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: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, Erik Rijkers <er@xs4all.nl>
Cc: Masahiko Sawada <sawada.mshk@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2018-12-16T14:31:52Z
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.

Attachments

Hi,

Attached is an updated version of this patch series. It's meant to be
applied on top of the 2pc decoding patch [1], because streaming of
in-progress transactions requires handling of concurrent aborts. So it
may or may not apply directly to master, I'm not sure - unfortunately
that's likely to confuse the cputube thing, but I don't want to include
the 2pc decoding bits here because that would be just confusing.

If needed, the part introducing logical_work_mem limit for ReorderBuffer
can be separated and committed independently, but I do expect this to be
committed after the 2pc decoding patch so I've left it like this.

This new version is mostly just a rebase to current master (or almost,
because 2pc decoding only applies to 29180e5d78 due to minor bitrot),
but it also addresses the new stuff committed since last version (most
importantly decoding of TRUNCATE). It also fixes a bug in WAL-logging of
subxact assignments, where the assignment was included in records with
XID=0, essentially failing to track the subxact properly.

For the logical_work_mem part, I think this is quite solid. The main
question is how to pick transactions for eviction. For now it uses the
same approach as master (i.e. picking the largest top-level transaction,
although measured by amount of memory and not just number of changes).

But I've realized that may not work with Generation context that great,
because unlike AllocSet it does not reuse the memory. That's nice as it
allows freeing old blocks (which AllocSet can't), but it means a small
transaction can have a change on old blocks preventing free(). That is
something we have in pg11 already, because that's where Generation
context got introduced - I haven't seen this issue in practice, but we
might need to do something about it.

In any case, I'm thinking we may need to pick a different eviction
algorithm - say using a transaction with the oldest change (and loop
until we release at least one block in the Generation context), or maybe
look for block mixing changes from the smallest number of transactions,
or something like that. Other ideas are welcome. I don't think the exact
algorithm is particularly critical, because it's meant to be triggered
only very rarely (i.e. pick logical_work_mem high enough).

The in-progress streaming is mostly mechanical extension of existing
functionality (new methods in various APIs, ...) and refactoring of
ReorderBuffer to handle incremental decoding. I'm sure it'd benefit from
reviews, of course.


regards

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