To: Anthony Presley <anthony@resolution.com>
cc: pgsql-performance@postgresql.org
Subject: Re: 64-bit vs 32-bit performance ... backwards? 
In-reply-to: <1150151282.14661.42.camel@tigger> 
References: <1150151282.14661.42.camel@tigger>
Comments: In-reply-to Anthony Presley <anthony@resolution.com>
	message dated "Mon, 12 Jun 2006 17:28:02 -0500"
Date: Mon, 12 Jun 2006 19:35:14 -0400
Message-ID: <13656.1150155314@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Anthony Presley <anthony@resolution.com> writes:
> I had an interesting discussion today w/ an Enterprise DB developer and
> sales person, and was told, twice, that the 64-bit linux version of
> Enterprise DB (which is based on the 64-bit version of PostgreSQL 8.1)
> is SIGNIFICANTLY SLOWER than the 32-bit version.  Since the guys of EDB
> are PostgreSQL ..... has anyone seen that the 64-bit is slower than the
> 32-bit version?

That is a content-free statement, since they didn't mention what
architectures they are comparing, what compilers (and compiler options)
they are using, or what test cases they are measuring on.

Theoretically speaking, 64-bit *should* be slower than 32-bit (because
more data to transfer between memory and CPU to accomplish the same
work), except when considering workloads that can profit from having
direct access to more than 4Gb of memory.  However the theoretical
advantage is probably completely swamped by implementation details,
ie, how tensely did the designers of your 64-bit chip optimize its
32-bit behavior.

I believe that Red Hat generally recommends using 32-bit mode for
small-memory applications on PPC machines, because PPC32 is indeed
measurably faster than PPC64, but finds no such advantage on x86_64,
ia64 or s390x.  I don't know what applications they tested to come
to that conclusion, though.

			regards, tom lane