Date: Fri, 27 Sep 2019 13:25:20 +0200
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>,
	PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: PATCH: logical_work_mem and logical streaming of large
 in-progress transactions
Message-ID: <20190927112520.3kco6tqbhcvlk6cd@development>
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On Fri, Sep 27, 2019 at 02:33:32PM +0530, Amit Kapila wrote:
>On Tue, Jan 9, 2018 at 7:55 AM Peter Eisentraut
><peter.eisentraut@2ndquadrant.com> wrote:
>>
>> On 1/3/18 14:53, Tomas Vondra wrote:
>> >> I don't see the need to tie this setting to maintenance_work_mem.
>> >> maintenance_work_mem is often set to very large values, which could
>> >> then have undesirable side effects on this use.
>> >
>> > Well, we need to pick some default value, and we can either use a fixed
>> > value (not sure what would be a good default) or tie it to an existing
>> > GUC. We only really have work_mem and maintenance_work_mem, and the
>> > walsender process will never use more than one such buffer. Which seems
>> > to be closer to maintenance_work_mem.
>> >
>> > Pretty much any default value can have undesirable side effects.
>>
>> Let's just make it an independent setting unless we know any better.  We
>> don't have a lot of settings that depend on other settings, and the ones
>> we do have a very specific relationship.
>>
>> >> Moreover, the name logical_work_mem makes it sound like it's a logical
>> >> version of work_mem.  Maybe we could think of another name.
>> >
>> > I won't object to a better name, of course. Any proposals?
>>
>> logical_decoding_[work_]mem?
>>
>
>Having a separate variable for this can give more flexibility, but
>OTOH it will add one more knob which user might not have a good idea
>to set.  What are the problems we see if directly use work_mem for
>this case?
>

IMHO it's similar to autovacuum_work_mem - we have an independent
setting, but most people use it as -1 so we use maintenance_work_mem as
a default value. I think it makes sense to do the same thing here.

It does ad an extra knob anyway (I don't think we should just use
maintenance_work_mem directly, the user should have an option to
override it when needed). But most users will not notice.

FWIW I don't think we should use work_mem, maintenace_work_mem seems
somewhat more appropriate here (not related to queries, etc.).

>If we can't use work_mem, then I think the name proposed by you
>(logical_decoding_work_mem) sounds good to me.
>

Yes, that name seems better.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services