From: | Simon Riggs <simon(at)2ndQuadrant(dot)com> |
---|---|
To: | Andres Freund <andres(at)2ndquadrant(dot)com> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Peter Geoghegan <peter(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: logical decoding - GetOldestXmin |
Date: | 2012-12-13 23:35:00 |
Message-ID: | CA+U5nMJKy=8qjJNNCpr1j=CXOZ=5P4nk-3qR8KM+sVEe02v-VQ@mail.gmail.com |
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On 13 December 2012 22:37, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> On 2012-12-13 17:29:06 -0500, Robert Haas wrote:
>> On Thu, Dec 13, 2012 at 3:03 PM, Andres Freund <andres(at)2ndquadrant(dot)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.
>
> But GetOldestXmin() should be called less frequently than
> GetSnapshotData() by several orders of magnitudes. I don't really see
> it being used in any really hot code paths?
Maybe, but that calculation doesn't *need* to be inside the lock, that
is just a consequence of the current coding.
--
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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