On Fri, 12 Jan 2001, Tim Wright wrote:
> On Sat, Jan 13, 2001 at 12:30:46AM +1100, Andrew Morton wrote:
> > what worries me about this is the Apache-flock-serialisation saga.
> >
> > Back in -test8, kumon@fujitsu demonstrated that changing this:
> >
> > lock_kernel()
> > down(sem)
> > <stuff>
> > up(sem)
> > unlock_kernel()
> >
> > into this:
> >
> > down(sem)
> > <stuff>
> > up(sem)
> >
> > had the effect of *decreasing* Apache's maximum connection rate
> > on an 8-way from ~5,000 connections/sec to ~2,000 conn/sec.
> >
> > That's downright scary.
> >
> > Obviously, <stuff> was very quick, and the CPUs were passing through
> > this section at a great rate.
> >
> > How can we be sure that converting spinlocks to semaphores
> > won't do the same thing? Perhaps for workloads which we
> > aren't testing?
> >
> > So this needs to be done with caution.
> >
>
> Hmmm...
> if <stuff> is very quick, and is guaranteed not to sleep, then a semaphore
> is the wrong way to protect it. A spinlock is the correct choice. If it's
> always slow, and can sleep, then a semaphore makes more sense, although if
> it's highly contented, you're going to serialize and throughput will die.
> At that point, you need to redesign :-)
> If it's mostly quick but occasionally needs to sleep, I don't know what the
> correct idiom would be in Linux. DYNIX/ptx has the concept of atomically
> releasing a spinlock and going to sleep on a semaphore, and that would be
> the solution there e.g.
>
> p_lock(lock);
> retry:
> ...
> if (condition where we need to sleep) {
> p_sema_v_lock(sema, lock);
> /* we got woken up */
> p_lock(lock);
> goto retry;
> }
> ...
>
> I'm stating the obvious here, and re-iterating what you said, and that is that
> we need to carefully pick the correct primitive for the job. Unless there's
> something very unusual in the Linux implementation that I've missed, a
> spinlock is a "cheaper" method of protecting a short critical section, and
> should be chosen.
>
> I know the BKL is a semantically a little unusual (the automatic release on
> sleep stuff), but even so, isn't
>
> lock_kernel()
> down(sem)
> <stuff>
> up(sem)
> unlock_kernel()
>
> actually equivalent to
>
> lock_kernel()
> <stuff>
> unlock_kernel()
>
> If so, it's no great surprise that performance dropped given that we replaced
> a spinlock (albeit one guarding somewhat more than the critical section) with
> a semaphore.
>
> Tim
>
> --
> Tim Wright - timw@splhi.com or timw@aracnet.com or twright@us.ibm.com
> IBM Linux Technology Center, Beaverton, Oregon
> "Nobody ever said I was charming, they said "Rimmer, you're a git!"" RD VI
>
Nigel Gamble nigel@nrg.org
Mountain View, CA, USA. http://www.nrg.org/
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This archive was generated by hypermail 2b29 : Mon Jan 15 2001 - 21:00:35 EST