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Re: advancing snapshot's xmin

From: Neil Conway <neilc(at)samurai(dot)com>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Cc: Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: advancing snapshot's xmin
Date: 2008-03-25 22:40:13
Message-ID: 1206484813.32670.7.camel@goldbach (view raw or flat)
Thread:
Lists: pgsql-hackers
On Tue, 2008-03-25 at 17:26 -0300, Alvaro Herrera wrote:
> There is one hole here: contention on ProcArrayLock.  Basically, for
> simple transactions we will need to update MyProc after every command.

If we're just updating MyProc->xmin, we only need to acquire
ProcArrayLock in shared mode, right?

> Another idea is to throttle the updating of Xmin so it only happens once
> in a while, but it's difficult to find a useful criterion and avoid
> falling into the trap that we just neglected to update it before a large
> command.

Using LWLockConditionalAcquire() might help also.

-Neil



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