MIME-Version: 1.0
From: Michail Nikolaev <michail.nikolaev@gmail.com>
Date: Sun, 19 Mar 2023 12:43:43 +0300
Message-ID: <CANtu0oh0si=jG5z_fLeFtmYcETssQ08kLEa8b6TQqDm_cinroA@mail.gmail.com>
Subject: Replace known_assigned_xids_lck by memory barrier
To: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Content-Type: multipart/mixed; boundary="0000000000001a707d05f73da65c"
Archived-At: <https://www.postgresql.org/message-id/CANtu0oh0si%3DjG5z_fLeFtmYcETssQ08kLEa8b6TQqDm_cinroA%40mail.gmail.com>
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Hello everyone and Tom.

Tom, this is about your idea (1) from 2010 to replace spinlock with a
memory barrier in a known assignment xids machinery.

It was mentioned by you again in (2) and in (3) we have decided to
extract this change into a separate commitfest entry.

So, creating it here with a rebased version of (4).

In a nutshell: KnownAssignedXids as well as the head/tail pointers are
modified only by the startup process, so spinlock is used to ensure
that updates of the array and head/tail pointers are seen in a correct
order. It is enough to pass the barrier after writing to the array
(but before updating the pointers) to achieve the same result.

Best regards.

[1]: https://github.com/postgres/postgres/commit/2871b4618af1acc85665eec0912c48f8341504c4#diff-8879f0173be303070ab7931db7c757c96796d84402640b9e386a4150ed97b179R2408-R2412

[2]: https://www.postgresql.org/message-id/flat/1249332.1668553589%40sss.pgh.pa.us#19d00eb435340f5c5455e3bf259eccc8

[3]: https://www.postgresql.org/message-id/flat/1225350.1669757944%40sss.pgh.pa.us#23ca1956e694910fd7795a514a3bc79f

[4]: https://www.postgresql.org/message-id/flat/CANtu0oiPoSdQsjRd6Red5WMHi1E83d2%2B-bM9J6dtWR3c5Tap9g%40mail.gmail.com#cc4827dee902978f93278732435e8521

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