| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Greg Stark <gsstark(at)mit(dot)edu> |
| Cc: | Stephen Frost <sfrost(at)snowman(dot)net>, Robert Haas <robertmhaas(at)gmail(dot)com>, Jeremy Kerr <jk(at)ozlabs(dot)org>, "<pgsql-hackers(at)postgresql(dot)org>" <pgsql-hackers(at)postgresql(dot)org>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Stefan Kaltenbrunner <stefan(at)kaltenbrunner(dot)cc>, Gurjeet Singh <singh(dot)gurjeet(at)gmail(dot)com> |
| Subject: | Re: [PATCH] backend: compare word-at-a-time in bcTruelen |
| Date: | 2009-06-16 14:23:56 |
| Message-ID: | 13985.1245162236@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Greg Stark <gsstark(at)mit(dot)edu> writes:
> On some architectures like intel accessing unaligned ints is just
> slow. On others (Alpha and PPC iirc?) it is an immediate bus error.
To a first approximation, Intel is the *only* popular architecture that
doesn't bus-error on unaligned accesses. (And I'm sure their chip
designers rue the day that their predecessors chose to allow that.)
There are some systems where the kernel trap handler then proceeds to
emulate the unaligned access for you, but that gives new meaning to the
word "slow". You definitely don't want to be doing it in a patch that's
alleged to give a performance improvement.
Speaking of which, what about some performance numbers? Like Heikki,
I'm quite suspicious of whether there is any real-world gain to be had
from this approach.
regards, tom lane
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