Hi,

On 05/04/2016 12:42 AM, Andres Freund wrote:
On 2016-05-03 20:57:13 +0200, Tomas Vondra wrote:
On 05/03/2016 07:41 PM, Andres Freund wrote:
...
I'm pretty sure that I said that somewhere else at least once: But to
be absolutely clear, I'm *not* really concerned with the performance
with the feature turned off. I'm concerned about the performance with
it turned on.


If you tell me how to best test it, I do have a 4-socket server sitting idly
in the corner (well, a corner reachable by SSH). I can get us some numbers,
but I haven't been following the snapshot_too_old so I'll need some guidance
on what to test.

I think it'd be cool if you could test the effect of the feature in
read-only (and additionally read-mostly?) workload with various client
counts and snapshot_too_old values. For the latter maybe -1, 0, 10, 60
or such?  I've done so (accidentally comparing 0 and 1 instead of -1 and
1) on a two socket machine in:
www.postgresql.org/message-id/20160413171955.i53me46fqqhdl...@alap3.anarazel.de

It'd be very interesting to see how big the penalty is on a bigger box.

OK. I do have results from mater with different values for the GUC (-1, 0, 10, 60), but I'm struggling with the reverts. Can you provide a patch against current master (commit 4bbc1a7e) that does the revert?

regards

--
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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