Horton

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

Peter Eisentraut <peter.eisentraut@2ndquadrant.com>

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From: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>, Greg Stark <stark@mit.edu>
Cc: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2018-01-18T13:24:06Z
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 1/12/18 23:19, Tomas Vondra wrote:
> Wouldn't the 'toplevel_by_lsn' be suitable for this? Subtransactions
> don't really commit independently, but as part of the toplevel xact. And
> that list is ordered by LSN, which is pretty much exactly the order in
> which we see the transactions.

Yes indeed.  There is even ReorderBufferGetOldestTXN().

> Another somewhat non-intuitive detail is that because ReorderBuffer
> switched to Generation allocator for changes (which usually represent
> 99% of the memory used during decoding), it does not reuse memory the
> way AllocSet does. Actually, it does not reuse memory at all, aiming to
> eventually give the memory back to libc (which AllocSet can't do).
> 
> Because of this evicting the youngest transactions seems like a quite
> bad idea, because those chunks will not be reused and there may be other
> chunks on the blocks, preventing their release.

Right.  But this raises the question whether we are doing the memory
accounting on the right level.  If we are doing all this tracking based
on ReorderBufferChanges, but then serializing changes possibly doesn't
actually free any memory in the operating system, that's no good.  Can
we get some usage statistics out of the memory context?  It seems like
we need to keep serializing transactions until we actually see the
memory context size drop.

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services