Re: Parallel CREATE INDEX for GIN indexes

On 4/18/25 03:03, Vinod Sridharan wrote:
> Hello,
> As part of testing this change I believe I found a scenario where the
> parallel build seems to trigger OOMs for larger indexes. Specifically,
> the calls for ginEntryInsert seem to leak memory into
> TopTransactionContext and OOM/crash the outer process.
> For serial build, the calls for ginEntryInsert tend to happen in a
> temporary memory context that gets reset at the end of the
> ginBuildCallback.
> For inserts, the call has a custom memory context and gets reset at
> the end of the insert.
> For parallel build, during the merge phase, the MemoryContext isn't
> swapped - and so this happens on the TopTransactionContext, and ends
> up growing (especially for larger indexes).
>
> I believe at the very least these should happen inside the tmpCtx
> found in the GinBuildState and reset periodically.
>
> In the attached patch, I've tried to do this, and I'm able to build
> the index without OOMing, and only consuming maintenance_work_mem
> through the merge process.
>
> Would appreciate your thoughts on this (and whether there's other approaches to
> resolve this too).
>

Thanks for the report. I didn't have time to look at this in detail yet,
but the fix looks roughly correct. I've added this to the list of open
items for PG18.

regards

--
Tomas Vondra