Re: Can we use parallel workers to create index without active/transaction snapshot?

On 7/19/24 09:11, Hao Zhang wrote:
> Hi hackers,
> I'm doing work related to creating an index with parallel workers. I found
> that SnapshotAny
> is used in table_beginscan_parallel() when indexInfo->ii_Concurrent Is set
> to false. So can we
> not pass the snapshot from the parallel worker creator to the parallel
> worker? like this:

Maybe, but I wonder why are you thinking about doing this. I'm guessing
you're trying to skip "unnecessary" stuff to make parallel workers
faster, or is the goal different? FWIW I doubt this will make measurable
difference, I'd expect the mere fork() to be way more expensive than
copying the SnapshotAny (which I think is pretty small).

Up to you, but I'd suggest doing some measurements first, to show how
much overhead this actually is.

> ```> InitializeParallelDSM()
> {
> ...
>
> if (is_concurrent == false)
> {
> /* Serialize the active snapshot. */
> asnapspace = shm_toc_allocate(pcxt->toc, asnaplen);
> SerializeSnapshot(active_snapshot, asnapspace);
> shm_toc_insert(pcxt->toc, PARALLEL_KEY_ACTIVE_SNAPSHOT,
> asnapspace);
> }
>
> ...
> }
>
> ParallelWorkerMain()
> {
> ...
>
> if(is_concurrent == false)
> {
> asnapspace = shm_toc_lookup(toc, PARALLEL_KEY_ACTIVE_SNAPSHOT,
> false);
> tsnapspace = shm_toc_lookup(toc, PARALLEL_KEY_TRANSACTION_SNAPSHOT,
> true);
> asnapshot = RestoreSnapshot(asnapspace);
> tsnapshot = tsnapspace ? RestoreSnapshot(tsnapspace) : asnapshot;
> RestoreTransactionSnapshot(tsnapshot,
> fps->parallel_leader_pgproc);
> PushActiveSnapshot(asnapshot);
> }
>
> ...
> }
> ```
>

It's not clear to me where you get the is_concurrent flag in those
places. Also, in ParallelWorkerMain() you probably should not skip
restoring the transaction snapshot.

regards

--
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
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