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From: Tomas Vondra <[email protected]>
To: [email protected]
Subject: pgsql: Enforce memory limit during parallel GIN builds
Date: Tue, 04 Mar 2025 19:45:26 +0000
Message-ID: <[email protected]> (raw)

Enforce memory limit during parallel GIN builds

Index builds are expected to respect maintenance_work_mem, just like
other maintenance operations. For serial builds this is done simply by
flushing the buffer in ginBuildCallback() into the index. But with
parallel builds it's more complicated, because there are multiple places
that can allocate memory.

ginBuildCallbackParallel() does the same thing as ginBuildCallback(),
except that the accumulated items are written into tuplesort. Then the
entries with the same key get merged - first in the worker, then in the
leader - and the TID lists may get (arbitrarily) long. It's unlikely it
would exceed the memory limit, but it's possible. We address this by
evicting some of the data if the list gets too long.

We can't simply dump the whole in-memory TID list. The GIN index bulk
insert code expects to see TIDs in monotonic order; it may fail if the
TIDs go backwards. If the TID lists overlap, evicting the whole current
TID list would break this (a later entry might add "old" TID values into
the already-written part).

In the workers this is not an issue, because the lists never overlap.
But the leader may see overlapping lists produced by the workers.

We can however derive a safe "horizon" TID - the entries (for a given
key) are sorted by (key, first TID), which means no future list can add
values before the last "first TID" we've seen. This patch tracks the
"frozen" part of the TID list, which we know can't change by merging
additional TID lists. If needed, we can evict this part of the list.

We don't want to do this too often - the smaller lists we evict, the
more expensive it'll be to merge them in the next step (especially in
the leader). Therefore we only trim the list if we have at least 1024
frozen items, and if the whole list is at least 64kB large.

These thresholds are somewhat arbitrary and conservative. We might
calculate the values from maintenance_work_mem, but tests show that does
not really improve anything (time, compression ratio, ...). So we stick
to these conservative values to release memory faster.

Author: Tomas Vondra
Reviewed-by: Matthias van de Meent, Andy Fan, Kirill Reshke
Discussion: https://postgr.es/m/6ab4003f-a8b8-4d75-a67f-f25ad98582dc%40enterprisedb.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b229c10164770769c3b5033785917ca7a43a2471

Modified Files
--------------
src/backend/access/gin/gininsert.c | 212 +++++++++++++++++++++++++++++++++++--
1 file changed, 204 insertions(+), 8 deletions(-)

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