Horton

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

Kuntal Ghosh <kuntalghosh.2007@gmail.com>

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From: Kuntal Ghosh <kuntalghosh.2007@gmail.com>
To: Alexey Kondratov <a.kondratov@postgrespro.ru>
Cc: Dilip Kumar <dilipbalaut@gmail.com>, 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: 2019-11-12T11:34:05Z
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 Tue, Nov 12, 2019 at 4:12 PM Alexey Kondratov
<a.kondratov@postgrespro.ru> wrote:
>
> 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.
>
Yeah, I've also found the same. With stream_start/stop message, it
writes 1 byte of checksum and 4 bytes of number of sub-transactions
which increases the write amplification significantly.


-- 
Thanks & Regards,
Kuntal Ghosh
EnterpriseDB: http://www.enterprisedb.com