Re: Flushing large data immediately in pqcomm

> Proposed change modifies socket_putmessage to send any data larger than
> 8K immediately without copying it into the send buffer. Assuming that
> the send buffer would be flushed anyway due to reaching its limit, the
> patch just gets rid of the copy part which seems unnecessary and sends
> data without waiting.

If there's already some data in PqSendBuffer, I wonder if it would be
better to fill it up with data, flush it, and then send the rest of the
data directly. Instead of flushing the partial data first. I'm afraid
that you'll make a tiny call to secure_write(), followed by a large one,
then a tine one again, and so forth. Especially when socket_putmessage
itself writes the msgtype and len, which are tiny, before the payload.

Perhaps we should invent a new pq_putmessage() function that would take
an input buffer with 5 bytes of space reserved before the payload.
pq_putmessage() could then fill in the msgtype and len bytes in the
input buffer and send that directly. (Not wedded to that particular API,
but something that would have the same effect)

> This change affects places where pq_putmessage is used such as
> pg_basebackup, COPY TO, walsender etc.
>
> I did some experiments to see how the patch performs.
> Firstly, I loaded ~5GB data into a table [1], then ran "COPY test TO
> STDOUT". Here are perf results of both the patch and HEAD > ...
> The patch brings a ~5% gain in socket_putmessage.
>
> [1]
> CREATE TABLE test(id int, name text, time TIMESTAMP);
> INSERT INTO test (id, name, time) SELECT i AS id, repeat('dummy', 100)
> AS name, NOW() AS time FROM generate_series(1, 100000000) AS i;

I'm surprised by these results, because each row in that table is < 600
bytes. PqSendBufferSize is 8kB, so the optimization shouldn't kick in in
that test. Am I missing something?