Horton

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

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Dilip Kumar <dilipbalaut@gmail.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>, Michael Paquier <michael@paquier.xyz>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-03-28T06:25:53Z
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 Wed, Mar 4, 2020 at 9:14 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Wed, Mar 4, 2020 at 3:16 AM Tomas Vondra
> <tomas.vondra@2ndquadrant.com> wrote:
> >
> >
> > The first thing I realized that WAL-logging of assignments in v12 does
> > both the "old" logging (using dedicated message) and "new" with
> > toplevel-XID embedded in the first message. Yes, the patch was wrong,
> > because it eliminated all calls to ProcArrayApplyXidAssignment() and so
> > it was trivial to crash the replica due to KnownAssignedXids overflow.
> > But I don't think re-introducing XLOG_XACT_ASSIGNMENT message is the
> > right fix.
> >
> > I actually proposed doing this (having both ways to log assignments) so
> > that there's no regression risk with (wal_level < logical). But IIRC
> > Andres objected to it, argumenting that we should not log the same piece
> > of information in two very different ways at the same time (IIRC it was
> > discussed on the FOSDEM dev meeting, so I don't have a link to share).
> > And I do agree with him ...
> >
> > The question is, why couldn't the replica use the same assignment info
> > we already write for logical decoding? The main challenge is that now
> > the assignment can be sent in many different xlog messages, from a bunch
> > of resource managers (essentially, any xlog message with a xid can have
> > embedded XID of the toplevel xact). So the handling would either need to
> > happen in every rmgr, or we need to move it before we call the rmgr.
> >
> > For exampple, we might do this e.g. in StartupXLOG() I think, per the
> > attached patch (FWIW this particular fix was written by Masahiko Sawada,
> > not me). This does the trick for me - I'm no longer able to reproduce
> > the KnownAssignedXids overflow.
> >
> > The one difference is that we used to call ProcArrayApplyXidAssignment
> > for larger groups of XIDs, as sent in the assignment message. Now we
> > call it for each individual assignment. I don't know if this is an
> > issue, but I suppose we might introduce some sort of local caching
> > (accumulate the assignments into a local array, call the function only
> > when we have enough of them).
>
> Thanks for the pointers,  I will think over these points.
>

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.
(b)  Calling ProcArrayApplyXidAssignment() for each subtransaction can
be costly from the perspective of concurrency because it acquires
ProcArrayLock in Exclusive mode, so concurrently running transactions
might start blocking at this lock.  Also, I see that
SubTransSetParent() makes the page dirty, so it might lead to more
writes if we spread out setting that by calling it separately for each
sub-transaction.

Apart from this, I don't see how the proposed fix is correct because
as far as I can see it tries to remove the Xid before we even record
it via RecordKnownAssignedTransactionIds().  It seems after patch
RecordKnownAssignedTransactionIds() will be called after
ProcArrayApplyXidAssignment(), how could that be correct.

Thoughts?

-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com