pgbench: option delaying queries till connections establishment?

Hi,

I am trying to run a few benchmarks measuring the effects of patch to
make GetSnapshotData() faster in the face of larger numbers of
established connections.

Before the patch connection establishment often is very slow due to
contention. The first few connections are fast, but after that it takes
increasingly long. The first few connections constantly hold
ProcArrayLock in shared mode, which then makes it hard for new
connections to acquire it exclusively (I'm addressing that to a
significant degree in the patch FWIW).

But for a fair comparison of the runtime effects I'd like to only
compare the throughput for when connections are actually usable,
otherwise I end up benchmarking few vs many connections, which is not
useful. And because I'd like to run the numbers for a lot of different
numbers of connections etc, I can't just make each run several hour
longs to make the initial minutes not matter much.

Therefore I'd like to make pgbench wait till it has established all
connections, before they run queries.

Does anybody else see this as being useful?

If so, should this be done unconditionally? A new option? Included in an
existing one somehow?

Greetings,

Andres Freund