Re: Overhead cost of Serializable Snapshot Isolation

On Mon, Oct 10, 2011 at 02:59:04PM -0500, Kevin Grittner wrote:
> I do have some concern about whether the performance improvements
> from reduced LW locking contention elsewhere in the code may (in
> whack-a-mole fashion) cause the percentages to go higher in SSI.
> The biggest performance issues in some of the SSI benchmarks were on
> LW lock contention, so those may become more noticeable as other
> contention is reduced. I've been trying to follow along on the
> threads regarding Robert's work in that area, with hopes of applying
> some of the same techniques to SSI, but it's not clear whether I'll
> have time to work on that for the 9.2 release. (It's actually
> looking improbably at this point.)

I spent some time thinking about this a while back, but didn't have
time to get very far. The problem isn't contention in the predicate
lock manager (which is partitioned) but the single lock protecting the
active SerializableXact state.

It would probably help things a great deal if we could make that lock
more fine-grained. However, it's tricky to do this without deadlocking
because the serialization failure checks need to examine a node's
neighbors in the dependency graph.

Dan

--
Dan R. K. Ports MIT CSAIL http://drkp.net/