Re: [HACKERS] Re: pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

On 2016-04-13 13:52:15 -0500, Kevin Grittner wrote:
> On Wed, Apr 13, 2016 at 12:25 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> > [test results with old_snapshot_threshold = 0 and 10]
>
> From the docs:
>
> | A value of -1 disables this feature, and is the default.
>
> > Yuck. Aside from the fact that performance tanks when the feature is
> > turned on, it seems that there is a significant effect even with it
> > turned off.
>
> No evidence of that has been provided. -1 is off; 0 is for testing
> very fast expiration.

Longer tests are running, but, again on the previous hardware with only
two sockets, the results for 128 clients are:

0:
progress: 100.0 s, 593351.0 tps, lat 0.215 ms stddev 0.118
progress: 200.0 s, 594035.9 tps, lat 0.215 ms stddev 0.118
progress: 300.0 s, 594013.3 tps, lat 0.215 ms stddev 0.117

-1:
progress: 100.0 s, 600835.3 tps, lat 0.212 ms stddev 0.049
progress: 200.0 s, 601466.1 tps, lat 0.212 ms stddev 0.048
progress: 300.0 s, 601529.5 tps, lat 0.212 ms stddev 0.047

reverted:
progress: 100.0 s, 612676.6 tps, lat 0.208 ms stddev 0.048
progress: 200.0 s, 613214.3 tps, lat 0.208 ms stddev 0.047
progress: 300.0 s, 613384.3 tps, lat 0.208 ms stddev 0.047

This is all on virtualized (though using a dedicated instance)
hardware. So they numbers are to be taken with a grain of salt. But I
did run shorter tests in various orders, and the runtime difference
apears to be very small.

- Andres