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Re: Hot Standby: too many KnownAssignedXids

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: Joachim Wieland <joe(at)mcknight(dot)de>, Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Hot Standby: too many KnownAssignedXids
Date: 2010-12-02 08:39:31
Message-ID: 4CF75B43.5030101@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On 01.12.2010 20:51, Heikki Linnakangas wrote:
> Another approach would be to revisit the way the running-xacts snapshot
> is taken. Currently, we first take a snapshot, and then WAL-log it.
> There is a small window between the steps where backends can begin/end
> transactions, and recovery has to deal with that. When this was
> designed, there was long discussion on whether we should instead grab
> WALInsertLock and ProcArrayLock at the same time, to ensure that the
> running-xacts snapshot represents an up-to-date situation at the point
> in WAL where it's inserted.
>
> We didn't want to do that because both locks can be heavily contended.
> But maybe we should after all. It would make the recovery code simpler.
>
> If we want to get fancy, we wouldn't necessarily need to hold both locks
> for the whole duration. We could first grab ProcArrayLock and construct
> the snapshot. Then grab WALInsertLock and release ProcArrayLock, and
> finally write the WAL record and release WALInsertLock. But that would
> require small changes to XLogInsert.

I took a look at that approach. We don't actually need to hold 
ProcArrayLock while the WAL-record is written, we need to hold 
XidGenLock. I believe that's less severe than holding the ProcArrayLock 
as there's already precedence for writing a WAL record while holding 
that: we do that when we advance to a new clog page and write a 
zero-clog-page record.

So this is what we should do IMHO.

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

Attachment: knownassignedxids-fix-with-locking-1.patch
Description: text/x-diff (14.2 KB)

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Subject: Re: Hot Standby: too many KnownAssignedXids
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