On 17/12/15 19:07, Robert Haas wrote:
On Mon, Dec 14, 2015 at 6:08 PM, Tomas Vondra
<tomas.von...@2ndquadrant.com> wrote:
So we know that we should expect about
(prev_wal_bytes - wal_bytes) + (prev_wal_fpw_bytes - wal_fpw_bytes)
( regular WAL ) + ( FPW WAL )
to be produced until the end of the current checkpoint. I don't have a clear
idea how to transform this into the 'progress' yet, but I'm pretty sure
tracking the two types of WAL is a key to a better solution. The x^1.5 is
probably a step in the right direction, but I don't feel particularly
confident about the 1.5 (which is rather arbitrary).
If it works well empirically, does it really matter that it's
arbitrary? I mean, the entire planner is full of fairly arbitrary
assumptions about which things to consider in the cost model and which
to ignore. The proof that we have made good decisions there is in the
query plans it generates. (The proof that we have made bad decisions
in some cases in the query plans, too.)
Agreed.
I think a bigger problem for this patch is that Heikki seems to have
almost completely disappeared.
Yeah, there's that problem too :-).
The reason I didn't commit this back then was lack of performance
testing. I'm fairly confident that this would be a significant
improvement for some workloads, and shouldn't hurt much even in the
worst case. But I did only a little testing on my laptop. I think Simon
was in favor of just committing it immediately, and Fabien wanted to see
more performance testing before committing.
I was hoping that Digoal would re-ran his original test case, and report
back on whether it helps. Fabien had a performance test setup, for
testing another patch, but he didn't want to run it to test this patch.
Amit did some testing, but didn't see a difference. We can take that as
a positive sign - no regression - or as a negative sign, but I think
that basically means that his test was just not sensitive to the FPW issue.
So Tomas, if you're willing to do some testing on this, that would be
brilliant!
- Heikki
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