Date: Wed, 24 Aug 2005 20:21:05 -0400
From: mark@mark.mielke.cc
To: pgsql-performance@postgresql.org
Subject: Re: Caching by Postgres
Message-ID: <20050825002105.GB18300@mark.mielke.cc>
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On Wed, Aug 24, 2005 at 05:09:09PM -0400, Michael Stone wrote:
> On Wed, Aug 24, 2005 at 03:34:41PM -0400, mark@mark.mielke.cc wrote:
> >It isn't an urban myth that 64-bit math on a 64-bit processor is
> >faster, at least if done using registers. It definately is faster.
> >It may be an urban myth, though, that most applications perform
> >a sufficient amount of 64-bit arithmetic to warrant the upgrade.
> The mjor problem is that the definition of "64bit processor" is fuzzy.
> The major slowdown of "64bitness" is the necessity of carting around 
> 64 bit pointers. It's not, however, necessary to do 64bit pointers to
> get 64bit registers & fast 64 bit ops. E.g., sgi has "n32" & "n64" abi's
> which can access exactly the same instruction set & registers, the
> difference between them is the size of pointers and whether a "long" is
> the same as a "long long". Any discussion of "64 bit processors" is
> doomed from the start because people tend to start making implementation
> assumptions on top of an already vague concept. Current & future
> discussions are tinged by the fact that amd64 *doubles* the number
> of registers in 64 bit mode, potentially providing a major speedup--but
> one that doesn't really have anything to do with being "64bit". 
> Pretty much any discussion of 64 bit mode really needs to be a
> discussion of a particular abi on a particular processor; talking about
> "64 bit processors" abstractly is a waste of time.

Agree. :-)

As this very thread has shown! Hehe...

There is no way the manufacturers would release two machines, side by
side that could easily show that the 64-bit version is slower for
regular application loads. They added these other things specifically
to mask this... :-)

Cheers,
mark

-- 
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