Re: logical decoding - GetOldestXmin - Mailing list pgsql-hackers

From Michael Paquier
Subject Re: logical decoding - GetOldestXmin
Date
Msg-id CAB7nPqSvyv1AsEFbopwQW6nb+7h8V7GdqqN=OfA3SzYiqEmfyA@mail.gmail.com
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In response to Re: logical decoding - GetOldestXmin  (Robert Haas <robertmhaas@gmail.com>)
List pgsql-hackers


On Fri, Dec 14, 2012 at 2:29 AM, Robert Haas <robertmhaas@gmail.com> wrote:
On Thu, Dec 13, 2012 at 3:03 PM, Andres Freund <andres@2ndquadrant.com> wrote:
> It moves a computation of the sort of:
>
> result -= vacuum_defer_cleanup_age;
> if (!TransactionIdIsNormal(result))
>    result = FirstNormalTransactionId;
>
> inside ProcArrayLock. But I can't really imagine that to be relevant...

I can.  Go look at some of the 9.2 optimizations around
GetSnapshotData().  Those made a BIG difference under heavy
concurrency and they were definitely micro-optimization.  For example,
the introduction of NormalTransactionIdPrecedes() was shockingly
effective.

The two commits coming to my mind are:
- ed0b409 (Separate PGPROC into PGPROC and PGXACT)
- 0d76b60 (introduction of NormalTransactionIdPrecedes)
Those ones really improved concurrency performance.
--
Michael Paquier
http://michael.otacoo.com

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