Re: advance local xmin more aggressively

On Wed, Dec 10, 2014 at 9:49 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> I guess this bears some further thought. I certainly don't like the
> fact that this makes the whole system crap out at a lower number of
> subtransactions than presently. The actual performance numbers don't
> bother me very much; I'm comfortable with the possibility that closing
> a cursor will be some modest percentage slower if you've got thousands
> of active savepoints.

Here's a new version with two changes:

1. I changed the traversal of the resource owner tree to iterate
instead of recurse. It now does a depth-first, pre-order traversal of
the tree; when we reach the last child of a node, we follow its parent
pointer to get back to where we were. That way, we don't need to keep
anything on the stack. That fixed the crash at 100k cursors, but it
was still 4x slower.

2. Instead of traversing the tree until we find an xmin equal to the
one we're currently advertising, the code now traverses the entire
tree each time it runs. However, it also keeps a record of how many
times the oldest xmin occurred in the tree, which is decremented each
time we unregister a snapshot with that xmin; the traversal doesn't
run again until that count reaches 0. That fixed the performance
regression on your test case.

With a million subtransactions:

master 34.464s 33.742s 34.317s
advance-xmin 34.516s 34.069s 34.196s

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company