From: Jeff Davis <pgsql(at)j-davis(dot)com>
To: Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Removing PD_ALL_VISIBLE
Date: 2013-01-21 06:52:13
Message-ID: 1358751133.992.75.camel@jdavis
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On Mon, 2013-01-21 at 11:27 +0530, Pavan Deolasee wrote:
> I tend to agree. When I looked at the patch, I thought since its
> removing a WAL record (and associated redo logic), it has some
> additional value. But that was kind of broken (sorry, I haven't looked
> at the latest patch if Jeff fixed it without requiring to reinstate
> the WAL logging). I also thought that bug invalidates the performance
> numbers reported.

I revalidated the performance numbers after reinstating the WAL logging.
No difference (which is unsurprising, since my tests were read-only).

> Of course, there is an argument that this patch will
> simplify the code, but I'm not sure if its enough to justify the
> additional contention which may or may not show up in the benchmarks
> we are running, but we know its there.

What additional contention? How do you know it's there?

The design is to keep the VM page pinned, and to read it without
requiring locks (like index-only scans already do). So I don't see any
obvious additional contention there, unless you're talking about the
work the CPU does for cache coherency (which did not show up in any of
my tests).

I understand that my patch is probably not going in, but I would like to
be clear about what is a known practical problem, what is a theoretical
problem that has eluded my testing capabilities, and what is no problem
at all.

Jeff Davis

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