Message-ID: <448DF692.3070906@boreham.org>
Date: Mon, 12 Jun 2006 17:19:46 -0600
From: David Boreham <david_list@boreham.org>
Reply-To: david_list@boreham.org
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To: pgsql-performance@postgresql.org
Subject: Re: 64-bit vs 32-bit performance ... backwards?
References: <1150151282.14661.42.camel@tigger>
In-Reply-To: <1150151282.14661.42.camel@tigger>
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Anthony Presley wrote:

>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?
>
>I was told that the added 32-bits puts a "strain" and extra "overhead"
>on the processor / etc.... which actually slows down the pointers and
>necessary back-end "stuff" on the database.
>
>I'm curious if anyone can back this up .... or debunk it.  It's about
>the polar opposite of everything I've heard from every other database
>vendor for the past several years, and would be quite an eye-opener for
>me.
>  
>
What they are saying is strictly true : 64-bit pointers tend to increase 
the working set size
of an application vs. 32-bit pointers. This means that any caches will 
have somewhat lower
hit ratio. Also the bytes/s between the CPU and memory will be higher 
due to moving those larger pointers.
In the case of a 32-bit OS this also applies to the kernel so the effect 
will be system-wide.

However, an application that needs to work on > around 2G of data will 
in the end be
much faster 64-bit due to reduced I/O (it can keep more of the data in 
memory).

I worked on porting a large database application from 32-bit to 64-bit. One
of our customers required us to retain the 32-bit version because of 
this phenomenon.

In measurements I conducted on that application, the performance 
difference wasn't
great (10% or so), but it was measurable. This was with Sun Sparc hardware.
It is possible that more modern CPU designs have more efficient 64-bit
implementation than 32-bit, so the opposite might be seen too.

Whether or not PG would show the same thing I can't say for sure. 
Probably it would though.