Horton

Re: Perfomance degradation 9.3 (vs 9.2) for FreeBSD

Alvaro Herrera <alvherre@2ndquadrant.com>

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From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Alfred Perlstein <alfred@freebsd.org>
Cc: Andrew Dunstan <andrew@dunslane.net>, Magnus Hagander <magnus@hagander.net>, Andres Freund <andres@2ndquadrant.com>, Tom Lane <tgl@sss.pgh.pa.us>, Palle Girgensohn <girgen@freebsd.org>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>, Robert Haas <robertmhaas@gmail.com>, Francois Tigeot <ftigeot@wolfpond.org>
Date: 2014-04-21T16:52:35Z
Lists: pgsql-hackers

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  1. Add shared_memory_type GUC.

Alfred Perlstein wrote:

> I am unsure of the true overhead of making this a runtime tunable so
> pardon if I'm asking for "a lot".  From the perspective of both an
> OS developer and postgresql user (I am both) it really makes more
> sense to have it a runtime tunable for the following reasons:
> 
> From an OS developer making this a runtime allows us to much more
> easily do the testing (instead of needing two compiled versions).
> From a sysadmin perspective it makes switching to/from a LOT easier
> in case the new mmap code exposes a stability or performance bug.

In this case, AFAICS the only overhead of a runtime option (what we call
a GUC) is the added potential for user confusion, and the necessary
documentation.  If we instead go for a compile-time option, both items
become smaller.

In any case, I don't see that there's much need for a runtime option,
really; you already know that the mmap code path is slower in FreeBSD.
You only need to benchmark both options once the FreeBSD vm code itself
is fixed, right?

In fact, it might not even need to be a configure option; I would
suggest a pg_config_manual.h setting instead, and perhaps tweaks to the
src/template/freebsd file to enable it automatically on the "broken"
FreeBSD releases.  We could then, in the future, have the template
itself turn the option off for the future FreeBSD release that fixes the
problem.

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services