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: Craig Ringer <craig@2ndquadrant.com>
Cc: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2017-12-24T13:43:49Z
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 12/24/2017 05:51 AM, Craig Ringer wrote:
> On 23 December 2017 at 12:57, Tomas Vondra <tomas.vondra@2ndquadrant.com
> <mailto:tomas.vondra@2ndquadrant.com>> wrote:
> 
>     Hi all,
> 
>     Attached is a patch series that implements two features to the logical
>     replication - ability to define a memory limit for the reorderbuffer
>     (responsible for building the decoded transactions), and ability to
>     stream large in-progress transactions (exceeding the memory limit).
> 
>     I'm submitting those two changes together, because one builds on the
>     other, and it's beneficial to discuss them together.
> 
> 
>     PART 1: adding logical_work_mem memory limit (0001)
>     ---------------------------------------------------
> 
>     Currently, limiting the amount of memory consumed by logical decoding is
>     tricky (or you might say impossible) for several reasons:
> 
>     * The value is hard-coded, so it's not quite possible to customize it.
> 
>     * The amount of decoded changes to keep in memory is restricted by
>     number of changes. It's not very unclear how this relates to memory
>     consumption, as the change size depends on table structure, etc.
> 
>     * The number is "per (sub)transaction", so a transaction with many
>     subtransactions may easily consume significant amount of memory without
>     actually hitting the limit.
> 
> 
> Also, even without subtransactions, we assemble a ReorderBufferTXN
> per transaction. Since transactions usually occur concurrently,
> systems with many concurrent txns can face lots of memory use.
> 

I don't see how that could be a problem, considering the number of
toplevel transactions is rather limited (to max_connections or so).

> We can't exclude tables that won't actually be replicated at the reorder
> buffering phase either. So txns use memory whether or not they do
> anything interesting as far as a given logical decoding session is
> concerned. Even if we'll throw all the data away we must buffer and
> assemble it first so we can make that decision.

Yep.

> Because logical decoding considers snapshots and cid increments even
> from other DBs (at least when the txn makes catalog changes) the memory
> use can get BIG too. I was recently working with a system that had
> accumulated 2GB of snapshots ... on each slot. With 7 slots, one for
> each DB.
> 
> So there's lots of room for difficulty with unpredictable memory use.
> 

Yep.

>     So the patch does two things. Firstly, it introduces logical_work_mem, a
>     GUC restricting memory consumed by all transactions currently kept in
>     the reorder buffer
> 
> 
> Does this consider the (currently high, IIRC) overhead of tracking
> serialized changes?
>  

Consider in what sense?


regards

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