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: Amit Kapila <amit.kapila16@gmail.com>
Cc: Dilip Kumar <dilipbalaut@gmail.com>, Michael Paquier <michael@paquier.xyz>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-03-29T00:59:05Z
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 Sat, Mar 28, 2020 at 03:29:34PM +0530, Amit Kapila wrote:
>On Sat, Mar 28, 2020 at 2:19 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>>
>> On Sat, Mar 28, 2020 at 11:56 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
>> >
>> >
>> > I have looked at the solution proposed and I would like to share my
>> > findings.  I think calling ProcArrayApplyXidAssignment for each
>> > subtransaction is not a good idea for a couple of reasons:
>> > (a) It will just beat the purpose of maintaining KnowAssignedXids
>> > array which is to avoid looking at pg_subtrans in
>> > TransactionIdIsInProgress() on standby.  Basically, if we remove it
>> > for each subXid, it will consider the KnowAssignedXids to be
>> > overflowed and check pg_subtrans frequently.
>>
>> Right, I also think this is a problem with this solution.  I think we
>> may try to avoid this by caching this information.  But, then we will
>> have to maintain this in some dimensional array which stores
>> sub-transaction ids per top transaction or we can maintain a list of
>> sub-transaction for each transaction.  I haven't thought about how
>> much complexity this solution will add.
>>
>
>How about if instead of writing an XLOG_XACT_ASSIGNMENT WAL, we set a
>flag in TransactionStateData and then log that as special information
>whenever we write next WAL record for a new subtransaction?  Then
>during recovery, we can only call ProcArrayApplyXidAssignment when we
>find that special flag is set in a WAL record.  One idea could be to
>use a flag bit in XLogRecord.xl_info.  If that is feasible then the
>solution can work as it is now, without any overhead or change in the
>way we maintain KnownAssignedXids.
>

Ummm, how is that different from what the patch is doing now? I mean, we
only write the top-level XID for the first WAL record in each subxact,
right? Or what would be the difference with your approach?

Anyway, I think you're right the ProcArrayApplyXidAssignment call was
done too early, but I think that can be fixed by moving it until after
the RecordKnownAssignedTransactionIds call, no? Essentially, right
before rm_redo().

You're right calling ProcArrayApplyXidAssignment() may be an issue,
because it exclusively acquires the ProcArrayLock. I've actually hinted
that might be an issue in my original message, suggesting we might add a
local cache of assigned XIDs (a small static array, doing essentially
the same thing we used to do on the upstream node). I haven't done that
in my WIP patch to keep it simple, but AFACS it'd work.

regards

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