Re: ALTER TABLE lock strength reduction patch is unsafe

On Mon, Dec 19, 2011 at 12:05:09PM -0500, Robert Haas wrote:
> Yet another option, which I wonder whether we're dismissing too
> lightly, is to just call GetSnapshotData() and do the scan using a
> plain old MVCC snapshot.  Sure, it's more expensive than SnapshotNow,
> but is it expensive enough to worry about?

Good point.  For the most part, we already regard a catalog scan as too
expensive for bulk use, hence relcache and catcache.  That's not license to
slow them down recklessly, but it's worth discovering how much of a hit we'd
actually face.  I created a function that does this in a loop:
    HeapTuple t;
    CatalogCacheFlushCatalog(ProcedureRelationId);    t = SearchSysCache1(PROCOID, ObjectIdGetDatum(42) /* int4in */);
 if (!HeapTupleIsValid(t))        elog(ERROR, "cache lookup failed for function 42");    ReleaseSysCache(t);
 

Then, I had pgbench call the function once per client with various numbers of
clients and a loop iteration count such that the total number of scans per run
was always 19200000.  Results for master and for a copy patched to use MVCC
snapshots in catcache.c only:
2 clients, master: 4:30.66elapsed4 clients, master: 4:26.82elapsed
32 clients, master: 4:25.30elapsed2 clients, master: 4:25.67elapsed4 clients, master: 4:26.58elapsed
32 clients, master: 4:26.40elapsed2 clients, master: 4:27.54elapsed4 clients, master: 4:26.60elapsed
32 clients, master: 4:27.20elapsed2 clients, mvcc-catcache: 4:35.13elapsed4 clients, mvcc-catcache: 4:30.40elapsed
32 clients, mvcc-catcache: 4:37.91elapsed2 clients, mvcc-catcache: 4:28.13elapsed4 clients, mvcc-catcache:
4:27.06elapsed
32 clients, mvcc-catcache: 4:32.84elapsed2 clients, mvcc-catcache: 4:32.47elapsed4 clients, mvcc-catcache:
4:24.35elapsed
32 clients, mvcc-catcache: 4:31.54elapsed

I see roughly a 2% performance regression.  However, I'd expect any bulk
losses to come from increased LWLock contention, which just doesn't
materialize in a big way on this 2-core box.  If anyone would like to rerun
this on a larger machine, I can package it up for reuse.