Horton

Re: Pluggable Storage - Andres's take

Amit Khandekar <amitdkhan.pg@gmail.com>

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From: Amit Khandekar <amitdkhan.pg@gmail.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Haribabu Kommi <kommi.haribabu@gmail.com>, Asim R P <apraveen@pivotal.io>, Pg Hackers <pgsql-hackers@postgresql.org>, Ashwin Agrawal <aagrawal@pivotal.io>, Dmitry Dolgov <9erthalion6@gmail.com>, Alvaro Herrera <alvherre@2ndquadrant.com>, Alexander Korotkov <a.korotkov@postgrespro.ru>
Date: 2019-02-21T11:43:45Z
Lists: pgsql-hackers 
On Thu, 21 Feb 2019 at 04:17, Robert Haas <robertmhaas@gmail.com> wrote:
>
> On Fri, Feb 8, 2019 at 5:18 AM Amit Khandekar <amitdkhan.pg@gmail.com> wrote:
> > In the attached v1 patch, the prefetch_distance is calculated as
> > effective_io_concurrency + 10. Also it has some cosmetic changes.
>
> I did a little brief review of this patch and noticed the following things.
>
> +} PrefetchState;
> That name seems too generic.

Ok, so something like XidHorizonPrefetchState ? On similar lines, does
prefetch_buffer() function name sound too generic as well ?

>
> +/*
> + * An arbitrary way to come up with a pre-fetch distance that grows with io
> + * concurrency, but is at least 10 and not more than the max effective io
> + * concurrency.
> + */
>
> This comment is kinda useless, because it only tells us what the code
> does (which is obvious anyway) and not why it does that.  Saying that
> your formula is arbitrary may not be the best way to attract support
> for it.

Well, I had checked the way the number of drive spindles
(effective_io_concurrency) is used to calculate the prefetch distance
for bitmap heap scans (ComputeIoConcurrency). Basically I think the
intention behind that method is to come up with a number that makes it
highly likely that we pre-fetch a block of each of the drive spindles.
But I didn't get how that exactly works, all the less for non-parallel
bitmap scans. Same is the case for the pre-fetching that we do here
for xid-horizon stuff, where we do the block reads sequentially. Me
and Andres discussed this offline, and he was of the opinion that this
formula won't help here, and instead we just keep a constant distance
that is some number greater than effective_io_concurrency. I agree
that instead of saying "arbitrary" we should explain why we have done
that, and before that, come up with an agreed-upon formula.


>
> + for (i = prefetch_state->next_item; i < nitems && count < prefetch_count; i++)
>
> It looks strange to me that next_item is stored in prefetch_state and
> nitems is passed around as an argument.  Is there some reason why it's
> like that?

We could keep the max count in the structure itself as well. There
isn't any specific reason for not keeping it there. It's just that
this function prefetch_state () is not a general function for
maintaining a prefetch state that spans across function calls; so we
might as well just pass the max count to that function instead of
having another field in that structure. I am not inclined specifically
towards either of the approaches.

>
> + /* prefetch a fixed number of pages beforehand. */
>
> Not accurate -- the number of pages we prefetch isn't fixed.  It
> depends on effective_io_concurrency.

Yeah, will change that in the next patch version, according to what we
conclude about the prefetch distance calculation.


-- 
Thanks,
-Amit Khandekar
EnterpriseDB Corporation
The Postgres Database Company

Commits

  1. Add default_table_access_method to postgresql.conf.sample.

  2. tableam: Avoid relying on relation size to determine validity of tids.

  3. tableam: Don't assume that every AM uses md.c style storage.

  4. Allow pg_class xid & multixid horizons to not be set.

  5. Fix slot type issue for fuzzy distance index scan over out-of-core table AM.

  6. tableam: comment and formatting fixes.

  7. table: docs: fix typos and grammar.

  8. tableam: basic documentation.

  9. Only allow heap in a number of contrib modules.

  10. tableam: Add table_finish_bulk_insert().

  11. tableam: sample scan.

  12. tableam: bitmap table scan.

  13. tableam: Move heap specific logic from estimate_rel_size below tableam.

  14. tableam: VACUUM and ANALYZE support.

  15. tableam: relation creation, VACUUM FULL/CLUSTER, SET TABLESPACE.

  16. tableam: Support for an index build's initial table scan(s).

  17. tableam: Add table_get_latest_tid, to wrap heap_get_latest_tid.

  18. tableam: Add helper for indexes to check if a corresponding table tuples exist.

  19. tableam: Add and use table_fetch_row_version().

  20. tableam: Use in CREATE TABLE AS and CREATE MATERIALIZED VIEW.

  21. Ensure sufficient alignment for ParallelTableScanDescData in BTShared.

  22. Don't reuse slots between root and partition in ON CONFLICT ... UPDATE.

  23. Use a virtual rather than a heap slot in two places where that suffices.

  24. Store tuples for EvalPlanQual in slots, rather than as HeapTuples.

  25. Use slots in trigger infrastructure, except for the actual invocation.

  26. Store table oid and tuple's tid in tuple slots directly.

  27. Allow to use HeapTupleData embedded in [Buffer]HeapTupleTableSlot.

  28. Add ExecStorePinnedBufferHeapTuple.

  29. Add ArchiveOpts to pass options to ArchiveEntry

  30. Rename RelationData.rd_amroutine to rd_indam.

  31. Rephrase references to "time qualification".

  32. Move remaining code from tqual.[ch] to heapam.h / heapam_visibility.c.

  33. Move generic snapshot related code from tqual.h to snapmgr.h.

  34. Remove superfluous tqual.h includes.

  35. Don't duplicate parallel seqscan shmem sizing logic in nbtree.

  36. Move vacuumlazy.c into access/heap.

  37. Support parallel bitmap heap scans.