Nigel Gamble wrote:
>
> On 29 Jan 2002, Robert Love wrote:
> > On Tue, 2002-01-29 at 20:26, Andrew Morton wrote:
> > > Just a little word of caution here. Remember the
> > > apache-flock-synchronisation fiasco, where removal
> > > of the BKL halved Apache throughput on 8-way x86.
> > >
> > > This was because the BKL removal turned serialisation
> > > on a quick codepath from a spinlock into a schedule().
>
> Yes, but the other factor to consider here is why did the extra schedule
> take place at all? I think this is a actually a scheduler issue, and
> I'm hoping that the new scheduler will behave better in this case. A
> call to schedule() should not happen unless the woken process has a
> higher priority than the process that did the unlock, but the old
> scheduler evidently always calculated this to be the case. But we
> really want the process that did the unlock to continue running (until
> the end of its timeslice, if not preempted or blocked before then), just
> as it would when the lock was a spinlock. It would be interesting to
> see whether the new scheduler gets this right.
>
> Am I remembering the problem correctly?
>
I don't think so :)
The problem was that the semaphore was highly contended, so the
losing process was explicitly scheduling away.
This doesn't necessarily mean that it was a long-held lock. In
this case, it was a short-held lock, but it was also very *frequently*
being held and released. This is a scenario where a spinlock is
heaps more appropriate than a semaphore.
I don't think we need any locking at all in the default lseek()
path, btw. Apart from the non-atomic i_size thing, which is
only an issue for 32-bit machines.
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This archive was generated by hypermail 2b29 : Thu Jan 31 2002 - 21:01:12 EST