From: | Bruce Momjian <bruce(at)momjian(dot)us> |
---|---|
To: | Alvaro Herrera <alvherre(at)commandprompt(dot)com> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Bug in autovacuum.c? |
Date: | 2011-04-01 21:48:16 |
Message-ID: | 201104012148.p31LmGP17089@momjian.us |
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Lists: | pgsql-hackers |
Alvaro Herrera wrote:
> Excerpts from Bruce Momjian's message of vie abr 01 16:50:29 -0300 2011:
>
> > To do the right thing every computation that passes over the xid
> > wraparound bounary should subtract FirstNormalTransactionId, not just
> > those that fall in the boundry. That would prevent the value from going
> > backward and still allow the mapping you liked; it isn't worth it, but
> > that is the right answer.
>
> This code is only concerned calculating an immediate the wrap horizon
> for the autovacuuming run that's about to take place. If it's wrong in
> one or three counts doesn't mean much. Consider what would happen if
> load was high and it would have taken 100 extra milliseconds to get to
> that bit: ReadNewTransactionId would have returned a value 3
> transactions later. Furthermore, before this value is even used at all
> for vacuuming, there has to be a whole lot of inter-process signalling,
> a fork, and a new backend startup.
>
> I think this should be left alone. As you said, it isn't worth it.
Agreed it is not worth it but I think we should at least C comment
something. I think at a minimum we should set it to
FirstNormalTransactionId.
I am not so concerned about this case but about other cases where we are
computing xid distances across the invalid range.
--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ It's impossible for everything to be true. +
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/pgpatches/autovacuum.diff | text/x-diff | 1.1 KB |
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