Re: B-tree parent pointer and checkpoints

On 08.11.2010 15:40, Heikki Linnakangas wrote:
> Here's a first draft of this, using the inCommit flag as is. It works,
> but suffers from starvation if you have a lot of concurrent
> multi-WAL-record actions. I tested that by running INSERTs to a table
> with tsvector field with a GiST index on it from five concurrent
> sessions, and saw checkpoints regularly busy-waiting for over a minute.
>
> To avoid that, we need something a little bit more complicated than a
> boolean flag. I'm thinking of adding a counter beside the inCommit flag
> that's incremented every time a new multi-WAL-record action begins, so
> that the checkpoint process can distinguish between a new action that
> was started after deciding the REDO pointer and an old one that's still
> running.
>
> (inCommit is a misnomer now, of course. Will need to find a better name..)

Here's a 2nd version, with an additional counter in PGPROC to avoid
starving checkpoint in the face of a constant stream e.g GiST inserts.

The new rule is that before you start a multi-WAL-record operation that
needs to be completed at end of recovery if you crash in the middle, you
call HoldCheckpoint(), and once you're finished, ResumeCheckpoint().
rm_safe_restartpoint() is gone.

This is a pre-existing bug, but given the lack of field reports and the
fact that it's pretty darn hard to run into this in real life, I'm
inclined to not backpatch this.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com