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
