Horton

Re: Parallel copy

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: vignesh C <vignesh21@gmail.com>
Cc: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Rafia Sabih <rafia.pghackers@gmail.com>, Ashutosh Sharma <ashu.coek88@gmail.com>, Andres Freund <andres@anarazel.de>, Robert Haas <robertmhaas@gmail.com>, Ants Aasma <ants@cybertec.at>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Alastair Turner <minion@decodable.me>, Thomas Munro <thomas.munro@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2020-10-15T09:10:18Z
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. Allow WaitLatch() to be used without a latch.

  2. Add %P to log_line_prefix for parallel group leader

  3. Include replication origins in SQL functions for commit timestamp

  4. Avoid useless buffer allocations during binary COPY FROM.

On Wed, Oct 14, 2020 at 6:51 PM vignesh C <vignesh21@gmail.com> wrote:
>
> On Fri, Oct 9, 2020 at 11:01 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > I am not able to properly parse the data but If understand the wal
> > data for non-parallel (1116 |       0 |   3587203) and parallel (1119
> > |       6 |   3624405) case doesn't seem to be the same. Is that
> > right? If so, why? Please ensure that no checkpoint happens for both
> > cases.
> >
>
> I have disabled checkpoint, the results with the checkpoint disabled
> are given below:
>                                            | wal_records | wal_fpi | wal_bytes
> Sequential Copy                   | 1116            |       0   |   3587669
> Parallel Copy(1 worker)         | 1116            |       0   |   3587669
> Parallel Copy(4 worker)         | 1121            |       0   |   3587668
> I noticed that for 1 worker wal_records & wal_bytes are same as
> sequential copy, but with different worker count I had noticed that
> there is difference in wal_records & wal_bytes, I think the difference
> should be ok because with more than 1 worker the order of records
> processed will be different based on which worker picks which records
> to process from input file. In the case of sequential copy/1 worker
> the order in which the records will be processed is always in the same
> order hence wal_bytes are the same.
>

Are all records of the same size in your test? If so, then why the
order should matter? Also, even the number of wal_records has
increased but wal_bytes are not increased, rather it is one-byte less.
Can we identify what is going on here? I don't intend to say that it
is a problem but we should know the reason clearly.

-- 
With Regards,
Amit Kapila.