Some interesting results from tweaking spinlocks

I have been experimenting with altering the SPINS_PER_DELAY number in
src/backend/storage/lmgr/s_lock.c.  My results suggest that the current
setting of 100 may be too small.

The attached graph shows pgbench results on the same 4-way Linux box
I described in my last message.  (The numbers are not exactly comparable
to the previous graph, because I recompiled with --enable-cassert off
for this set of runs.)  All runs use current CVS plus the second LWLock
patch under discussion.

Evidently, on this hardware and test case the optimal SPINS_PER_DELAY
value is somewhere in the low thousands, not 100.  I find this rather
surprising given that spinlocks are never held for more than a few
dozen instructions, but the results seem quite stable.

On the other hand, increasing SPINS_PER_DELAY could hardly fail to be
a loser on a single-CPU machine.

Would it be worth making this value a GUC parameter, so that it could
be tuned conveniently on a per-installation basis?

            regards, tom lane