Date: Tue, 13 Jun 2006 14:43:38 -0500
From: "Jim C. Nasby" <jnasby@pervasive.com>
To: David Boreham <david_list@boreham.org>
Cc: pgsql-performance@postgresql.org
Subject: Re: 64-bit vs 32-bit performance ... backwards?
Message-ID: <20060613194337.GY34196@pervasive.com>
References: <1150151282.14661.42.camel@tigger> <448DF692.3070906@boreham.org>
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On Mon, Jun 12, 2006 at 05:19:46PM -0600, David Boreham wrote:
> 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.

While bytes/s will go up what really matters is words/s, where a word is
the size of a memory transfer to the CPU. Taking a simplistic view, 8
bit CPUs move data into the CPU one byte at a time; 16 bit CPUs, 2
bytes; 32 bit, 4 bytes, and 64 bit, 8 bytes. The reality is a bit more
complicated, but I'm 99.9% certain that you won't see a modern 64 bit CPU
tranfering data in less than 64 bit increments.

> 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).
 
There's not an automatic correlation between word size and address
space, just look at the 8088, so this depends entirely on the CPU.

> 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.

It's going to depend entirely on the CPU and the compiler. I can say
that in the 32 vs 64 bit benchmarking I've done using dbt2, I wasn't
able to find a difference at all on Sunfire Opteron machines.
-- 
Jim C. Nasby, Sr. Engineering Consultant      jnasby@pervasive.com
Pervasive Software      http://pervasive.com    work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461