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

Alexey Kondratov <a.kondratov@postgrespro.ru>

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From: Alexey Kondratov <a.kondratov@postgrespro.ru>
To: Kuntal Ghosh <kuntalghosh.2007@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>
Cc: vignesh C <vignesh21@gmail.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Amit Kapila <amit.kapila16@gmail.com>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2019-11-12T10:42:35Z
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 04.11.2019 13:05, Kuntal Ghosh wrote:
> On Mon, Nov 4, 2019 at 3:32 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>> So your result shows that with "streaming on", performance is
>> degrading?  By any chance did you try to see where is the bottleneck?
>>
> Right. But, as we increase the logical_decoding_work_mem, the
> performance improves. I've not analyzed the bottleneck yet. I'm
> looking into the same.

My guess is that 64 kB is just too small value. In the table schema used 
for tests every rows takes at least 24 bytes for storing column values. 
Thus, with this logical_decoding_work_mem value the limit should be hit 
after about 2500+ rows, or about 400 times during transaction of 1000000 
rows size.

It is just too frequent, while ReorderBufferStreamTXN includes a whole 
bunch of logic, e.g. it always starts internal transaction:

/*
  * Decoding needs access to syscaches et al., which in turn use
  * heavyweight locks and such. Thus we need to have enough state around to
  * keep track of those.  The easiest way is to simply use a transaction
  * internally.  That also allows us to easily enforce that nothing writes
  * to the database by checking for xid assignments. ...
  */

Also it issues separated stream_start/stop messages around each streamed 
transaction chunk. So if streaming starts and stops too frequently it 
adds additional overhead and may even interfere with current in-progress 
transaction.

If I get it correctly, then it is rather expected with too small values 
of logical_decoding_work_mem. Probably it may be optimized, but I am not 
sure that it is worth doing right now.


Regards

-- 
Alexey Kondratov

Postgres Professional https://www.postgrespro.com
Russian Postgres Company