Re: Rewrite sinval messaging to reduce contention

I wrote:
> ... enabling LWLOCK_STATS shows that the contention
> rate on the sinval locks is now completely negligible --- one block
> per thousand acquisitions on SInvalWriteLock, and less than one in
> 10000 on SInvalReadLock.  The vast majority of the LWLock contention
> now comes from WALInsertLock and the LockMgr locks:

> 	Lock			# acquisitions	# times blocked

> 	SInvalReadLock		6469840		380
> 	SInvalWriteLock		240567		163
> 	WALInsertLock		2388805		89142
> 	LockMgr partition locks	8253142		177715

For comparison's sake I rebuilt CVS HEAD with LWLOCK_STATS enabled
and repeated the same test.  I got

	SInvalLock		81090044	505750
	WALInsertLock		2382254		62747
	LockMgr locks		10657480	171799

The change in sinval numbers is gratifying, but for awhile I didn't
believe these results because of the discrepancy in LockMgr acquisition
figures.  I think though that what we are seeing here is that CVS HEAD
has the reset-everyone-on-sinval-queue-overflow behavior, which results
in a whole lot of useless cache resets, which results in a lot of
unnecessary cache reloads, and every one of those requires taking
AccessShareLock on one or more system catalogs in order to suck the data
back in.  So the reduction in LockMgr traffic is explained by not doing
so many cache resets.

			regards, tom lane

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