]
IBM Linux Technology Center, Beaverton, Oregon
Interested in Linux scalability ? Look at http://lse.sourceforge.net/
"Nobody ever said I was charming, they said "Rimmer, you're a git!"" RD VI
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