Re: [PATCH] Reuse Workers and Replication Slots during Logical Replication

Hi Melih,

BACKGROUND
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We wanted to compare performance for the 2 different reuse-worker
designs, when the apply worker is already busy handling other
replications, and then simultaneously the test table tablesyncs are
occurring.

To test this scenario, some test scripts were written (described
below). For comparisons, the scripts were then run using a build of
HEAD; design #1 (v21); design #2 (0718).

HOW THE TEST WORKS
------------------

Overview:
1. The apply worker is made to subscribe to a 'busy_tbl'.
2. After the SUBSCRIPTION is created, the publisher-side then loops
(forever) doing INSERTS into that busy_tbl.
3. While the apply worker is now busy, the subscriber does an ALTER
SUBSCRIPTION REFRESH PUBLICATION to subscribe to all the other test
tables.
4. We time how long it takes for all tablsyncs to complete
5. Repeat above for different numbers of empty tables (10, 100, 1000,
2000) and different numbers of sync workers (2, 4, 8, 16)

Scripts
-------

(PSA 4 scripts to implement this logic)

testrun script
- this does common setup (do_one_test_setup) and then the pub/sub
scripts (do_one_test_PUB and do_one_test_SUB -- see below) are run in
parallel
- repeat 10 times

do_one_test_setup script
- init and start instances
- ipc setup tables and procedures

do_one_test_PUB script
- ipc setup pub/sub
- table setup
- publishes the "busy_tbl", but then waits for the subscriber to
subscribe to only this one
- alters the publication to include all other tables (so subscriber
will see these only after the ALTER SUBSCRIPTION PUBLICATION REFRESH)
- enter a busy INSERT loop until it informed by the subscriber that
the test is finished

do_one_test_SUB script
- ipc setup pub/sub
- table setup
- subscribes only to "busy_tbl", then informs the publisher when that
is done (this will cause the publisher to commence the stay_busy loop)
- after it knows the publishing busy loop has started it does
- ALTER SUBSCRIPTION REFRESH PUBLICATION
- wait until all the tablesyncs are ready <=== This is the part that
is timed for the test RESULT

PROBLEM
-------

Looking at the output files (e.g. *.dat_PUB and *.dat_SUB)  they seem
to confirm the tests are working how we wanted.

Unfortunately, there is some slot problem for the patched builds (both
designs #1 and #2). e.g. Search "ERROR" in the *.log files and see
many slot-related errors.

Please note - running these same scripts with HEAD build gave no such
errors. So it appears to be a patch problem.

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Kind Regards
Peter Smith.
Fujitsu Australia