Message-ID: <ffd67cc0a7b6b4c06ad36967c4d7eb7825bf293a.camel@j-davis.com>
Subject: Re: Improve WALRead() to suck data directly from WAL buffers when
 possible
From: Jeff Davis <pgsql@j-davis.com>
To: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Alvaro
	Herrera <alvherre@alvh.no-ip.org>
Cc: Andres Freund <andres@anarazel.de>, Dilip Kumar <dilipbalaut@gmail.com>,
  Kyotaro Horiguchi <horikyota.ntt@gmail.com>,
 pgsql-hackers@lists.postgresql.org, Nathan Bossart
 <nathandbossart@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>
Date: Mon, 12 Feb 2024 15:56:19 -0800
In-Reply-To: <db1e59dbdaf0c2fe722f1c5dd534c7cb048ba606.camel@j-davis.com>
References: 
	<CALj2ACW4oZzmg6joL0ppQN=cCGOLaVKJknc3OLqmo8Q+cGDRKw@mail.gmail.com>
	 <202401301731.o3cthlcry4ve@alvherre.pgsql>
	 <CALj2ACVSR9cgFW=OMxd4mtLceAmucVYHJ2anRAoG3y6OSyLR+g@mail.gmail.com>
	 <db1e59dbdaf0c2fe722f1c5dd534c7cb048ba606.camel@j-davis.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
User-Agent: Evolution 3.44.4-0ubuntu2 
MIME-Version: 1.0
Archived-At: <https://www.postgresql.org/message-id/ffd67cc0a7b6b4c06ad36967c4d7eb7825bf293a.camel%40j-davis.com>
Precedence: bulk

On Mon, 2024-02-12 at 11:33 -0800, Jeff Davis wrote:
> For 0002 & 0003, I'd like more clarity on how they will actually be
> used by an extension.

In patch 0002, I'm concerned about calling
WaitXLogInsertionsToFinish(). It loops through all the locks, but
doesn't have any early return path or advance any state.

So if it's repeatedly called with the same or similar values it seems
like it would be doing a lot of extra work.

I'm not sure of the best fix. We could add something to LogwrtResult to
track a new LSN that represents the highest known point where all
inserters are finished (in other words, the latest return value of
WaitXLogInsertionsToFinish()). That seems invasive, though.

Regards,
	Jeff Davis