Horton

Re: walsender performance regression due to logical decoding on standby changes

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

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From: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
To: Andres Freund <andres@anarazel.de>, Jeff Davis <pgsql@j-davis.com>
Cc: pgsql-hackers@postgresql.org, Amit Kapila <amit.kapila16@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>
Date: 2023-05-10T06:39:08Z
Lists: pgsql-hackers 
Hi,

On 5/9/23 11:00 PM, Andres Freund wrote:
> Hi,
> 
> On 2023-05-09 13:38:24 -0700, Jeff Davis wrote:
>> On Tue, 2023-05-09 at 12:02 -0700, Andres Freund wrote:
>>> I don't think the approach of not having any sort of "registry" of
>>> whether
>>> anybody is waiting for the replay position to be updated is
>>> feasible. Iterating over all walsenders slots is just too expensive -
>>
>> Would it work to use a shared counter for the waiters (or, two
>> counters, one for physical and one for logical), and just early exit if
>> the count is zero?
> 
> That doesn't really fix the problem - once you have a single walsender
> connected, performance is bad again.
> 

Just to clarify, do you mean that if there is only one remaining active walsender that, say,
has been located at slot n, then we’d still have to loop from 0 to n in WalSndWakeup()?

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com

Commits

  1. Optimize walsender wake up logic using condition variables

  2. For cascading replication, wake physical and logical walsenders separately