Re: lazy vxid locks, v1

On Sun, Jun 12, 2011 at 10:39 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> I hacked up the system to
> report how often each lwlock spinlock exceeded spins_per_delay.

I don't doubt the rest of your analysis but one thing to note, number
of spins on a spinlock is not the same as the amount of time spent
waiting for it.

When there's contention on a spinlock the actual test-and-set
instruction ends up taking a long time while cache lines are copied
around. In theory you could have processes spending an inordinate
amount of time waiting on a spinlock even though they never actually
hit spins_per_delay or you could have processes that quickly exceed
spins_per_delay.

I think in practice the results are the same because the code the
spinlocks protect is always short so it's hard to get the second case
on a multi-core box without actually having contention anyways.

--
greg