From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
cc: Andres Freund <andres@2ndquadrant.com>, Peter Geoghegan <pg@heroku.com>,
 Pg Hackers <pgsql-hackers@postgresql.org>, Ants Aasma <ants@cybertec.at>
Subject: Re: better atomics - v0.5
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Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
 message dated "Mon, 30 Jun 2014 13:15:23 -0400"
Date: Mon, 30 Jun 2014 13:45:52 -0400
Message-ID: <12903.1404150352@sss.pgh.pa.us>
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Sender: pgsql-hackers-owner@postgresql.org

Robert Haas <robertmhaas@gmail.com> writes:
> I'm personally not convinced that we're approaching this topic in the
> right way.  I'm not convinced that it's at all reasonable to try to
> emulate atomics on platforms that don't have them.  I would punt the
> problem into the next layer and force things like lwlock.c to have
> fallback implementations at that level that don't require atomics, and
> remove those fallback implementations if and when we move the
> goalposts so that all supported platforms must have working atomics
> implementations.  People who write code that uses atomics are not
> likely to think about how those algorithms will actually perform when
> those atomics are merely emulated, and I suspect that means that in
> practice platforms that have only emulated atomics are going to
> regress significantly vs. the status quo today.

I think this is a valid objection, and I for one am not prepared to
say that we no longer care about platforms that don't have atomic ops
(especially not if it's not a *very small* set of atomic ops).

Also, just because a platform claims to have atomic ops doesn't mean that
those ops perform well.  If there's a kernel trap involved, they don't,
at least not for our purposes.  We're only going to be bothering with
installing atomic-op code in places that are contention bottlenecks
for us already, so we are not going to be happy with the results for any
atomic-op implementation that's not industrial strength.  This is one
reason why I'm extremely suspicious of depending on gcc's intrinsics for
this; that will not make the issue go away, only make it beyond our
power to control.

			regards, tom lane


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