Re: Hmmm... why does CPU-intensive pl/pgsql code parallelise so badly when queries parallelise fine? Anyone else seen this?

On 2015-07-08 13:46:53 -0500, Merlin Moncure wrote:
> On Wed, Jul 8, 2015 at 12:48 PM, Craig James <cjames@emolecules.com> wrote:
> > On Tue, Jul 7, 2015 at 10:31 PM, Joshua D. Drake <jd@commandprompt.com>
> >> Using Apache Fast-CGI, you are going to fork a process for each instance
> >> of the function being executed and that in turn will use all CPUs up to the
> >> max available resource.
> >>
> >> With PostgreSQL, that isn't going to happen unless you are running (at
> >> least) 8 functions across 8 connections.
> >
> >
> > Well, right, which is why I mentioned "even with dozens of clients."
> > Shouldn't that scale to at least all of the CPUs in use if the function is
> > CPU intensive (which it is)?
>
> only in the absence of inter-process locking and cache line bouncing.

And addititionally memory bandwidth (shared between everything, even in
the numa case), cross socket/bus bandwidth (absolutely performance
critical in multi-socket configurations), cache capacity (shared between
cores, and sometimes even sockets!).