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: Dilip Kumar <dilipbalaut@gmail.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, a.kondratov@postgrespro.ru, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2019-10-22T17:22:10Z
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 Tue, Oct 22, 2019 at 11:01:48AM +0530, Dilip Kumar wrote:
>On Tue, Oct 22, 2019 at 10:46 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
>>
>> On Thu, Oct 3, 2019 at 1:18 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>> >
>> > I have attempted to test the performance of (Stream + Spill) vs
>> > (Stream + BGW pool) and I can see the similar gain what Alexey had
>> > shown[1].
>> >
>> > In addition to this, I have rebased the latest patchset [2] without
>> > the two-phase logical decoding patch set.
>> >
>> > Test results:
>> > I have repeated the same test as Alexy[1] for 1kk and 1kk data and
>> > here is my result
>> > Stream + Spill
>> > N           time on master(sec)   Total xact time (sec)
>> > 1kk               6                               21
>> > 3kk             18                               55
>> >
>> > Stream + BGW pool
>> > N          time on master(sec)  Total xact time (sec)
>> > 1kk              6                              13
>> > 3kk            19                              35
>> >
>>
>> I think the test results for the master are missing.
>Yeah, That time, I was planning to compare spill vs bgworker.
>  Also, how about
>> running these tests over a network (means master and subscriber are
>> not on the same machine)?
>
>Yeah, we should do that that will show the merit of streaming the
>in-progress transactions.
>

Which I agree it's an interesting feature, I think we need to stop
adding more stuff to this patch series - it's already complex enough, so
making it even more (unnecessary) stuff is a distraction and will make
it harder to get anything committed. Typical "scope creep".

I think the current behavior (spill to file) is sufficient for v0 and
can be improved later - that's fine. I don't think we need to bother
with comparisons to master very much, because while it might be a bit
slower in some cases, you can always disable streaming (so if there's a
regression for your workload, you can undo that).

>   In general, yours and Alexy's test results
>> show that there is merit by having workers applying such transactions.
>>   OTOH, as noted above [1], we are also worried about the performance
>> of Rollbacks if we follow that approach.  I am not sure how much we
>> need to worry about Rollabcks if commits are faster, but can we think
>> of recording the changes in memory and only write to a file if the
>> changes are above a certain threshold?  I think that might help saving
>> I/O in many cases.  I am not very sure if we do that how much
>> additional workers can help, but they might still help.  I think we
>> need to do some tests and experiments to figure out what is the best
>> approach?  What do you think?
>I agree with the point.  I think we might need to do some small
>changes and test to see what could be the best method to handle the
>streamed changes at the subscriber end.
>
>>
>> Tomas, Alexey, do you have any thoughts on this matter?  I think it is
>> important that we figure out the way to proceed in this patch.
>>
>> [1] - https://www.postgresql.org/message-id/b25ce80e-f536-78c8-d5c8-a5df3e230785%40postgrespro.ru
>>
>

I think the patch should do the simplest thing possible, i.e. what it
does today. Otherwise we'll never get it committed.

regards

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