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: 2022-09-28T03:49:07Z
Lists: pgsql-hackers 
On Fri, Sep 23, 2022 at 12:11 AM John Naylor
<john.naylor@enterprisedb.com> wrote:
>
>
> On Thu, Sep 22, 2022 at 11:46 AM John Naylor <john.naylor@enterprisedb.com> wrote:
> > One thing I want to try soon is storing fewer than 16/32 etc entries, so that the whole node fits comfortably inside a power-of-two allocation. That would allow us to use aset without wasting space for the smaller nodes, which would be faster and possibly would solve the fragmentation problem Andres referred to in
>
> > https://www.postgresql.org/message-id/20220704220038.at2ane5xkymzzssb%40awork3.anarazel.de
>
> While calculating node sizes that fit within a power-of-two size, I noticed the current base node is a bit wasteful, taking up 8 bytes. The node kind only has a small number of values, so it doesn't really make sense to use an enum here in the struct (in fact, Andres' prototype used a uint8 for node_kind). We could use a bitfield for the count and kind:
>
> uint16 -- kind and count bitfield
> uint8 shift;
> uint8 chunk;
>
> That's only 4 bytes. Plus, if the kind is ever encoded in a pointer tag, the bitfield can just go back to being count only.

Good point, agreed.

>
> Here are the v6 node kinds:
>
> node4:   8 +   4 +(4)    +   4*8 =   48 bytes
> node16:  8 +  16         +  16*8 =  152
> node32:  8 +  32         +  32*8 =  296
> node128: 8 + 256 + 128/8 + 128*8 = 1304
> node256: 8       + 256/8 + 256*8 = 2088
>
> And here are the possible ways we could optimize nodes for space using aset allocation. Parentheses are padding bytes. Even if my math has mistakes, the numbers shouldn't be too far off:
>
> node3:   4 +   3 +(1)    +   3*8 =   32 bytes
> node6:   4 +   6 +(6)    +   6*8 =   64
> node13:  4 +  13 +(7)    +  13*8 =  128
> node28:  4 +  28         +  28*8 =  256
> node31:  4 + 256 +  32/8 +  31*8 =  512 (XXX not good)
> node94:  4 + 256 +  96/8 +  94*8 = 1024
> node220: 4 + 256 + 224/8 + 220*8 = 2048
> node256:                         = 4096
>
> The main disadvantage is that node256 would balloon in size.

Yeah, node31 and node256 are bloated.  We probably could use slab for
node256 independently. It's worth trying a benchmark to see how it
affects the performance and the tree size.

BTW We need to consider not only aset/slab but also DSA since we
allocate dead tuple TIDs on DSM in parallel vacuum cases. FYI DSA uses
the following size classes:

static const uint16 dsa_size_classes[] = {
    sizeof(dsa_area_span), 0,   /* special size classes */
    8, 16, 24, 32, 40, 48, 56, 64,  /* 8 classes separated by 8 bytes */
    80, 96, 112, 128,           /* 4 classes separated by 16 bytes */
    160, 192, 224, 256,         /* 4 classes separated by 32 bytes */
    320, 384, 448, 512,         /* 4 classes separated by 64 bytes */
    640, 768, 896, 1024,        /* 4 classes separated by 128 bytes */
    1280, 1560, 1816, 2048,     /* 4 classes separated by ~256 bytes */
    2616, 3120, 3640, 4096,     /* 4 classes separated by ~512 bytes */
    5456, 6552, 7280, 8192      /* 4 classes separated by ~1024 bytes */
};

node256 will be classed as 2616, which is still not good.

Anyway, I'll implement DSA support for radix tree.

Regards,

-- 
Masahiko Sawada
PostgreSQL Contributors Team
RDS Open Source Databases
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