Re: SAVEPOINTs and COMMIT performance

On Sun, 2011-02-06 at 12:11 -0500, Bruce Momjian wrote:
> Did this ever get addressed?

Patch attached.

Seems like the easiest fix I can come up with.

> Simon Riggs wrote:
> >
> > As part of a performance investigation for a customer I've noticed an
> > O(N^2) performance issue on COMMITs of transactions that contain many
> > SAVEPOINTs. I've consistently measured COMMIT times of around 9 seconds,
> > with 49% CPU, mostly in LockReassignCurrentOwner().
> >
> > BEGIN;
> > INSERT...
> > SAVEPOINT ...
> > INSERT...
> > SAVEPOINT ...
> > ... (repeat 10,000 times)
> > COMMIT;
> >
> > The way SAVEPOINTs work is that each is nested within the previous one,
> > so that at COMMIT time we must recursively commit all the
> > subtransactions before we issue final commit.
> >
> > That's a shame because ResourceOwnerReleaseInternal() contains an
> > optimisation to speed up final commit, by calling ProcReleaseLocks().
> >
> > What we actually do is recursively call LockReassignCurrentOwner() which
> > sequentially scans LockMethodLocalHash at each level of transaction. The
> > comments refer to this as "retail" rather than the wholesale method,
> > which never gets to execute anything worthwhile in this case.
> >
> > This issue does NOT occur in PLpgSQL functions that contain many
> > EXCEPTION clauses in a loop, since in that case the subtransactions are
> > started and committed from the top level so that the subxact nesting
> > never goes too deep.
> >
> > Fix looks like we need special handling for the depth-first case, rather
> > than just a recursion loop in CommitTransactionCommand().
> >
> > Issues looks like it goes all the way back, no fix for 9.0.
> >
> > I notice also that the nesting model of SAVEPOINTs also means that
> > read-only subtransactions will still generate an xid when followed by a
> > DML statement. That's unnecessary, but required given current design.
> >
> > --
> > Simon Riggs www.2ndQuadrant.com
> > PostgreSQL Development, 24x7 Support, Training and Services
> >
> >
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--
Simon Riggs http://www.2ndQuadrant.com/books/
PostgreSQL Development, 24x7 Support, Training and Services