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Re: pg_last_xact_insert_timestamp

From: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Simon Riggs <simon(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Chris Redekop <chris(at)replicon(dot)com>
Subject: Re: pg_last_xact_insert_timestamp
Date: 2011-09-11 04:24:10
Message-ID: CAHGQGwEsMSOCmgeOVte5WwHgPTXNr=2aYSDjSQi9f28UBXRT9Q@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Sat, Sep 10, 2011 at 12:32 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Thu, Sep 8, 2011 at 8:42 PM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
>> Another idea to avoid spinlock contention is save the timestamp in
>> PgBackendStatus (which contains information for pg_stat_activity).
>> This enables us to write and read the timestamp without spinlock.
>> Comments?
>
> That seems like a possibly promising approach, in that each backend
> could update the information separately, and it's the reader's job to
> go find the maximum of all those values when needed.  So the overhead
> is (properly, in this case) placed on the reader instead of the
> writer.  But it's a bit tricky, because when the reader wants that
> maximum, it has to take into account inactive backends that may have
> committed transactions before exiting, not just the ones that are
> still connected.

Yes, that's what I was thinking. The attached patch has adopted that
approach.

Regards,

-- 
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center

Attachment: pg_last_xact_insert_timestamp_v2.patch
Description: application/octet-stream (13.0 KB)

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