Re: BUG #17005: Enhancement request: Improve walsender throughput by aggregating multiple messages in one send

Hi,

On 2021-05-13 00:31:53 +0000, PG Bug reporting form wrote:
> I measured the throughput of reading the logical replication slot and found
> that in smaller row size (512 bytes) the throughput is 50% lower compared to
> 1024 bytes.

Huh, that is interesting.

> tcpdump shows that ethernet packets sent by the replication server contain
> only one message per packet (see tcpdump output below).
> May be this is the intended design to achieve low latency but this is not
> favorable in application that requires high throughput.

What kind of network is this? I would have expected that if the network
can't keep up the small sends would end up getting aggregated into
larger packets anyway? Are you hitting a PPS limit due to the small
packages, but not yet the throughput limit?

> Is it possible for PostgreSQL to enable Nagle's algorithm on the streaming
> socket for replication?
> Or aggegate the messages manually before sending them in one send()?

I think we can probably do better in cases a transaction is more than a
single change - but I don't think either enabling nagle's or aggregation
are really an option in case of single row transactions. The latency
impact for some scenarios seems too high.

Greetings,

Andres Freund