Re: RFC: Async query processing

On 01/02/2014 07:52 PM, Claudio Freire wrote:

>> No, because this doesn't scale automatically with the bandwidth-delay
>> product. It also requires that the client buffers queries and their
>> parameters even though the network has to do that anyway.
>
> Why not? I'm talking about transport-level packets, btw, not libpq
> frames/whatever.
>
> Yes, the network stack will sometimes do that. But the it doesn't have
> to do it. It does it sometimes, which is not the same.

The network inevitably buffers because the speed of light is not infinite.

Here's a concrete example. Suppose the server is 100ms away, and you
want to send data at a constant rate of 10 Mbps. The server needs to
acknowledge the data you sent, but this acknowledgment arrives after 200
ms. As a result, you've sent 2 Mbits before the acknowledgment arrives,
so the network appears to have buffered 250 KB. This effect can
actually be used for data storage, called "delay line memory", but it is
somewhat out of fashion now.

> And buffering algorithms are quite platform-dependent anyway, so it's
> not the best idea to make libpq highly reliant on them.

That is why I think libpq needs to keep sending until the first response
from the server arrives. Batching a fixed number of INSERTs together in
a single conceptual query does not achieve auto-tuning to the buffering
characteristics of the path.

> So, trusting the network start to do the quick start won't work. For
> steady streams of queries, it will work. But not for short bursts,
> which will be the most heavily used case I believe (most apps create
> short bursts of inserts and not continuous streams at full bandwidth).

Loading data into the database isn't such an uncommon task. Not
everything is OLTP.

--
Florian Weimer / Red Hat Product Security Team