From: Nathan Bossart <nathandbossart@gmail.com>
To: Corey Huinker <corey.huinker@gmail.com>
Cc: Ilya Gladyshev <ilya.v.gladyshev@gmail.com>
Cc: Daniel Gustafsson <daniel@yesql.se>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Jeff Davis <pgsql@j-davis.com>
Cc: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: optimizing pg_upgrade's once-in-each-database steps
Date: Sat, 10 Aug 2024 10:17:27 -0500
Message-ID: <ZreEh54Xr6D3Qy8k@nathan> (raw)
In-Reply-To: <CADkLM=fc-wujxNtGpLSVrBAtWQgsW-8RYrMxVyQ1buJfQX9CHw@mail.gmail.com>
References: <ZprY4e23vR95Nc51@nathan>
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<CADkLM=fc-wujxNtGpLSVrBAtWQgsW-8RYrMxVyQ1buJfQX9CHw@mail.gmail.com>
On Fri, Aug 09, 2024 at 04:06:16PM -0400, Corey Huinker wrote:
>> I'll admit I hadn't really considered pipelining, but I'm tempted to say
>> that it's probably not worth the complexity. Not only do most of the tasks
>> have only one step, but even tasks like the data types check are unlikely
>> to require more than a few queries for upgrades from supported versions.
>
> Can you point me to a complex multi-step task that you think wouldn't work
> for pipelining? My skimming of the other patches all seemed to be one query
> with one result set to be processed by one callback.
I think it would work fine. I'm just not sure it's worth it, especially
for tasks that run one exactly one query in each connection.
>> Furthermore, most of the callbacks should do almost nothing for a given
>> upgrade, and since pg_upgrade runs on the server, client/server round-trip
>> time should be pretty low.
>
> To my mind, that makes pipelining make more sense, you throw out N queries,
> most of which are trivial, and by the time you cycle back around and start
> digesting result sets via callbacks, more of the queries have finished
> because they were waiting on the query ahead of them in the pipeline, not
> waiting on a callback to finish consuming its assigned result set and then
> launching the next task query.
My assumption is that the "waiting for a callback before launching the next
query" time will typically be pretty short in practice. I could try
measuring it...
>> Perhaps pipelining would make more sense if we consolidated the tasks a bit
>> better, but when I last looked into that, I didn't see a ton of great
>> opportunities that would help anything except for upgrades from really old
>> versions. Even then, I'm not sure if pipelining is worth it.
>
> I think you'd want to do the opposite of consolidating the tasks. If
> anything, you'd want to break them down in known single-query operations,
> and if the callback function for one of them happens to queue up a
> subsequent query (with subsequent callback) then so be it.
By "consolidating," I mean combining tasks into fewer tasks with additional
steps. This would allow us to reuse connections instead of creating N
connections for every single query. If we used a task per query, I'd
expect pipelining to provide zero benefit.
--
nathan
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To: pgsql-hackers@postgresql.org
Cc: nathandbossart@gmail.com, corey.huinker@gmail.com, ilya.v.gladyshev@gmail.com, daniel@yesql.se, robertmhaas@gmail.com, pgsql@j-davis.com
Subject: Re: optimizing pg_upgrade's once-in-each-database steps
In-Reply-To: <ZreEh54Xr6D3Qy8k@nathan>
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