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Re: cheaper snapshots redux

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: cheaper snapshots redux
Date: 2011-08-23 17:14:30
Message-ID: 215.1314119670@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> That's certainly a fair concern, and it might even be worse than
> O(n^2).  On the other hand, the current approach involves scanning the
> entire ProcArray for every snapshot, even if nothing has changed and
> 90% of the backends are sitting around playing tiddlywinks, so I don't
> think I'm giving up something for nothing except perhaps in the case
> where there is only one active backend in the entire system.  On the
> other hand, you could be entirely correct that the current
> implementation wins in the uncontended case.  Without testing it, I
> just don't know...

Sure.  Like I said, I don't know that this can't be made to work.
I'm just pointing out that we have to keep an eye on the single-backend
case as well as the many-backends case.

			regards, tom lane

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