Horton

Re: 040_pg_createsubscriber.pl is slow and unstable (was Re: speed up a logical replica setup)

Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>

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From: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>, Peter Eisentraut <peter@eisentraut.org>, Noah Misch <noah@leadboat.com>, Alexander Lakhin <exclusion@gmail.com>, Euler Taveira <euler@eulerto.com>, Shlok Kyal <shlok.kyal.oss@gmail.com>, Tomas Vondra <tomas.vondra@enterprisedb.com>, "kuroda.hayato@fujitsu.com" <kuroda.hayato@fujitsu.com>, Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>, Michael Paquier <michael@paquier.xyz>, Andres Freund <andres@anarazel.de>, Fabrízio de Royes Mello <fabriziomello@gmail.com>, vignesh C <vignesh21@gmail.com>
Date: 2024-07-30T05:58:41Z
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. pg_createsubscriber: Remove obsolete comment

  2. pg_createsubscriber: Fix an unpredictable recovery wait time.

  3. Fix unstable test in 040_pg_createsubscriber.

  4. Fix the testcase introduced in commit 81d20fbf7a.

  5. Further weaken new pg_createsubscriber test on Windows.

  6. Temporarily(?) weaken new pg_createsubscriber test on Windows.

  7. Make pg_createsubscriber warn if publisher has two-phase commit enabled.

  8. Make pg_createsubscriber more wary about quoting connection parameters.

  9. pg_createsubscriber: Remove failover replication slots on subscriber

  10. pg_createsubscriber: Remove replication slot check on primary

  11. pg_createsubscriber: Only --recovery-timeout controls the end of recovery process

  12. pg_createsubscriber: creates a new logical replica from a standby server

  13. Add some const decorations

  14. Add option force_initdb to PostgreSQL::Test::Cluster:init()

  15. Remove MSVC scripts

On Tue, Jul 30, 2024 at 9:25 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Tue, Jul 30, 2024 at 1:48 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >
> > Robert Haas <robertmhaas@gmail.com> writes:
> > > On Sun, Jun 30, 2024 at 2:40 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > >> ... However, I added a new open item about how the
> > >> 040_pg_createsubscriber.pl test is slow and still unstable.
> >
> > > But that said, I see no commits in the commit history which purport to
> > > improve performance, so I guess the performance is probably still not
> > > what you want, though I am not clear on the details.
> >
> > My concern is described at [1]:
> >
> > >> I have a different but possibly-related complaint: why is
> > >> 040_pg_createsubscriber.pl so miserably slow?  On my machine it
> > >> runs for a bit over 19 seconds, which seems completely out of line
> > >> (for comparison, 010_pg_basebackup.pl takes 6 seconds, and the
> > >> other test scripts in this directory take much less).  It looks
> > >> like most of the blame falls on this step:
> > >>
> > >> [12:47:22.292](14.534s) ok 28 - run pg_createsubscriber on node S
> > >>
> > >> AFAICS the amount of data being replicated is completely trivial,
> > >> so that it doesn't make any sense for this to take so long --- and
> > >> if it does, that suggests that this tool will be impossibly slow
> > >> for production use.  But I suspect there is a logic flaw causing
> > >> this.  Speculating wildly, perhaps that is related to the failure
> > >> Alexander spotted?
> >
> > The followup discussion in that thread made it sound like there's
> > some fairly fundamental deficiency in how wait_for_end_recovery()
> > detects end-of-recovery.  I'm not too conversant with the details
> > though, and it's possible that pg_createsubscriber is just falling
> > foul of a pre-existing infelicity.
> >
> > If the problem can be correctly described as "pg_createsubscriber
> > takes 10 seconds or so to detect end-of-stream",
> >
>
> The problem can be defined as: "pg_createsubscriber waits for an
> additional (new) WAL record to be generated on primary before it
> considers the standby is ready for becoming a subscriber". Now, on
> busy systems, this shouldn't be a problem but for idle systems, the
> time to detect end-of-stream can't be easily defined.

AFAIU, the server will emit running transactions WAL record at least
15 seconds. So the subscriber should not have to wait longer than 15
seconds. I understand that it would be a problem for tests, but will
it be a problem for end users? Sorry for repetition, if this has been
discussed.

-- 
Best Wishes,
Ashutosh Bapat