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>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2019-09-26T21:24:55Z
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 Thu, Sep 26, 2019 at 04:36:20PM -0300, Alvaro Herrera wrote:
>On 2019-Sep-26, Alvaro Herrera wrote:
>
>> How certain are you about the approach to measure memory used by a
>> reorderbuffer transaction ... does it not cause a measurable performance
>> drop?  I wonder if it would make more sense to use a separate contexts
>> per transaction and use context-level accounting (per the patch Jeff
>> Davis posted elsewhere for hash joins ... though I see now that that
>> only works fot aset.c, not other memcxt implementations), or something
>> like that.
>
>Oh, I just noticed that that patch was posted separately in its own
>thread, and that that improved version does include support for other
>memory context implementations.  Excellent.
>

Unfortunately, that won't fly, for two simple reasons:

1) The memory accounting patch is known to perform poorly with many
child contexts - this was why array_agg/string_agg were problematic,
before we rewrote them not to create memory context for each group.

It could be done differently (eager accounting) but then the overhead
for regular/common cases (with just a couple of contexts) is higher. So
that seems like a much inferior option.

2) We can't actually have a single context per transaction. Some parts
(REORDER_BUFFER_CHANGE_INTERNAL_TUPLECID) of a transaction are not
evicted, so we'd have to keep them in a separate context.

It'd also mean higher allocation overhead, because now we can reuse
chunks cross-transaction. So one transaction commits or gets serialized,
and we reuse the chunks for something else. With per-transaction
contexts we'd lose some of this benefit - we could only reuse chunks
within a transaction (i.e. large transactions that get spilled to disk)
but not across commits.

I don't have any numbers, of course, but I wouldn't be surprised if it
was significant e.g. for small transactions that don't get spilled. And
creating/destroying the contexts is not free either, I think.


regards

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