Re: Suggestions on message transfer among backends

Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> On Tue, Mar 12, 2019 at 4:34 AM Antonin Houska <ah(at)cybertec(dot)at> wrote:
> > Andy Fan <zhihui(dot)fan1213(at)gmail(dot)com> wrote:
> > > I just don't know why shm_mq is designed to single-reader & single-writer.
> >
> > shm_mq was implemented as a part of infrastructure for parallel query
> > processing. The leader backend launches multiple parallel workers and sets up
> > a few queues to communicate with each. One queue is used to send request
> > (query plan) to the worker, one queue is there to receive data from it, and I
> > think there's one more queue to receive error messages.
>
> No, the queues aren't used to send anything to the worker. We know
> the size of the query plan before we create the DSM, so we can just
> allocate enough space to store the whole thing.

ok, I forgot that. (Last time I saw this part was when reading the parallel
sequential scan patch a few years ago.)

--
Antonin Houska
https://www.cybertec-postgresql.com