Horton

Re: PATCH: logical_work_mem and logical streaming of large in-progress transactions

Tomas Vondra <tomas.vondra@2ndquadrant.com>

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From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2019-09-29T17:56:30Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Tighten the concurrent abort check during decoding.

  2. Improve hash_create()'s API for some added robustness.

  3. Use HASH_BLOBS for xidhash.

  4. Fix initialization of RelationSyncEntry for streaming transactions.

  5. Remove unused function declaration in logicalproto.h.

  6. Add additional tests to test streaming of in-progress transactions.

  7. Fix inline marking introduced in commit 464824323e.

  8. Add support for streaming to built-in logical replication.

  9. Fix the SharedFileSetUnregister API.

  10. Fix comment in procarray.c

  11. Suppress compiler warning in non-cassert builds.

  12. Extend the BufFile interface.

  13. Mark a few logical decoding related variables with PGDLLIMPORT.

  14. Implement streaming mode in ReorderBuffer.

  15. Extend the logical decoding output plugin API with stream methods.

  16. WAL Log invalidations at command end with wal_level=logical.

  17. Immediately WAL-log subtransaction and top-level XID association.

  18. Allow logical replication to transfer data in binary format.

  19. Only superuser can set sslcert/sslkey in postgres_fdw user mappings

  20. Track statistics for spilling of changes from ReorderBuffer.

  21. Add logical_decoding_work_mem to limit ReorderBuffer memory usage.

  22. logical decoding: process ASSIGNMENT during snapshot build

  23. Emit invalidations to standby for transactions without xid.

On Sun, Sep 29, 2019 at 02:30:44PM -0300, Alvaro Herrera wrote:
>On 2019-Sep-29, Amit Kapila wrote:
>
>> On Sun, Sep 29, 2019 at 12:39 AM Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:
>
>> > So that's what I did in the attached patch - I've renamed the GUC to
>> > logical_decoding_work_mem, detached it from m_w_m and set the default to
>> > 64MB (i.e. the same default as m_w_m).
>>
>> Fair enough, let's not argue more on this unless someone else wants to
>> share his opinion.
>
>I just read this part of the conversation and I agree that having a
>separate GUC with its own value independent from other GUCs is a good
>solution.  Tying it to m_w_m seemed reasonable, but it's true that
>people frequently set m_w_m very high, and it would be undesirable to
>propagate that value to logical decoding memory usage.
>
>
>I wonder what would constitute good advice on how to set this value, I
>mean what is the metric that the user needs to be thinking about.   Is
>it the total of memory required to keep all concurrent write transactions
>in memory?  (Quick example: if you do 2048 wTPS and each transaction
>lasts 1s, and each transaction does 1kB of logically-decoded changes,
>then ~2MB are sufficient for the average case.  Is that correct? 

Yes, something like that. Essentially we'd like to keep all concurrent
transactions decoded in memory, to eliminate the need to spill to disk.
One of the subsequent patches adds some subscription-level stats, so
maybe we don't need to worry about this too much - the stats seem like a
better source of information for tuning.

>I *think* that full-page images do not count, correct?  With these
>things in mind users could go through pg_waldump output and figure out
>what to set the value to.)
>

Right, FPW do not matter here.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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