Message-ID: <430CC0AD.5040400@rentec.com>
Date: Wed, 24 Aug 2005 14:47:09 -0400
From: Alan Stange <stange@rentec.com>
Reply-To: stange@rentec.com
Organization: Renaissance Technologies Corp.
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To: mark@mark.mielke.cc
Cc: Donald Courtney <Donald.Courtney@Sun.COM>,
	William Yu <wyu@talisys.com>, pgsql-performance@postgresql.org
Subject: Re: Caching by Postgres
References: <20050823171045.12112.qmail@web51309.mail.yahoo.com>
	<20050823124323.710a9dd3.frank@wiles.org>
	<430B6DE3.6040109@sun.com> <deg68k$16gh$1@news.hub.org>
	<430C7448.7030103@sun.com> <20050824173055.GA11788@mark.mielke.cc>
In-Reply-To: <20050824173055.GA11788@mark.mielke.cc>
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mark@mark.mielke.cc wrote:
> So then we move on to what 64-bit is really useful for. Obviously,
> there is the arithmetic. If you were previously doing 64-bit
> arithmetic through software, you will notice an immediate speed
> improvement when doing it through hardware instead. If you have
> a program that is scanning memory in any way, it may benefit from
> 64-bit instructions (for example - copying data 64-bit words at
> a time instead of 32-bit words at a time). PostgreSQL might benefit
> slightly from either of these, slightly balancing the performance
> degradation of using more memory to store the pointers, and more
> memory bandwidth the access the pointers.
>   
At least on Sparc processors, v8 and newer, any double precision math 
(including longs) is performed with a single instruction, just like for 
a 32 bit datum.  Loads and stores of 8 byte datums are also handled via 
a single instruction.   The urban myth that 64bit math is 
different/better on a 64 bit processor is just that;  yes, some lower 
end processors would emulate/trap those instructions but that an 
implementation detail, not architecture.    I believe that this is all 
true for other RISC processors as well.

The 64bit API on UltraSparcs does bring along some extra FP registers IIRC.

> If, however, you happen to have a very large amount of physical memory
> - more memory than is supported by a 32-bit system, but is supported
> by your 64-bit system, then the operating system should be able to use
> this additional physical memory to cache file system data pages, which
> will benefit PostgreSQL if used with tables that are larger than the
> memory supported by your 32-bit system, and which have queries which
> require more pages than the memory supported by your 32-bit system to
> be frequently accessed. If you have a huge database, with many clients
> accessing the data, this would be a definate yes. With anything less,
> it is a maybe, or a probably not.
>   
Solaris, at least, provided support for far more than 4GB of physical 
memory on 32 bit kernels.  A newer 64 bit kernel might be more 
efficient, but that's just because the time was taken to support large 
page sizes and more efficient data structures.  It's nothing intrinsic 
to a 32 vs 64 bit kernel.

-- Alan