Max-open-requests and natural back-pressure

Akka Akka HTTP
1

Hello guys,

here goes pseudo code just to give background about composition I have at hand.

Source
      .queue(…)
      .via {
        Http().newHostConnectionPool(…)
      }
      .toMat(Sink.ignore)(Keep.left)
      .run()

So as you can see I have a queue with own buffer settings, however then there is newHostConnectionPool under the hood creating another buffer with max-open-requests size and throwing exceptions upon reaching the limit.

I tried to disable additional buffer ensuring that settings.maxOpenRequests <= settings.maxConnections

val targetBufferSize = settings.maxOpenRequests - settings.maxConnections
      if (targetBufferSize > 0) Flow[RequestContext].buffer(targetBufferSize, OverflowStrategy.backpressure)

but I’m occasionally getting:

BufferOverflowException("Exceeded configured max-open-requests…)

I’d like to have a natural back-pressure of my stream without PoolInterface’s buffer amortising spikes.
Which looks like impossible as otherwise with low values of max-open-requests I would get BufferOverflowException instead of back-pressure.

Am I missing something?
How can I achieve natural back-pressure without exceptions?

Thanks for the help in advance.