On Thu, 4 Jun 2009, Robert Haas wrote:
On Wed, Jun 3, 2009 at 5:09 PM, Scott Carey <scott@xxxxxxxxxxxxxxxxx> wrote:
On 6/3/09 11:39 AM, "Robert Haas" <robertmhaas@xxxxxxxxx> wrote:
On Wed, Jun 3, 2009 at 2:12 PM, Scott Carey <scott@xxxxxxxxxxxxxxxxx> wrote:
Postgres could fix its connection scalability issues -- that is entirely
independent of connection pooling.
I think I see the distinction you're drawing here. IIUC, you're
arguing that other database products use connection pooling to handle
rapid connect/disconnect cycles and to throttle the number of
simultaneous queries, but not to cope with the possibility of large
numbers of idle sessions. My limited understanding of why PostgreSQL
has a problem in this area is that it has to do with the size of the
process array which must be scanned to derive an MVCC snapshot. I'd
be curious to know if anyone thinks that's correct, or not.
Assuming for the moment that it's correct, databases that don't use
MVCC won't have this problem, but they give up a significant amount of
scalability in other areas due to increased blocking (in particular,
writers will block readers). So how do other databases that *do* use
MVCC mitigate this problem? The only one that we've discussed here is
Oracle, which seems to get around the problem by having a built-in
connection pooler. That gets me back to thinking that the two issues
are related, unless there's some other technique for dealing with the
need to derive snapshots.
if this is the case, how hard would it be to have threads add and remove
themselves from some list as they get busy/become idle?
I've been puzzled as I've been watching this conversation on what internal
locking/lookup is happening that is causing the problems with idle threads
(the pure memory overhead isn't enough to account for the problems that
are being reported)
David Lang
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