Message-ID: <559CB5BF.2080509@commandprompt.com>
Date: Tue, 07 Jul 2015 22:31:43 -0700
From: "Joshua D. Drake" <jd@commandprompt.com>
Organization: Command Prompt, Inc.
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To: Craig James <cjames@emolecules.com>,
 "Graeme B. Bell" <graeme.bell@nibio.no>
CC: postgres performance list <pgsql-performance@postgresql.org>
Subject: Re: Hmmm... why does CPU-intensive pl/pgsql code parallelise
 so badly when queries parallelise fine? Anyone else seen this?
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On 07/07/2015 08:05 PM, Craig James wrote:
>
>
> No ideas, but I ran into the same thing. I have a set of C/C++ functions
> that put some chemistry calculations into Postgres as extensions (things
> like, "calculate the molecular weight of this molecule"). As SQL
> functions, the whole thing bogged down, and we never got the scalability
> we needed. On our 8-CPU setup, we couldn't get more than 2 CPUs busy at
> the same time, even with dozens of clients.
>
> When I moved these same functions into an Apache fast-CGI HTTP service
> (exact same code, same network overhead), I could easily scale up and
> use the full 100% of all eight CPUs.
>
> I have no idea why, and never investigated further. The convenience of
> having the functions in SQL wasn't that important.

I admit that I haven't read this whole thread but:

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.

JD

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