Horton

Re: [PoC] Improve dead tuple storage for lazy vacuum

John Naylor <john.naylor@enterprisedb.com>

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From: John Naylor <john.naylor@enterprisedb.com>
To: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: Nathan Bossart <nathandbossart@gmail.com>, Andres Freund <andres@anarazel.de>, Matthias van de Meent <boekewurm+postgres@gmail.com>, Yura Sokolov <y.sokolov@postgrespro.ru>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2023-03-11T15:54:40Z
Lists: pgsql-hackers 
On Fri, Mar 10, 2023 at 9:30 PM Masahiko Sawada <sawada.mshk@gmail.com>
wrote:
>
> On Fri, Mar 10, 2023 at 3:42 PM John Naylor
> <john.naylor@enterprisedb.com> wrote:

> > I'd suggest sharing your todo list in the meanwhile, it'd be good to
discuss what's worth doing and what is not.
>
> Apart from more rounds of reviews and tests, my todo items that need
> discussion and possibly implementation are:

Quick thoughts on these:

> * The memory measurement in radix trees and the memory limit in
> tidstores. I've implemented it in v30-0007 through 0009 but we need to
> review it. This is the highest priority for me.

Agreed.

> * Additional size classes. It's important for an alternative of path
> compression as well as supporting our decoupling approach. Middle
> priority.

I'm going to push back a bit and claim this doesn't bring much gain, while
it does have a complexity cost. The node1 from Andres's prototype is 32
bytes in size, same as our node3, so it's roughly equivalent as a way to
ameliorate the lack of path compression. I say "roughly" because the loop
in node3 is probably noticeably slower. A new size class will by definition
still use that loop.

About a smaller node125-type class: I'm actually not even sure we need to
have any sub-max node bigger about 64 (node size 768 bytes). I'd just let
65+ go to the max node -- there won't be many of them, at least in
synthetic workloads we've seen so far.

> * Node shrinking support. Low priority.

This is an architectural wart that's been neglected since the tid store
doesn't perform deletion. We'll need it sometime. If we're not going to
make this work, why ship a deletion API at all?

I took a look at this a couple weeks ago, and fixing it wouldn't be that
hard. I even had an idea of how to detect when to shrink size class within
a node kind, while keeping the header at 5 bytes. I'd be willing to put
effort into that, but to have a chance of succeeding, I'm unwilling to make
it more difficult by adding more size classes at this point.

--
John Naylor
EDB: http://www.enterprisedb.com

Commits

  1. radixtree: Fix SIGSEGV at update of embeddable value to non-embeddable.

  2. Get rid of anonymous struct

  3. Teach radix tree to embed values at runtime

  4. Teach TID store to skip bitmap for small numbers of offsets

  5. Use bump context for TID bitmaps stored by vacuum

  6. Fix alignment of stack variable

  7. Use TidStore for dead tuple TIDs storage during lazy vacuum.

  8. Rethink create and attach APIs of shared TidStore.

  9. Fix inconsistent function prototypes with function definitions.

  10. Fix a calculation in TidStoreCreate().

  11. Fix potential integer handling issue in radixtree.h.

  12. Add TIDStore, to store sets of TIDs (ItemPointerData) efficiently.

  13. Fix link error for test_radixtree module on Windows

  14. Blind attempt to fix ODR violations

  15. Fix incorrect format specifier for int64

  16. Fix redefinition of typedefs

  17. Add template for adaptive radix tree

  18. Fix signedness error in 9f225e992 for gcc

  19. Introduce helper SIMD functions for small byte arrays

  20. Optimize vacuuming of relations with no indexes.

  21. Add bound check before bsearch() for performance

  22. Allocate consecutive blocks during parallel seqscans