Re: Locking B-tree leafs immediately in exclusive mode

On Mon, Jul 9, 2018 at 9:43 AM, Alexander Korotkov
<a(dot)korotkov(at)postgrespro(dot)ru> wrote:
> In this case it also looks like we observed 1% regression. Despite 1%
> may seem to be very small, I think we should clarify whether it really
> exists. I have at least two hypothesis about this.
>
> 1) There is no real regression, observed difference of TPS is less
> than error of measurements. In order to check that we need to retry
> the experiment multiple times. Also, if you run benchmark on master
> before patched version (or vice versa) you should also try to swap the
> order to make sure there is no influence of the order of benchmarks.
> 2) If we consider relation between TPS and number of clients, TPS is
> typically growing with increasing number of clients until reach some
> saturation value. After the saturation value, there is some
> degradation of TPS. If patch makes some latency lower, that my cause
> saturation to happen earlier. In order to check that, we need run
> benchmarks with various number of clients and draw a graph: TPS
> depending on clients.
>
> So, may I ask you to make more experiments in order to clarify the
> observed regression?

It would be nice to actually see script_duplicated.sql. I don't know
exactly what the test case was.

Here is my wild guess: You may end up moving right more often within
_bt_findinsertloc(), which is actually worse than moving right within
_bt_moveright(), even when you _bt_moveright() in exclusive mode.
_bt_findinsertloc() couples/crabs exclusive buffer locks because the
unique case requires it, even when we're not inserting into a unique
index. Whereas _bt_moveright() holds at most one buffer lock at a
time.

--
Peter Geoghegan