Subject: Re: PATCH: logical_work_mem and logical streaming of large
 in-progress transactions
To: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>,
 PostgreSQL Hackers <pgsql-hackers@postgresql.org>
References: <688b0b7f-2f6c-d827-c27b-216a8e3ea700@2ndquadrant.com>
 <3530384f-f73a-186a-a728-8eeeb5533a10@2ndquadrant.com>
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Message-ID: <262cc9df-0d19-3fca-f899-0890bd2cd4be@2ndquadrant.com>
Date: Wed, 3 Jan 2018 20:53:59 +0100
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On 01/02/2018 04:07 PM, Peter Eisentraut wrote:
> On 12/22/17 23:57, Tomas Vondra wrote:
>> PART 1: adding logical_work_mem memory limit (0001)
>> ---------------------------------------------------
> 
> The documentation in this patch contains some references to later
> features (streaming).  Perhaps that could be separated so that the
> patches can be applied independently.
> 

Yeah, that's probably a good idea. But now that you mention it, I wonder
if "streaming" is really a good term. We already use it for "streaming
replication" and it may be quite confusing to use it for another feature
(particularly when it's streaming within logical streaming replication).

But I can't really think of a better name ...

> 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.

> 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?

> I think we need a way to report on how much memory is actually used,
> so the setting can be tuned. Something analogous to log_temp_files
> perhaps.
> 

Yes, I agree. I'm just about to submit an updated version of the patch
series, that also introduces a few columns pg_stat_replication, tracking
this type of stats (amount of data spilled to disk or streamed, etc.).

regards

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