From: | Ian Barwick <ian(dot)barwick(at)2ndquadrant(dot)com> |
---|---|
To: | Andres Freund <andres(at)anarazel(dot)de> |
Cc: | Michael Paquier <michael(at)paquier(dot)xyz>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Daniel Gustafsson <daniel(at)yesql(dot)se>, "Jonathan S(dot) Katz" <jkatz(at)postgresql(dot)org>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, Peter Geoghegan <pg(at)bowt(dot)ie>, Bruce Momjian <bruce(at)momjian(dot)us>, David Rowley <dgrowleyml(at)gmail(dot)com> |
Subject: | Re: Improving connection scalability: GetSnapshotData() |
Date: | 2020-09-08 04:23:01 |
Message-ID: | 0006ffe6-6f76-c074-3e58-2801cee1c05f@2ndquadrant.com |
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Lists: | pgsql-hackers |
On 2020/09/08 13:11, Andres Freund wrote:
> Hi,
>
> On 2020-09-08 13:03:01 +0900, Ian Barwick wrote:
(...)
>> I wonder if it's possible to increment "xactCompletionCount"
>> during replay along these lines:
>>
>> *** a/src/backend/access/transam/xact.c
>> --- b/src/backend/access/transam/xact.c
>> *************** xact_redo_commit(xl_xact_parsed_commit *
>> *** 5915,5920 ****
>> --- 5915,5924 ----
>> */
>> if (XactCompletionApplyFeedback(parsed->xinfo))
>> XLogRequestWalReceiverReply();
>> +
>> + LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
>> + ShmemVariableCache->xactCompletionCount++;
>> + LWLockRelease(ProcArrayLock);
>> }
>>
>> which seems to work (though quite possibly I've overlooked something I don't
>> know that I don't know about and it will all break horribly somewhere,
>> etc. etc.).
>
> We'd also need the same in a few more places. Probably worth looking at
> the list where we increment it on the primary (particularly we need to
> also increment it for aborts, and 2pc commit/aborts).
Yup.
> At first I was very confused as to why none of the existing tests have
> found this significant issue. But after thinking about it for a minute
> that's because they all use psql, and largely separate psql invocations
> for each query :(. Which means that there's no cached snapshot around...
>
> Do you want to try to write a patch?
Sure, I'll give it a go as I have some time right now.
Regards
Ian Barwick
--
Ian Barwick https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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