Horton

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

Masahiko Sawada <sawada.mshk@gmail.com>

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From: Masahiko Sawada <sawada.mshk@gmail.com>
To: John Naylor <john.naylor@enterprisedb.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: 2023-02-28T13:20:17Z
Lists: pgsql-hackers 
On Tue, Feb 28, 2023 at 3:42 PM John Naylor
<john.naylor@enterprisedb.com> wrote:
>
>
> On Fri, Feb 24, 2023 at 12:50 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > On Wed, Feb 22, 2023 at 6:55 PM John Naylor
> > <john.naylor@enterprisedb.com> wrote:
> > >
> > > That doesn't seem useful to me. If we've done enough testing to reassure us the new way always gives the same answer, the old way is not needed at commit time. If there is any doubt it will always give the same answer, then the whole patchset won't be committed.
>
> > My idea is to make the bug investigation easier but on
> > reflection, it seems not the best idea given this purpose.
>
> My concern with TIDSTORE_DEBUG is that it adds new code that mimics the old tid array. As I've said, that doesn't seem like a good thing to carry forward forevermore, in any form. Plus, comparing new code with new code is not the same thing as comparing existing code with new code. That was my idea upthread.
>
> Maybe the effort my idea requires is too much vs. the likelihood of finding a problem. In any case, it's clear that if I want that level of paranoia, I'm going to have to do it myself.
>
> > What do you think
> > about the attached patch? Please note that it also includes the
> > changes for minimum memory requirement.
>
> Most of the asserts look logical, or at least harmless.
>
> - int max_off; /* the maximum offset number */
> + OffsetNumber max_off; /* the maximum offset number */
>
> I agree with using the specific type for offsets here, but I'm not sure why this change belongs in this patch. If we decided against the new asserts, this would be easy to lose.

Right. I'll separate this change as a separate patch.

>
> This change, however, defies common sense:
>
> +/*
> + * The minimum amount of memory required by TidStore is 2MB, the current minimum
> + * valid value for the maintenance_work_mem GUC. This is required to allocate the
> + * DSA initial segment, 1MB, and some meta data. This number is applied also to
> + * the local TidStore cases for simplicity.
> + */
> +#define TIDSTORE_MIN_MEMORY (2 * 1024 * 1024L) /* 2MB */
>
> + /* Sanity check for the max_bytes */
> + if (max_bytes < TIDSTORE_MIN_MEMORY)
> + elog(ERROR, "memory for TidStore must be at least %ld, but %zu provided",
> + TIDSTORE_MIN_MEMORY, max_bytes);
>
> Aside from the fact that this elog's something that would never get past development, the #define just adds a hard-coded copy of something that is already hard-coded somewhere else, whose size depends on an implementation detail in a third place.
>
> This also assumes that all users of tid store are limited by maintenance_work_mem. Andres thought of an example of some day unifying with tidbitmap.c, and maybe other applications will be limited by work_mem.
>
> But now that I'm looking at the guc tables, I am reminded that work_mem's minimum is 64kB, so this highlights a design problem: There is obviously no requirement that the minimum work_mem has to be >= a single DSA segment, even though operations like parallel hash and parallel bitmap heap scan are limited by work_mem.

Right.

>  It would be nice to find out what happens with these parallel features when work_mem is tiny (maybe parallelism is not even considered?).

IIUC both don't care about the allocated DSA segment size. Parallel
hash accounts actual tuple (+ header) size as used memory but doesn't
consider how much DSA segment is allocated behind. Both parallel hash
and parallel bitmap scan can work even with work_mem = 64kB, but when
checking the total DSA segment size allocated during these operations,
it was 1MB.

I realized that there is a similar memory limit design issue also on
the non-shared tidstore cases. We deduct 70kB from max_bytes but it
won't work fine with work_mem = 64kB.  Probably we need to reconsider
it. FYI 70kB comes from the maximum slab block size for node256.

Regards,

-- 
Masahiko Sawada
Amazon Web Services: https://aws.amazon.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